1 ，安装 Chromium 的depot_tools

我的目录是在/opt下面

cd /opt
git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git

修改环境变量

vi ~/.bash_profile
export PATH=$PATH:/opt/depot_tools

2, 下载 buildroot

我测试flutter engine编译的目录是/Volumes/huc/dartvm/flutter_engine

在此目录下面创建名为.gclient的文件， 提前到github上fork engine项目到github.com/hc2088/engine.git自己的账号下：

solutions = [
{
  "managed": False,
  "name": "src/flutter",
  "url": "git@github.com:hc2088/engine.git",
  "custom_deps": {},
  "deps_file": "DEPS",
  "safesync_url": "",
},
]

执行

cd /Volumes/huc/dartvm/flutter_engine/
gclient sync 

执行完成之后，会生成一个src目录，对应的就是buildroot 的git仓库

cd src
 git remote -v
origin  https://github.com/flutter/buildroot.git (fetch)
origin  https://github.com/flutter/buildroot.git (push)

src/flutter 对应就是flutter engine的git仓库

cd src/flutter
git remote -v
origin  git@github.com:hc2088/engine.git (fetch)
origin  git@github.com:hc2088/engine.git (push)

3，确认flutter engine与flutter 版本是否一致

我的flutter 装在/Volumes/huc/dartvm/flutter目录下

which flutter 
/Volumes/huc/dartvm/flutter/bin/flutter
cd /Volumes/huc/dartvm/flutter
cat bin/internal/engine.version 
3a0a63c2d2098fffe4dc2fa161e265420bdfbb25

修改flutter engine仓库的版本

cd  /Volumes/huc/dartvm/flutter_engine/src/flutter
git reset --hard 3a0a63c2d2098fffe4dc2fa161e265420bdfbb25

4， 下载 依赖

在src目录下，重新拉取依赖

pwd
/Volumes/huc/dartvm/flutter_engine/src
gclient sync

这一步会把dart sdk的源代码项目下载到下面目录：

src/third_party/dart  

pwd
/Volumes/huc/dartvm/flutter_engine/src/third_party/dart

git remote -v
origin  https://dart.googlesource.com/sdk.git (fetch)
origin  https://dart.googlesource.com/sdk.git (push)

image.png

注意执行这一步，会比较慢:Syncing projects done完成之后，不要终止进程，继续等后面任务执行完：

huchu58:src huchu$ gclient sync
23>WARNING: subprocess '"git" "-c" "core.deltaBaseCacheLimit=2g" "clone" "--no-checkout" "--progress" "https://dart.googlesource.com/sdk.git" "/Volumes/huc/dartvm/flutter_engine/src/third_party/_gclient_dart_0aepvjce"' in /Volumes/huc/dartvm/flutter_engine failed; will retry after a short nap...
53>WARNING: subprocess '"git" "-c" "core.deltaBaseCacheLimit=2g" "fetch" "origin" "--no-tags"' in /Volumes/huc/dartvm/flutter_engine/src/third_party/pkg/vector_math failed; will retry after a short nap...
Syncing projects:  50% (65/129) src/third_party/pkg/vector_math
[0:01:40] Still working on:
[0:01:40]   src/third_party/dart
//......
Syncing projects: 100% (129/129), done.                                         
///这里需要等
Hook 'python3 src/flutter/tools/pub_get_offline.py' took 21.91 secs
Running hooks:  88% ( 8/ 9) Download prebuilt Dart SDK
Hook 'python3 src/flutter/tools/download_dart_sdk.py' took 195.20 secs
Running hooks: 100% (9/9), done.  

5， 准备构建文件

使用预先构建的dart sdk: (默认的是--prebuilt-dart-sdk)

cd src
./flutter/tools/gn --ios --unoptimized --prebuilt-dart-sdk
./flutter/tools/gn --unoptimized --prebuilt-dart-sdk

使用dart sdk源代码，现编译的dart sdk:
改成--no-prebuilt-dart-sdk

huchu58:src huchu$ ./flutter/tools/gn --ios --unoptimized --no-prebuilt-dart-sdk
GOMA usage was specified but can't be found, falling back to local builds. Set the GOMA_DIR environment variable to fix GOMA.
Generating GN files in: out/ios_debug_unopt
Generating Xcode projects took 405ms
Generating compile_commands took 79ms
Done. Made 564 targets from 213 files in 9560ms
huchu58:src huchu$ ./flutter/tools/gn --unoptimized --no-prebuilt-dart-sdk
GOMA usage was specified but can't be found, falling back to local builds. Set the GOMA_DIR environment variable to fix GOMA.
Generating GN files in: out/host_debug_unopt
Generating Xcode projects took 447ms
Generating compile_commands took 91ms
Done. Made 931 targets from 260 files in 5531ms

6， 构建 engine

huchu58:src huchu$ ninja -C out/ios_debug_unopt && ninja -C out/host_debug_unopt
ninja: Entering directory `out/ios_debug_unopt'
[1362/1362] STAMP obj/default.stamp
ninja: Entering directory `out/host_debug_unopt'
[6208/6208] STAMP obj/default.stamp

输出：

src/out/host_debug_unopt
src/out/ios_debug_unopt

7， flutter app集成自定义engine

创建flutter app

cd ~/Desktop
flutter create xxapp

flutter run --local-engine=ios_debug_unopt --local-engine-src-path=/Volumes/huc/dartvm/flutter_engine/src

使用--config-local，不编译只是修改xcode LOCAL_ENGINE 和相关插件的配置，加快速度。

flutter build ios --local-engine=ios_debug_unopt --config-only --local-engine-src-path=/Volumes/huc/dartvm/flutter_engine/src

flutter app native端确认用的flutter engine

打开Runner.xcworkspace工程：
/Users/huchu/Desktop/xxapp/ios/Runner.xcworkspace

问题：怎么都不进断点 -[FlutterEngine runWithEntrypoint:]

8，xcode集成原理

run 项目时候，xcode先后调用以下脚本：

/bin/sh "$FLUTTER_ROOT/packages/flutter_tools/bin/xcode_backend.sh" build
/bin/sh "$FLUTTER_ROOT/packages/flutter_tools/bin/xcode_backend.sh" embed_and_thin

这里的FLUTTER_ROOT是/Volumes/huc/dartvm/flutter
主要作用是确认dart 是/Volumes/huc/dartvm/flutter/bin/dart
读取在flutter build/run这一步配置好的相关变量值：

image.png
然后会生成flutter_export_environment.sh 文件，flutter_export_environment.sh 使得变量有效：

#!/bin/sh
# This is a generated file; do not edit or check into version control.
export "FLUTTER_ENGINE=/Volumes/huc/dartvm/flutter_engine/src"
export "LOCAL_ENGINE=ios_debug_unopt"
#其他内容这里就不粘贴了
....

可以看到/Volumes/huc/dartvm/flutter/packages/flutter_tools/bin/xcode_backend.dart文件有用到：

如果没有用自定flutter_engine

final String frameworkPath = '${environmentEnsure('FLUTTER_ROOT')}/bin/cache/artifacts/engine/$artifactVariant';
String flutterFramework = '$frameworkPath/Flutter.xcframework';

对应使用的engine 就是 flutter 自己同步的

image.png

咱们这里用了--local-engine

final String? localEngine = environment['LOCAL_ENGINE'];
 flutterFramework = '${environmentEnsure('FLUTTER_ENGINE')}/out/$localEngine/Flutter.xcframework';

该路径指定了使用我们自定义的flutter_engine的产物：

image.png
flutter App dart端如何确认dart sdk
修改pubspec.yaml

dependency_overrides:
  sky_engine:
    path: /Volumes/huc/dartvm/flutter_engine/src/out/ios_debug_unopt/gen/dart-pkg/sky_engine

默认为：

/Volumes/huc/dartvm/flutter/bin/cache/pkg/sky_engine