在 Android 中运行 Rust

原文

为了我目前的一位客户,我们决定将 Rust 作为我们主要的编程语言。做出这个决定的原因有很多:除了技术优势 (merit) 外,还有一个无可争议的 (undisputable) 事实就是:Rust 仍然是一门相对较新的语言,花哨 (fancy) 且时髦 (hip) - 当你是一家初创公司 (startup) 时,使用十几年前的技术可能会让你陷入困境。我的意思是 - 不使用创新的技术如何进行创新?最快的成功方式就是对其大肆宣传 (aboard the hype train)。

”用户持有自己的数据“应该是产品的一个卖点,它不能是一个完全通过浏览器访问的服务,而应该是一种可以分发给用户,并让其运行在用户设备上的某个东西。我们在内部已经运行了一些 headless (一种无界面的软件运行模式) 实例,只要再完成一些琐碎的 (trivial) 工作,就可以为 Windows 和 Linux 系统制作可重新分发的程序包。但是我们知道如果程序包只能运行在桌面操作系统中时,将会严重阻碍应用的普及 - 如果我们想让它脱颖而出 (take off),则需要该应用程序的移动版本。这意味着我们必须要知道如何让我们的程序运行在 Android 或者 iOS 系统中。因为我对交叉编译与自动化构建已经有了一些经验,我主动的研究了这个主题。

获取工具

先从基础开始,我需要获取 Rust 交叉编译器。幸运的是,Rust 让此操作变得十分简单,因为只需要调用以下命令:

 rustup target add armv7-linux-androideabi  # For 32-bit ARM.
 rustup target add aarch64-linux-android    # For 64-bit ARM.

# x86_64 is mainly useful for running your app in the emulator.
# Speaking of hardware, there are some commercial x86-based tablets,
# and there's also hobbyists running Android-x86 on their laptops.
 rustup target add x86_64-linux-android

(注意:以后只会显示 aarch64 架构的所有示例)

我还需要 Android 的构建工具。在经过一番调研之后,我来到 Android Studio 的下载页面 并抓取了归档的命令行工具。尽管 SDK 包有 80+ MiB 大小,却依然只有所需工具的最小子集,所以我听从了互联网的建议并使用sdkmanager来安装额外的部件。

 cd ~/android/sdk/cmdline-tools/bin/
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">./</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord mathnormal">kmana</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord">−</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.151392em;"><span style="top:-2.5500000000000003em;margin-left:-0.03148em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">r</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal">oo</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord">&quot;</span></span></span></span>{HOME}/android/sdk" --install 'build-tools;29.0.2'
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">./</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord mathnormal">kmana</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord">−</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.151392em;"><span style="top:-2.5500000000000003em;margin-left:-0.03148em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">r</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal">oo</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord">&quot;</span></span></span></span>{HOME}/android/sdk" --install 'cmdline-tools;latest'
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">./</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord mathnormal">kmana</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord">−</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.151392em;"><span style="top:-2.5500000000000003em;margin-left:-0.03148em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">r</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal">oo</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord">&quot;</span></span></span></span>{HOME}/android/sdk" --install 'platform-tools'
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">./</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord mathnormal">kmana</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord">−</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.151392em;"><span style="top:-2.5500000000000003em;margin-left:-0.03148em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.02778em;">r</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal">oo</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord">&quot;</span></span></span></span>{HOME}/android/sdk" --install 'platforms;android-29'

尽管 Android 支持运行 native 代码,但是大多数应用还是采用 Java 或者 Kotlin 来编写,SDK 反应了这一点。为了能够使用 native 代码,我还需要一个工具 - 原生开发工具套件 (Native Development kit)。NDK 下载页面 提供了几个版本以供选择 - 在经过一段深思熟虑后,我决定使用 LTS 版本:r21e。

足够简单!或想太多?

搞定了开发工具之后,我决定试着直接编译项目。

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">o</span><span class="mord mathnormal">b</span><span class="mord mathnormal">u</span><span class="mord mathnormal">i</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.80952em;vertical-align:-0.19444em;"></span><span class="mord">−</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">e</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">‘‘‘</span><span class="mord cjk_fallback">和预期的一样,构建失败了,并且错误信息占满了整个屏幕。经过筛选</span><span class="mopen">(</span><span class="mord mathnormal">s</span><span class="mord mathnormal">i</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">t</span><span class="mclose">)</span><span class="mord cjk_fallback">后,显示存在一个链接错误:</span><span class="mord">‘‘‘</span><span class="mord mathnormal">co</span><span class="mord mathnormal">n</span><span class="mord mathnormal">so</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal" style="margin-right:0.02778em;">eerror</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">inkin</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.02691em;">w</span><span class="mord mathnormal">i</span><span class="mord mathnormal">t</span><span class="mord mathnormal">h</span><span class="mord">‘</span><span class="mord mathnormal">cc</span><span class="mord">‘</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">ai</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">e</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal">e</span><span class="mord mathnormal">x</span><span class="mord mathnormal">i</span><span class="mord mathnormal">t</span><span class="mord mathnormal">co</span><span class="mord mathnormal">d</span><span class="mord mathnormal">e</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">1/</span><span class="mord mathnormal">u</span><span class="mord mathnormal" style="margin-right:0.02778em;">sr</span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">t</span><span class="mord mathnormal">u</span><span class="mord mathnormal">p</span><span class="mord">.48656</span><span class="mord mathnormal">c</span><span class="mord">6</span><span class="mord mathnormal">c</span><span class="mord">6</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord">20546865726521.</span><span class="mord mathnormal">o</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.00773em;">R</span><span class="mord mathnormal">e</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">oc</span><span class="mord mathnormal">a</span><span class="mord mathnormal">t</span><span class="mord mathnormal">i</span><span class="mord mathnormal">o</span><span class="mord mathnormal">n</span><span class="mord mathnormal">s</span><span class="mord mathnormal">in</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">e</span><span class="mord mathnormal">n</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mord mathnormal">i</span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.05764em;">E</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.13889em;">F</span><span class="mopen">(</span><span class="mord mathnormal" style="margin-right:0.10903em;">EM</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">183</span><span class="mclose">)</span><span class="mord">/</span><span class="mord mathnormal">u</span><span class="mord mathnormal" style="margin-right:0.02778em;">sr</span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">t</span><span class="mord mathnormal">u</span><span class="mord mathnormal">p</span><span class="mord">.48656</span><span class="mord mathnormal">c</span><span class="mord">6</span><span class="mord mathnormal">c</span><span class="mord">6</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord">20546865726521.</span><span class="mord mathnormal">o</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.02778em;">error</span><span class="mord mathnormal">a</span><span class="mord mathnormal">dd</span><span class="mord mathnormal">in</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.03588em;">sy</span><span class="mord mathnormal">mb</span><span class="mord mathnormal">o</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">s</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal">i</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">e</span><span class="mord mathnormal">in</span><span class="mord mathnormal" style="margin-right:0.02691em;">w</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">n</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mord mathnormal" style="margin-right:0.02778em;">or</span><span class="mord mathnormal">ma</span><span class="mord mathnormal">t</span><span class="mord mathnormal">co</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mord mathnormal">ec</span><span class="mord mathnormal">t</span><span class="mord">2</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.43056em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.02778em;">error</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mord mathnormal">re</span><span class="mord mathnormal">t</span><span class="mord mathnormal">u</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">n</span><span class="mord mathnormal">e</span><span class="mord mathnormal">d</span><span class="mord">1</span><span class="mord mathnormal">e</span><span class="mord mathnormal">x</span><span class="mord mathnormal">i</span><span class="mord mathnormal">t</span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal">t</span><span class="mord mathnormal">u</span><span class="mord mathnormal">s</span><span class="mord">‘‘‘</span><span class="mord cjk_fallback">我认为这</span><span class="mopen">(</span><span class="mord cjk_fallback">错误提示</span><span class="mclose">)</span><span class="mord cjk_fallback">足够简单</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">o</span><span class="mord cjk_fallback">试图使用系统的链接器</span><span class="mopen">(</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal" style="margin-right:0.03148em;">ink</span><span class="mord mathnormal" style="margin-right:0.02778em;">er</span><span class="mclose">)</span><span class="mord cjk_fallback">而不是</span><span class="mord mathnormal">A</span><span class="mord mathnormal">n</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span><span class="mord mathnormal" style="margin-right:0.07153em;">DK</span><span class="mord cjk_fallback">的链接器。我可以使用</span><span class="mord">‘</span><span class="mord mathnormal" style="margin-right:0.07153em;">CC</span><span class="mord">‘</span><span class="mord cjk_fallback">和</span><span class="mord">‘</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord">‘</span><span class="mord cjk_fallback">环境变量让</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">o</span><span class="mord cjk_fallback">指向正确的链接器。</span><span class="mord">‘‘‘</span><span class="mord mathnormal">co</span><span class="mord mathnormal">n</span><span class="mord mathnormal">so</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">e</span></span></span></span> export ANDROID_NDK_ROOT="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal" style="margin-right:0.05764em;">OME</span></span><span class="mord">/</span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">n</span><span class="mord mathnormal">d</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord">&quot;</span></span></span></span> export TOOLCHAIN="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord mathnormal" style="margin-right:0.02778em;">RO</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.02778em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.10903em;">N</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.07153em;">K</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.07153em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.00773em;">R</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.13889em;">OOT</span></span><span class="mord">/</span><span class="mord mathnormal">t</span><span class="mord mathnormal">oo</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">c</span><span class="mord mathnormal">hain</span><span class="mord mathnormal">s</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mord mathnormal" style="margin-right:0.03588em;">v</span><span class="mord mathnormal">m</span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mord mathnormal">re</span><span class="mord mathnormal">b</span><span class="mord mathnormal">u</span><span class="mord mathnormal">i</span><span class="mord mathnormal">lt</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord mathnormal">x</span><span class="mord">8</span><span class="mord"><span class="mord">6</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.30110799999999993em;"><span style="top:-2.5500000000000003em;margin-left:0em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight">6</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord">4&quot;</span></span></span></span> export CC="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">29</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">an</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord">&quot;</span></span></span></span> export LD="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mord">&quot;</span></span></span></span> cargo build --target=aarch64-linux-android

让我失望的是,这并没有起作用。我不愿意花费一天的时间来和 Cargo 纠缠 (wrestle),因此我决定寻找是否有其他人给出了解决方案 - 很快,我便找到看似十分完美的工具。

cargo-apk

cargo-apk 是一个可以简单的将 Cargo 项目构建成.apk的工具。你所需要做得就是安装这个工具,在Cargo.toml文件中添加一些配置,然后你就可以继续了。

# cargo-apk compiles your code to an .so file,
# which is then loaded by the Android runtime
[lib]
path = "src/main.rs"
crate-type = ["cdylib"]
 
# Android-specic configuration follows.
[package.metadata.android]
# Name of your APK as shown in the app drawer and in the app switcher
apk_label = "Hip Startup"
 
# The target Android API level.
target_sdk_version = 29
min_sdk_version = 26
 
# See: https://developer.android.com/guide/topics/manifest/activity-element#screen
orientation = "portrait"

有了上面添加的配置,我试图使用cargo-apk来构建项目。

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">o</span><span class="mord mathnormal">in</span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mord mathnormal">c</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">o</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal">p</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span></span></span></span> export ANDROID_SDK_ROOT="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal" style="margin-right:0.05764em;">OME</span></span><span class="mord">/</span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">s</span><span class="mord mathnormal">d</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord">&quot;</span></span></span></span> export ANDROID_NDK_ROOT="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal" style="margin-right:0.05764em;">OME</span></span><span class="mord">/</span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">n</span><span class="mord mathnormal">d</span><span class="mord mathnormal" style="margin-right:0.03148em;">k</span><span class="mord">&quot;</span></span></span></span> cargo apk build --target aarch64-linux-android

令人惊奇的是,它成功了!(等等) 额,好吧,我再一次遇到了链接错误。但是这一次,它不是关于重定位和文件格式的神秘错误,而是一个缺少链接库的错误提示:

error: linking with `aarch64-linux-android29-clang` failed: exit code: 1
    aarch64-linux-android/bin/ld: cannot find -lsqlite3
    clang: error: linker command failed with exit code 1 (use -v to see invocation)

依赖,依赖,依赖

我们的项目使用 SQLite,这是一个 C 库。尽管 Rust 社区在每个可能的场合都吹捧 (tout) ”用 Rust 重写“在某种程度上是臭名昭著的,但是实际上某些与流行库一起使用的 crate 并不需要重新实现,因为这需要大量的 (colossal) 工作。相反,它们仅提供在 Rust 代码中调用库的方式,既可以作为 C 函数重新导出,也可以提供更加友好的 API 并稍微抽象化 FFI 调用。我们使用的 rusqlite 并没有什么不同,意味着我们也需要构建 SQLite。

SQLite 使用 GNU Autotool 进行构建。在对环境变量和用于配置的选项有了一些了解之后,我仔细浏览了 NDK 的文档 - 我找到了一个在各种构建系统(包括 Autotools) 中使用 NDK 的文档页面。尽管 Google 提供了 LTS 版本的 NDK,以及最新版本的文档,但在 r21 LTS 和最新的 r22 之间发生了变化,事情变得稍微有点棘手。幸运的是,Wayback 机器具有该页面的历史版本,让我能够找到合适的 NDK r21 的说明。

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.83333em;vertical-align:-0.15em;"></span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord mathnormal" style="margin-right:0.02778em;">RO</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.02778em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">A</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.13889em;">P</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.64444em;vertical-align:0em;"></span><span class="mord">29</span></span></span></span> TOOLCHAIN="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord mathnormal" style="margin-right:0.02778em;">RO</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.02778em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.10903em;">N</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.07153em;">K</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.07153em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.00773em;">R</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.13889em;">OOT</span></span><span class="mord">/</span><span class="mord mathnormal">t</span><span class="mord mathnormal">oo</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">c</span><span class="mord mathnormal">hain</span><span class="mord mathnormal">s</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">ll</span><span class="mord mathnormal" style="margin-right:0.03588em;">v</span><span class="mord mathnormal">m</span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mord mathnormal">re</span><span class="mord mathnormal">b</span><span class="mord mathnormal">u</span><span class="mord mathnormal">i</span><span class="mord mathnormal">lt</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.84444em;vertical-align:-0.15em;"></span><span class="mord mathnormal">x</span><span class="mord">8</span><span class="mord"><span class="mord">6</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.30110799999999993em;"><span style="top:-2.5500000000000003em;margin-left:0em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight">6</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord">4&quot;</span><span class="mord mathnormal">i</span></span></span></span> export CC="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span></span></span></span>{ANDROID_API}-clang"
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8777699999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">e</span><span class="mord mathnormal">x</span><span class="mord mathnormal">p</span><span class="mord mathnormal" style="margin-right:0.02778em;">or</span><span class="mord mathnormal" style="margin-right:0.07847em;">tCXX</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord">&quot;</span></span></span></span>{TOOLCHAIN}/bin/aarch64-linux-android<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.83333em;vertical-align:-0.15em;"></span><span class="mord"><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="mord mathnormal" style="margin-right:0.02778em;">RO</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">D</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.32833099999999993em;"><span style="top:-2.5500000000000003em;margin-left:-0.02778em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">A</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.15em;"><span></span></span></span></span></span></span><span class="mord mathnormal" style="margin-right:0.13889em;">P</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span></span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">an</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord">+</span><span class="mord">&quot;</span></span></span></span> export AR="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord">&quot;</span></span></span></span> export AS="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal">s</span><span class="mord">&quot;</span></span></span></span> export LD="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">d</span><span class="mord">&quot;</span></span></span></span> export RANLIB="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.69444em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">an</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">ib</span><span class="mord">&quot;</span></span></span></span> export STRIP="<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.02778em;">TOO</span><span class="mord mathnormal">L</span><span class="mord mathnormal" style="margin-right:0.07153em;">C</span><span class="mord mathnormal" style="margin-right:0.08125em;">H</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.07847em;">I</span><span class="mord mathnormal" style="margin-right:0.10903em;">N</span></span><span class="mord">/</span><span class="mord mathnormal">bin</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">i</span><span class="mord mathnormal">p</span><span class="mord">&quot;</span></span></span></span> ./configure --host=aarch64-linux-android --with-pic
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.03148em;">mak</span><span class="mord mathnormal">e</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.85396em;vertical-align:-0.19444em;"></span><span class="mord mathnormal" style="margin-right:0.05724em;">j</span></span></span></span>(nproc)

Pick me up, Scotty

使用上述方法,成功构建了 SQLite,生成了libsqlite3.so。现在只需要知道如何让 Cargo 使用它即可。在浏览 Cargo Book 时,我遇到了讲述环境变量的一个章节,它提及了RUSTFLAGS。和 Make 或 CMake 对待CFLAGSCXXFLAGS一样,RUSTFLAGS的内容被 Cargo 传递给rustc编译器,允许它影响编译器的行为。

尽管这种方式十分简单,但是对我来说不是很优雅,因此我进一步深入研究了其他选项。继续浏览 Cargo Book,我遇到了描述项目配置的章节,可以肯定的是,有一种方法可以指定 RUSTFLAGS。然而,无论我如何尝试,我始终都会收到来自 Cargo 的提示,告诉我关于未使用的 manifest 键的信息。

warning: unused manifest key: target.aarch64-linux-android.rustflags

浏览 Cargo Book 的更多章节,我遇到了关于构建脚本的章节。它们毫无疑问是一个强大的工具,但是我已经花费了很多时间摸索 (fumble) 学习 Cargo 的配置,不想再花更多的时间阅读关于如何编写构建脚本的内容,因此,最终我选择了环境变量的解决方案,并且可能会在之后尝试使用构建脚本的方式(不可能)。

我在终端中输入命令,并焦急的观察它的执行过程。

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.68333em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.00773em;">R</span><span class="mord mathnormal" style="margin-right:0.10903em;">U</span><span class="mord mathnormal" style="margin-right:0.13889em;">STF</span><span class="mord mathnormal">L</span><span class="mord mathnormal">A</span><span class="mord mathnormal" style="margin-right:0.05764em;">GS</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2777777777777778em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord">&quot;</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.68333em;vertical-align:0em;"></span><span class="mord mathnormal">L</span></span></span></span>(pwd)/sqlite-autoconf-3340000/.libs/" cargo apk build --target aarch64-linux-android

再一次,它。。。在某种程度上成功了。虽然链接器不再将错误解释成缺失库,但是cargo-apk无法找到该链接器并将其添加到最终的 APK 文件中。

 'lib/arm64-v8a/libstartup.so'...
Shared library "libsqlite3.so" not found.
Verifying alignment of target/debug/apk/statup.apk (4)...
      49 AndroidManifest.xml (OK - compressed)
     997 lib/arm64-v8a/libstartup.so (OK - compressed)
Verification succesful

当我还没有编译libsqlite3.so时,我返回上一步仔细阅读了链接器产生的错误信息。链接器组合了很多的目标文件,这些目标文件都位于target/aarch64-linux-android/debug/deps目录下。如果我将.so文件放在这里会发生什么?

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal">p</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:0.01968em;">ql</span><span class="mord mathnormal">i</span><span class="mord mathnormal">t</span><span class="mord mathnormal">e</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.8888799999999999em;vertical-align:-0.19444em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal">u</span><span class="mord mathnormal">t</span><span class="mord mathnormal">oco</span><span class="mord mathnormal">n</span><span class="mord mathnormal" style="margin-right:0.10764em;">f</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">3340000/.</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">ib</span><span class="mord mathnormal">s</span><span class="mord">/</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:0.01968em;">ql</span><span class="mord mathnormal">i</span><span class="mord mathnormal">t</span><span class="mord mathnormal">e</span><span class="mord">3.</span><span class="mord mathnormal">so</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">e</span><span class="mord mathnormal">t</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">d</span><span class="mord mathnormal">e</span><span class="mord mathnormal">b</span><span class="mord mathnormal" style="margin-right:0.03588em;">ug</span><span class="mord">/</span><span class="mord mathnormal">d</span><span class="mord mathnormal">e</span><span class="mord mathnormal">p</span><span class="mord mathnormal">s</span></span></span></span> cargo apk build --target aarch64-linux-android

让我惊讶的是,它成功了!

 'lib/arm64-v8a/libstartup.so'...
 'lib/arm64-v8a/libsqlite3.so'...
Verifying alignment of target/debug/apk/startup.apk (4)...
      49 AndroidManifest.xml (OK - compressed)
     997 lib/arm64-v8a/libstatup.so (OK - compressed)
15881608 lib/arm64-v8a/libsqlite3.so (OK - compressed)
Verification succesful

我现在有了一个可以安装在 Android 手机上的.apk文件。真是个巨大的成功!

应用 和 Activity

将 Rust 代码编译进.apk中后,我们剩下要做的就是要搞清楚如何将 Rust 代码与编写 UI 的 Java 代码合并。我天真的在 DuckDuckGo 中输入“如何组合 APK”。在阅读顶层几个结果后,明白了这明显是不可能的,至少在对 Android 应用的工作原理没有更深的了解的情况下是不可能的。但是,并不是说没有其他的方法,因为文章提出了另一种方法 - 将 Activity 组合到一个应用程序里。

如果你像我一样,之前从未开发过 Android,可能会疑惑“什么是 Activity”:当你设计一个应用时,它就是所谓的“界面”或者“视图”。例如,在购物应用中:

  • 登陆页面是一个 Activity
  • 产品搜索页面是一个 Activity
  • 所选产品的详情页面是一个 Activity
  • 购物车页面是一个 Activity
  • 结账页面是一个 Activity

这里的每个页面可能都包含一些交互元素,如无处不在的汉堡包菜单。如果你愿意,从理论上来讲,你可以将整个应用程序放在一个单独的 Activity 中,但是开发难度比较大。当然,关于 Activity 还有很多内容可以介绍,但是目前和我们要讲的内容关系不大。

让我们继续介绍有关 Rust 的内容。虽然我的问题的解决方案是将 Activity 组合到一个应用程序中,但是我不确定用 Rust 构建的.apk文件是如何与所有这些联系在一起的。在仔细研究了 cargo-apk 代码之后,我意识到它本质是将我的代码封装进一些胶水 (glue) 代码中,并为 Android 的运行创建 NativeActivity

为了将 Activity 组合进一个应用中,我需要修改 (tinker with) 应用程序的AndroidManifest.xml文件,在文档中添加合适的 Activity 节点。但是我应该如何知道cargo-apk生成的 NativeActivity 的属性呢?幸运的是,当cargo-apk工作时,它会生成一个最小版的AndroidManifest.xml文件,并将其放在生成的.apk旁边。其中 NativeActivity 的声明如下所示:

<activity
    android:name="android.app.NativeActivity"
    android:label="startup"
    android:screenOrientation="portrait"
    android:launchMode="standard"
    android:configChanges="orientation|keyboardHidden|screenSize">
    <meta-data android:name="android.app.lib_name" android:value="startup" />
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

我要做的就是将上面的代码片段复制并粘贴到 Java 应用程序的 manifest 中。

当然,这只是在应用的 manifest 中添加了一条语句,告诉应用将要包含哪些 Activity。Java 应用程序的构建过程不会知道libstartup.so文件的位置,并自动的将其包含在内。幸运的是,我只需要将库文件复制到指定的文件夹下即可,Gradle (Android 应用的构建工具) 会自动将它们采集起来。

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.03148em;">mk</span><span class="mord mathnormal">d</span><span class="mord mathnormal">i</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">p</span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">a</span><span class="mord mathnormal">pp</span><span class="mord">/</span><span class="mord mathnormal">src</span><span class="mord">/</span><span class="mord mathnormal">main</span><span class="mord">/</span><span class="mord mathnormal">jni</span><span class="mord mathnormal">L</span><span class="mord mathnormal">ib</span><span class="mord mathnormal">s</span><span class="mord">/</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">m</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.64444em;vertical-align:0em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">v</span><span class="mord">8</span><span class="mord mathnormal">a</span></span></span></span> cp sqlite-autoconf-3340000/.libs/libsqlite3.so android/app/src/main/jniLibs/arm64-v8a/
<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal">pt</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal" style="margin-right:0.03588em;">g</span><span class="mord mathnormal">e</span><span class="mord mathnormal">t</span><span class="mord">/</span><span class="mord mathnormal">aa</span><span class="mord mathnormal">rc</span><span class="mord mathnormal">h</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:0.77777em;vertical-align:-0.08333em;"></span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">in</span><span class="mord mathnormal">ux</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">d</span><span class="mord mathnormal">e</span><span class="mord mathnormal">b</span><span class="mord mathnormal" style="margin-right:0.03588em;">ug</span><span class="mord">/</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">ib</span><span class="mord mathnormal">s</span><span class="mord mathnormal">t</span><span class="mord mathnormal">a</span><span class="mord mathnormal">t</span><span class="mord mathnormal">u</span><span class="mord mathnormal">p</span><span class="mord">.</span><span class="mord mathnormal">so</span><span class="mord mathnormal">an</span><span class="mord mathnormal">d</span><span class="mord mathnormal">ro</span><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span><span class="mord">/</span><span class="mord mathnormal">a</span><span class="mord mathnormal">pp</span><span class="mord">/</span><span class="mord mathnormal">src</span><span class="mord">/</span><span class="mord mathnormal">main</span><span class="mord">/</span><span class="mord mathnormal">jni</span><span class="mord mathnormal">L</span><span class="mord mathnormal">ib</span><span class="mord mathnormal">s</span><span class="mord">/</span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.02778em;">r</span><span class="mord mathnormal">m</span><span class="mord">64</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord mathnormal" style="margin-right:0.03588em;">v</span><span class="mord">8</span><span class="mord mathnormal">a</span><span class="mord">/</span></span></span></span> cd android/ && ./gradlew && ./gradlew build

这些都完成后,我启动了构建,它成功了!我将.apk安装在我闲置的 (laying around) Android 设备中,但是...好像有哪里不太对劲呢!

two-launcher-activities

我的应用一旦安装成功后,会在应用的启动界面产生两个快捷方式。其中一个启动 Java 的 UI 界面,而另一个启动包含 Rust 代码的 NativeActivity。在阅读了更多关于 Activity 和 AndroidManifest 的内容后,我了解到,造成此问题的部分是 NativeActivity 的 - 即 (namely) category 节点声明应在启动器中显示它。一旦我将它移除,一切就会恢复正常,NativeActivity 不再显示在启动器中。

但是,仍然存在一个问题:我如何让 Java 的 Activity 要求 Rust 的Activity 为其工作?

恶意的 (Malicious) Intent

Android 中的 Activity 可以毫无问题的相互启动 - 如果这不可能,则无法真正在两者之间传递用户信息。调用另一个 Activity 的标准方法是通过 startActivity() 方法,该方法接收一个参数:Intent 类实例。

尽管 Intent 类的名称是不言而喻的 (self-explanatory),但是起初它的用法可能有点不直观。在它最基本的形式中,它仅包含对调用 Activity 实例的引用,以及我们要调用的 Activity 的类句柄。(确切的说,一个 Intent 需要调用一个 Context。Activity 只是 Context 的一种类型)。

但是,Intent 也可以用于传达为什么一个 Activity 会调用另一个 Activity 的信息(例如 action),可以用来区分例如“显示某些内容”和“编辑某些内容”;或要操作的数据 URI 及其 MIME 类型。除了 get/set 方法,Intent 还可以容纳几乎任何数量的“额外”数据,这些数据通常作为键值对存储。

Intent 提供了一种在 Activity 之间传递信息的标准化方式。调用者向被调用者提供处理其请求所需的一切信息,并且它可以接收包含所有请求信息的另一个 Intent 作为返回值。使用 Java 编写代码时,没有什么问题,但是,将 Rust 代码放入 NativeActivity 会发生什么?

如果你查看继承树,你可以看到 NativeActivity 继承了 Activity - 这意味着它可以访问 Activity 所有非私有方法。我可以调用getIntent()并从调用者中获取数据。除此之外,由于这是 Java 方法,并且我是在 native 代码中运行,因此需要使用 JNI (Java Native Interface) 执行函数调用。不幸的是,NativeActivity 没有任何其他的机制来传递信息或使用 Intent。这让我十分沮丧 (dismay),因为这意味着我必须要与 JNI 一起工作。

JNI 之旅

在这一点上,我花了太多时间却没有取得明显的 (tangible) 成果,这让我感到十分沮丧。另一方面,我意识到使用 JNI 带来了一些新的可能 - 不必使用 Activity 和 Intent,我可以将代码粘贴在函数中,并通过调用参数和返回值进行通信。有了这个新思路,我开始了对 JNI 的研究。

因为在 Java 中,万物皆对象,并且代码不能存在于类之外的部分 - native 代码也必须是类的一部分。因为我不需要持久化,因此简单的使用静态方法就很有用。

package com.startup.hip;
 
public class RustCode {
    public static native void doStuff();
}

上面是一个 Java 类的最小示例,其中带有一个标记为native的静态方法。有了这个,我需要实现相应的功能。但是我应该如何正确的使用函数签名呢?

幸运的是,Java 具有为 JNI 生成 C 语言头文件的功能。在 Java SE9 之前,它是一个独立的工具 - javah;后来,它作为-h选项合并到了主要的javac编译器可执行文件中。该选项需要一个目录参数,用来放置生成的.h文件。用法十分简单。

 javac -h ./ RustCode.java

调用上面的命令将创建一个com_startup_hip_RustCode.h文件,其中包含函数定义。

#include <jni.h>
JNIEXPORT void JNICALL Java_com_startup_hip_RustCode_doStuff(JNIEnv *, jclass);

有了这些知识,我就可以继续在 Rust 中创建适当的函数了。

C++ 闪回

当处理外部代码时,Rust 和 C 很像,主要是使用 extern 块。此外,与 C++ 一样,Rust 可以使用 name mangling - 这不足为奇,因为这门语言对范型和宏提供了强大的支持。幸运的是,Rust 提供了一种简单的方式来禁用 name mangling - 使用 #[no mangle] 注解。


#![allow(unused)]
fn main() {
use jni::{objects::JClass, JNIEnv};
 
#[no_mangle]
pub extern "C" fn Java_com_startup_hip_RustCode_doStuff(
    _env: JNIEnv,
    _class: JClass,
) {}
}

创建了函数声明之后,接下来我需要编写对应的实现。

接收参数

通常,native 函数需要接收一些参数。在本例中,它是一个包含代码的字符串,该代码随后将被传递给服务端。

package com.startup.hip;
 
public class RustCode {
    public static native void doStuff(String code);
}

修改 Java 代码之后,我重新生成了 C 语言的头文件并据此编辑了 Rust 代码。


#![allow(unused)]
fn main() {
use jni::{objects::JClass, JNIEnv};
 
#[no_mangle]
pub extern "C" fn Java_com_startup_hip_RustCode_doStuff(
    _env: JNIEnv,
    _class: JClass,
    code: JString,
) {}
}

这很简单。现在我需要从 Java 字符串中提取文本并将其传递给 Rust 代码。这比我预期 (anticipate) 要复杂的多。问题在于,JVM 内部使用 UTF-8 的修改版本存储字符串,而 Rust 字符串必须是有效的 UTF-8。尽管 Rust 具有用于处理任意字符串的类型,但是我们的代码仅使用“经典”的字符串类型,对其进行全部修改需要大量工作。

幸运的是,jni库带有内置的机制,可以通过特殊的 JNIStr 类型在标准 UTF-8 和 JVM 修改后的 UTF-8 之间进行转换。在仔细阅读了文档之后,我想到了以下代码:


#![allow(unused)]
fn main() {
// Convert from JString – a thinly wrapped JObject – to a JavaStr
let code_jvm = env.get_string(code).unwrap();
// Create a String from JavaStr, causing text conversion
let code_rust = String::from(code_jvm);
}

现在我有了一个 Rust 字符串,可以将其传递给之后的 Rust 代码。又是个巨大的成功!

返回值

接收参数只是故事的一半,我还需要一个返回值,巧的是,它也是一个字符串 - 一个代表服务端返回值的字符串。

package com.startup.hip;
 
public class RustCode {
    public static native String doStuff(String code);
}

我再一次修改了 Java 代码,重新生成了 C 语言的头文件,并据此编辑 Rust 代码。

use jni::{objects::JClass, JNIEnv};
 
#[no_mangle]
pub extern "C" fn Java_com_startup_hip_RustCode_doStuff<'a>(
    env: JNIEnv<'a>,
    _class: JClass,
    code: JString,
) -> JString<'a>
{
    // function body here
}

如你所见,JNI 中的返回值仍然作为返回值处理。剩下要做的事就是创建保存结果的 JString。与get_string()类似,JNIEnv结构还包含一个new_string()函数,该函数的功能与名称指代的完全相同。


#![allow(unused)]
fn main() {
// Copy-pasted from earlier snippet 
let code_rust = String::from(env.get_string(code_jni).unwrap());
 
let result = match some_rust_function(code_rust) {
    Ok(value) => format!("OK {}", value),
    Err(e) => format!("ER {:?}", e),
};
return env.new_string(result).unwrap();
}

就像这样,我的 JNI 封装器就完成了。现在,我可以在 Java 代码中调用 Rust 函数,将值传递给调用并接收返回值。

Rust 的错误处理

尽管代码如预期一样执行,但是我不喜欢出现的.unwrap()调用次数。毕竟,错误处理是 Rust 的重要组成部分,仅仅因为我正在进行语言的互操作,并不意味着就可以忽略此事。恰恰相反,我认为两种语言的接触面应该尽可能简单,以防止最终发现一些晦涩的错误是由于互操作性差而引起的。而且,必须检查 Java 的返回值以确定调用是否成功,这使得整个过程使用起来有些笨拙 (clunky)。

我没有重复造轮子,而是对如何更好的将 Rust 的 Result<A, B> 方式转换成 Java 侧的代码进行了思考。幸运的是,我的 Rust 函数的返回值都是字符串。至于错误,大多数错误要么是不可恢复的,要么是由错误的输入引起的 - 这意味着我可以放弃 (forego) 使用精确的错误代码,而仅仅依靠正确格式的错误信息 - 这又是指字符串。因此Result<A, B>可以变成Result<String, String>

定义 Java 类

尽管 Java 支持范型(虽然有点欺骗的感觉),但是我不想从 JNI 中深入了解使用范型的细节。我决定创建一个 Java 类,大致表示Result<String, String>语义。

public class Result {
    private boolean ok;
    private String value;
 
    public Result(boolean is_ok, String value) {
        this.ok = is_ok;
        this.value = value;
    }
 
    public boolean isOk() {
        return this.ok;
    }
 
    public boolean isError() {
        return !this.ok;
    }
 
    public String getValue() {
        return this.ok ? this.value : null;
    }
 
    public String getError() {
        return this.ok ? null : this.value;
    }
}

尽管完成了这项工作,但与 Rust 相比,它有一些缺点 - 最严重的就是当访问错误的结果变量时返回null。由于 null 对于 Java 字符串来说是一个没有问题的值,因此调用getValue()可能没有注意并将其传递给其他地方导致在无关紧要的代码中弹出 NullPointerException。尽管可以通过抛出异常来轻松地改善这个问题,我仍决定使用最好的方式来处理这个问题,以便此处以后永远也不需要添加新的内容。

从 JNI 中返回一个对象

剩下的唯一一件事就是从 Rust 函数中返回 Result 类的实例。经过一番搜索后,我找到了名为 NewObject() 的 JNI 函数。该函数有四个参数:

  • JNI 环境的句柄
  • 我们想要创建的类的句柄
  • 构造函数签名
  • 构造函数的参数

Rust 函数将 JNI 环境句柄作为其参数之一,因此已经进行了处理。构造函数参数可以作为数组传递,我需要找到另外两个函数参数。

为了获取该函数的句柄,JNI 提供了FindClass()函数。它有两个参数:环境句柄和类的完全限定名 - 简单的说就是类的“导入名”,但是./代替。例如java.lang.String变成java/lang/String。在本例中是指,com.startup.hip.Result变成com/startup/hip/Result

构造函数签名是一个字符串,它很好的描述了构造函数签名需要多少个参数以及哪些类型。乍一看,这让人有些困惑 - 但后来我想起 Java 支持函数重载,并且还包括构造函数。由于一个类可能有多个构造函数,所以我必须让 JNI 知道我想使用哪个构造函数。在互联网上搜索了之后,我发现最简单的学习函数签名的方法就是编译 Java 类。然后使用 Java 的反汇编工具:javap

 javac android/app/src/main/java/com/startup/hip/Result.java
 javap -s android/app/src/main/java/com/startup/hip/Result.class
Compiled from "Result.java"
public class com.startup.hip.Result {
  public com.startup.hip.Result(boolean, java.lang.String);
    descriptor: (ZLjava/lang/String;)V

  public boolean isOk();
    descriptor: ()Z

  public boolean isError();
    descriptor: ()Z

  public java.lang.String getValue();
    descriptor: ()Ljava/lang/String;

  public java.lang.String getError();
    descriptor: ()Ljava/lang/String;
}

执行了上述的命令,现在我知道了我想要使用的函数签名是(ZLjava/lang/String;)V

在所有步骤都准备就绪之后,是时候创建持有构造函数参数的数组,并调用NewObject()


#![allow(unused)]
fn main() {
fn create_java_result<'e>(
    env: &JNIEnv<'e>,
    is_ok: bool,
    value: &str,
) -> JObject<'e>
{
    let class = env
        .find_class("com/startup/hip/Result")
        .unwrap();
    let args: [JValue<'e>; 2] = [
        JValue::Bool(u8::from(is_ok)),
        JValue::Object(JObject::from(env.new_string(value).unwrap())),
    ];
    env.new_object(class, "(ZLjava/lang/String;)V", &args)
        .unwrap()
}
}

现在,我可以从 native 函数中返回自定义的Result Java 类了。

使用更通用的解决方案

尽管上面的代码可以很好的实现这个目的,但是它有一个缺点:它显示地采用了布尔值和字符串,要求调用者自己处理 Result 并使用适当的参数调用函数。编写“错误应该尽早返回”的逻辑很繁琐 (tedious),但是幸运的是,Rust 为此提供了一个解决方案 - ? 运算符。但是我们的代码从不同的库中调用函数,这些函数又使用了不同的错误类型 - 这意味着我们无法使用Result<OurType, OurError>,并且必须执行类似 (akin) Result<OurType, std::error::Error>的操作 - 这是不可能的,因为 Rust 不允许将特征用作函数的返回类型。

解决此问题的标准方法是使用 Box,但为了使事情变得更加简单,我决定使用 anyhow 库,该库允许按我的喜好混合和匹配错误。不管怎样,我可以这样编写代码:


#![allow(unused)]
fn main() {
fn rust_result_to_java_result<'e, T>(
    env: &JNIEnv<'e>,
    result: anyhow::Result<T>,
) -> JObject<'e>
where
    T: Display,
{
    let (is_ok, value) = match result {
        Ok(v) => (true, format!("{}", v)),
        Err(e) => (false, format!("{:?}", e)),
    };
    create_java_result(env, is_ok, value)
}
 
fn actually_do_stuff<'a>(
    env: JNIEnv<'a>,
    code: JString,
) -> anyhow::Result<String>
{
    let code = String::from(env.get_string(code)?);
    let intermediate_value = some_rust_function(code)?;
    other_rust_function(intermediate_value)
}
 
#[no_mangle]
pub extern "C" fn Java_com_startup_hip_RustCode_doStuff<'a>(
    env: JNIEnv<'a>,
    _class: JClass,
    code: JString,
) -> JObject<'a>
{
    rust_result_to_java_result(actually_do_stuff(env, code))
}
}

更简单了!现在我可以返回任何想要的结果,并将其转换为 Java 类的实例,以供 Java 代码使用。

封装

在 Android 中运行 Rust 并不是一件容易的事,但是我对最终找到的解决方案感到满意。我们使用及其普通 (bog-standard) 的 Rust 编写代码并将其编译到共享库中,然后由 JVM 在运行时加载。尽管 JNI 乍一看有点令人生畏 (intimidate),但是使用此标准化解决方案意味着 Java 代码和 Gradle 构建系统都不关心我们的 native 代码是用 Rust 编写的。使用 Cargo 进行交叉编译仍然有些棘手 (tricky),因为事实证明cargo-apk设置了许多环境变量以使整个过程正常运行。我们的代码还依赖于外部库 - 但是所有的这些都可以通过一堆 shell 脚本来解决。

如果你想要自己尝试一下,我已经准备了一个公共 Github 仓库,其中包含了一个最小的 Android 应用程序,既包含用 Rust 编写的部分,还依赖于外部的 C 库。该项目的许可证是 zlib。因此可以随意的获取源代码并将其用于你的个人目的。

参考