Flutter源码剖析(一):源码获取与构建

概述

本文介绍了Flutter源码的获取与构建，后面会另有文章介绍Flutter源码的版本管理、开发环境搭建等主题。

准备工作

Flutter源码分为两个部分：

• flutter/flutter是框架层，为开发者提供各种接口，主要是dart代码。
• flutter/engine是引擎层，负责Flutter的渲染以及宿主的交互。

相关依赖的安装可参考官方文档：Setting up the Engine development environment · flutter/flutter Wiki。以我的Mac为例，如JDK等一般都已经安装，无需担心。

源码下载

flutter/flutter可以直接通过git下载，但是flutter/engine需要通过gclient工具获取，因为engine有很多依赖，gclient可以很好地处理这些依赖，简化源码管理流程。

首先，新建一个目录，下载flutter框架代码：

$ mkdir flutter_source_code
$ cd flutter_source_code
$ git clone https://github.com/flutter/flutter.git
Cloning into 'flutter'...
remote: Enumerating objects: 12, done.
remote: Counting objects: 100% (12/12), done.
remote: Compressing objects: 100% (12/12), done.
remote: Total 272396 (delta 0), reused 6 (delta 0), pack-reused 272384
Receiving objects: 100% (272396/272396), 116.98 MiB | 2.48 MiB/s, done.
Resolving deltas: 100% (210440/210440), done.

获取depot_tools工具（这个一开始是用来管理chromium源码的）：

$ git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
Cloning into 'depot_tools'...
remote: Sending approximately 34.14 MiB ...
remote: Total 40539 (delta 27803), reused 40539 (delta 27803)
Receiving objects: 100% (40539/40539), 34.14 MiB | 5.04 MiB/s, done.
Resolving deltas: 100% (27803/27803), done.

设置环境变量（每次构建之前都要设置，也可以写入系统配置）：

export PATH=$PATH:`pwd`/depot_tools

开始拉取代码（这一步比较耗时）

$ gclient sync                                                                                               [18:04:43] 

......

remote: Enumerating objects: 25, done. 
remote: Counting objects: 100% (25/25), done.
remote: Compressing objects: 100% (22/22), done.
remote: Total 209672 (delta 10), reused 13 (delta 3), pack-reused 209647
Receiving objects: 100% (209672/209672), 196.61 MiB | 3.74 MiB/s, done.
Resolving deltas: 100% (153791/153791), done.
Syncing projects:  31% (33/104) src/third_party/vulkan
[0:03:59] Still working on:
[0:03:59]   src/ios_tools
[0:03:59]   src/third_party/angle
[0:03:59]   src/third_party/dart
[0:03:59]   src/third_party/icu

......

[0:12:48] Still working on:
[0:12:48]   src/third_party/dart

Syncing projects: 100% (104/104), done.
Running hooks: 100% ( 9/ 9) dart package config
________ running 'vpython src/flutter/tools/run_third_party_dart.py' in '/Users/vimerzhao/WorkProject/flutter_source_code'
Resolving dependencies... (1.7s)
+ charcode 1.1.3
+ collection 1.14.13
+ meta 1.2.3
+ package_config 1.9.3
+ path 1.7.0
+ pub_semver 1.4.4
+ source_span 1.7.0
+ string_scanner 1.0.5
+ term_glyph 1.1.0
+ yaml 2.2.1
Changed 10 dependencies!

需要注意的是，Syncing projects: 100% (104/104), done之后，会继续下载一些大文件，可能命令行没有输出，一定不能强制退出，可以通过资源管理器查看网络的流量，确定cipd是否在下载：

（因为git不是很擅长下载大文件，所以产生了cipd这个程序来做这些工作）

此时的目录结构：

$ tree -L 1
.
├── depot_tools # 源码管理工具
├── flutter     # flutter framework目录
└── src         # flutter engine以及相关依赖所在目录

framework的版本和engine的版本是一一对应的，framework的分支规则如下：

• stable是当前的稳定分支，无特殊情况，推荐开发者使用该分支作为flutter sdk
• master包含最新的特性，但是不稳定
• 每个版本会打上对应的tag

截止到 2020-10-29 ,最新的较稳定版本是 1.22.0，于是我们也先切到这个版本（不是必选的，但是个人认为：基于一个明确的版本编译和修改源代码似乎更合适）。

$ cd flutter
$ git checkout 1.22.0
$ cat bin/internal/engine.version
5babba6c4d25fa237bbf755ab85c9a0c50b3c6ec

engine.version这个文件指定了framework对应的engine版本，接下来，我们进入engine目录切换到这次commit。

$ cd ../src/flutter
$ git reset --hard 5babba6c4d25fa237bbf755ab85c9a0c50b3c6ec
HEAD is now at 5babba6c4 Flutter 1.22.0-12.3.pre engine cherrypicks (#21466)

此时，我们需要执行以下命令：

$ gclient sync --with_branch_heads --with_tags
Syncing projects: 100% (104/104), done.
......
Running hooks: 100% ( 8/ 8) dart package config
......
Running hooks: 100% (8/8), done. 

后面这两个参数的含义比较晦涩，参考Chromium的说明，其含义是：

Checkout all the submodules at their branch DEPS revisions

因为切换分支之后，某些依赖的版本可能有更改，所以需要再次sync一下，直接在src/flutter目录执行即可。

以上就完成了源码环境的搭建，下面正式开始编译。

源码编译

首先我们退回到src目录，然后通过gn生成ninja需要的元数据：

$ cd ..
$ pwd
/Users/vimerzhao/WorkProject/flutter_source_code/src
$ ./flutter/tools/gn --unoptimized --android --runtime-mode debug --android-cpu arm
Generating GN files in: out/android_debug_unopt
Generating Xcode projects took 75ms
Done. Made 438 targets from 197 files in 1960ms

对于编译参数，Compiling the engine · flutter/flutter Wiki有详细介绍，在此不做赘述，这里我构建的是一个未优化、Android平台、debug版本、arm 32位的 engine。

此时，查看out目录，可以看到：

$ ls out/
android_debug_unopt   compile_commands.json

compile_commands.json可以作为IDE的索引文件，提供类/函数/变量的跳转等能力，后面会说到。

然后就可以开始正式的编译了：

$ ninja -C out/android_debug_unopt
ninja: Entering directory `out/android_debug_unopt'
[38/3844] ACTION //flutter/shell/platform/android:flutter_shell_java(//build/toolchain/android:clang_arm)
警告: ../../third_party/android_tools/sdk/build-tools/30.0.1/core-lambda-stubs.jar(java/lang/invoke/LambdaMetafactory.class): 主版本 53 比 52 新, 此编译器
支持最新的主版本。
  建议升级此编译器。
注: 某些输入文件使用或覆盖了已过时的 API。
注: 有关详细信息, 请使用 -Xlint:deprecation 重新编译。
1 个警告
[3844/3844] STAMP obj/default.stamp

$ ls out/android_debug_unopt                                                                 [19:35:15]
all.xcodeproj                                 flutter_embedding_debug-sources.jar.md5.stamp lib.stripped
args.gn                                       flutter_embedding_debug.jar                   libflutter.so
armeabi_v7a_debug.jar                         flutter_embedding_debug.jar.md5.stamp         libflutter.so.TOC
armeabi_v7a_debug.pom                         flutter_embedding_debug.pom                   obj
build.ninja                                   flutter_icu                                   toolchain.ninja
build.ninja.d                                 flutter_patched_sdk                           vm_outline_strong.dill
clang_x64                                     gen                                           vm_platform_strong.dill
flutter.jar                                   gyp-mac-tool                                  vm_platform_strong.dill.d
flutter_embedding_debug-sources.jar           icudtl.dat                                    zip_archives

其中，flutter_embedding_debug.jar是Android嵌入层代码，libflutter.so是flutter的引擎层代码，通过这两个文件，可以在Android工程混合接入Flutter代码。

源码使用

创建一个工程：

$ pwd
/Users/vimerzhao/WorkProject/flutter_source_code
$ ls
depot_tools flutter     src
$ ./flutter/bin/flutter create flutter_demo
Downloading Dart SDK from Flutter engine 5babba6c4d25fa237bbf755ab85c9a0c50b3c6ec...
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  172M  100  172M    0     0  4381k      0  0:00:40  0:00:40 --:--:-- 4723k
Building flutter tool...
Downloading Material fonts...                                       0.5s
Downloading Gradle Wrapper...                                       0.1s
Downloading package sky_engine...                                   0.3s
Downloading flutter_patched_sdk tools...                            2.7s
Downloading flutter_patched_sdk_product tools...                    2.1s
Downloading darwin-x64 tools...                                     8.2s
Downloading libimobiledevice...                                     0.0s
Downloading usbmuxd...                                              0.7s
Downloading libplist...                                             0.0s
Downloading openssl...                                              0.2s

......
Creating project flutter_demo...
  flutter_demo/ios/Runner.xcworkspace/contents.xcworkspacedata (created)
  flutter_demo/ios/Runner.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist (created)

  ......

  flutter_demo/.idea/runConfigurations/main_dart.xml (created)
  flutter_demo/.idea/libraries/Dart_SDK.xml (created)
  flutter_demo/.idea/libraries/KotlinJavaRuntime.xml (created)
  flutter_demo/.idea/modules.xml (created)
  flutter_demo/.idea/workspace.xml (created)
Running "flutter pub get" in flutter_demo...                        2.3s
Wrote 71 files.

All done!

......

Run "flutter doctor" for information about installing additional components.

In order to run your application, type:

  $ cd flutter_demo
  $ flutter run

使用flutter并指定本地engine运行（不指定则会拉远程的、已经构建好的engine）。

$  ../flutter/bin/flutter run --local-engine-src-path ~/WorkProject/flutter_source_code/src --local-engine=android_debug_unopt
No Flutter engine build found at /Users/vimerzhao/WorkProject/flutter_source_code/src/out/host_debug_unopt. 

$ ../flutter/bin/flutter run --local-engine-src-path ~/WorkProject/flutter_source_code/src --local-engine=host_debug_unopt
Launching lib/main.dart on DUK AL20 in debug mode...
Oops; flutter has exited unexpectedly: "Invalid argument(s): Cannot find executable for
/Users/vimerzhao/WorkProject/flutter_source_code/src/out/host_debug_unopt/dart-sdk/bin/dart.".
A crash report has been written to /Users/vimerzhao/WorkProject/flutter_source_code/flutter_demo/flutter_02.log.
...
FAILURE: Build failed with an exception
* Where:
Script '/Users/vimerzhao/WorkProject/flutter_source_code/flutter/packages/flutter_tools/gradle/flutter.gradle' line: 904
* What went wrong:
Execution failed for task ':app:compileFlutterBuildDebug'.
> Process 'command '/Users/vimerzhao/WorkProject/flutter_source_code/flutter/bin/flutter'' finished with non-zero exit value 1
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.
* Get more help at https://help.gradle.org
BUILD FAILED in 1s
Running Gradle task 'assembleDebug'...
Running Gradle task 'assembleDebug'... Done                         1.8s
https://git.io/JTDf0

$../flutter/bin/flutter run --local-engine-src-path ~/WorkProject/flutter_source_code/src --local-engine=h
ost_debug_unopt
Launching lib/main.dart on DUK AL20 in debug mode...
Error: Error when reading '../src/out/host_debug_unopt/gen/frontend_server.dart.snapshot': No such file or directory
the Dart compiler exited unexpectedly.
the Dart compiler exited unexpectedly.
Running Gradle task 'assembleDebug'...

总的来说，遇到一些官方文档上没有提到的问题：

1. 输入android_debug_unopt 却提示找不到 host_debug_unopt，这个莫名其妙，只能先改一下之前构建的文件夹名称了。
2. 找不到dart-sdk，flutter会下载到bin/cache目录，只能自己手动copy一份到报错的目录。
3. 找不到frontend_server.dart.snapshot，在flutter目录用find . -name "frontend_server.dart.snapshot"找到，然后手动copy一份。

解决这三个问题之后，终于可以运行了。

$ ../flutter/bin/flutter run --local-engine-src-path ~/WorkProject/flutter_source_code/src --local-engine=host_debug_unopt
Launching lib/main.dart on DUK AL20 in debug mode...
Running Gradle task 'assembleDebug'...
Running Gradle task 'assembleDebug'... Done                        18.1s
✓ Built build/app/outputs/flutter-apk/app-debug.apk.
Installing build/app/outputs/flutter-apk/app.apk...                 3.5s
Waiting for DUK AL20 to report its views...                          8ms
Syncing files to device DUK AL20...                                210ms
Flutter run key commands.
r Hot reload. ???
R Hot restart.
h Repeat this help message.
d Detach (terminate "flutter run" but leave application running).
c Clear the screen
q Quit (terminate the application on the device).
An Observatory debugger and profiler on DUK AL20 is available at: http://127.0.0.1:63146/LE7Uc6cshds=/
D/AwareBitmapCacher(24977): handleInit switch not opened pid=24977

Application finished.

总结

以上完成源码的获取与构建，那么：

• 如何高效率的编辑源码？用Vim/VS Code还是Android Studio？
• 如何构建自己的版本？同时，如何保持和官方代码保持同步？
• 如何修改框架层代码并重新编译？

等等。

其实问题还有很多，后面再一一讲解。

参考

• 搭建Flutter Engine源码编译环境
• Flutter Engine 编译指北
• Flutter Engine与SDK的定制化与编译
• Setting up the Engine development environment · flutter/flutter Wiki
• Compiling the engine · flutter/flutter Wiki
• The flutter tool · flutter/flutter Wiki
• Using depot_tools - The Chromium Projects
• Working with Release Branches - The Chromium Projects
• CIPD for chromium dependencies
• Ninja, a small build system with a focus on speed
• gn - Git at Google

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