Flutter 命令本质之 Flutter tools 机制源码深入分析
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Flutter 系列文章连载～
- 《Flutter Android 工程结构及应用层编译源码深入分析》
- 《Flutter 命令本质之 Flutter tools 机制源码深入分析》
- 《Flutter 的 runApp 与三棵树诞生流程源码分析》
- 《Flutter Android 端 Activity/Fragment 流程源码分析》
- 《Flutter Android 端 FlutterInjector 及依赖流程源码分析》
- 《Flutter Android 端 FlutterEngine Java 相关流程源码分析》
- 《Flutter Android 端 FlutterView 相关流程源码分析》
- 《Flutter 绘制动机 VSYNC 流程源码全方位分析》
- 《Flutter 安卓 Platform 与 Dart 端消息通信方式 Channel 源码解析》
背景
上一篇《Flutter Android 工程结构及应用层编译源码深入分析》我们分析了 Flutter android 相关的应用层主要编译流程，其中分析到底层本质命令工具【Flutter SDK 下bin/flutter编译命令分析】小节时只提到，我们执行任何 flutter 命令的本质都是把参数传递到了FLUTTER_SDK_DIR/packages/flutter_tools/bin/flutter_tools.dart源码的 main 方法中，没有对这里面进行深入分析。本文要做的事就是层层递进揭开这里的本质，并与上篇呼应解释编译产物的由来。
flutter_tools 介绍
通过flutter -h命令我们可以直观全局感受都支持哪些参数，有些参数还有子参数。我们所执行的所有参数本质都走进了下面模块的源码入口中。
因此我们如果直接想从源码方式使用 flutter tools，则可以直接当前目录中如下命令：
# ARGS 就是一堆参数，譬如我们上篇的 build apk
dart bin/flutter_tools.dart ARGS
如果想重新生成 Flutter Tools snapshot，可以直接当前目录中执行如下命令：
rm ../../bin/cache/flutter_tools.stamp ../../bin/cache/flutter_tools.snapshot
这样就成功删除了上篇中 shell 脚本调用的 Flutter Tools snapshot，然后在执行时会自动重新生成一个。
源码分析
上面既然交代了整个背景，那么我们接下来就基于 Flutter SDK 入口packages/flutter_tools/bin/flutter_tools.dart开始分析，整个分析继续承接上篇flutter build apk命令，如下：
//1、导入packages/flutter_tools/lib/executable.dart文件
import 'package:flutter_tools/executable.dart' as executable;
//2、入口重点，执行executable.main方法，并将我们`build apk`参数传入
void main(List<String> args)
executable.main(args);
接下来我们去packages/flutter_tools/lib/executable.dart看看他的 main 方法，如下：
Future<void> main(List<String> args) async
//一堆参数解析判断啥的，譬如解析flutter doctor的doctor参数啥的
......
//1、重点！runner的本质是import 'runner.dart' as runner;
// 本质就是调用run方法的各种参数传递，重点关注第一个和第二个参数即可
await runner.run(
args,
() => generateCommands(
verboseHelp: verboseHelp,
verbose: verbose,
),
......,
);
//2、步骤1中runner.run的第二个核心参数方法定义
//FlutterCommand为packages/flutter_tools/lib/src/runner/flutter_command.dart中定义的抽象类
//这个方法本质就是把flutter执行的命令参数列表全部加入列表，类似命令模式
List<FlutterCommand> generateCommands(
@required bool verboseHelp,
@required bool verbose,
) => <FlutterCommand>[
AnalyzeCommand(
......
),
AssembleCommand(verboseHelp: verboseHelp, buildSystem: globals.buildSystem),
AttachCommand(verboseHelp: verboseHelp),
BuildCommand(verboseHelp: verboseHelp),
ChannelCommand(verboseHelp: verboseHelp),
CleanCommand(verbose: verbose),
ConfigCommand(verboseHelp: verboseHelp),
CreateCommand(verboseHelp: verboseHelp),
DaemonCommand(hidden: !verboseHelp),
DevicesCommand(verboseHelp: verboseHelp),
DoctorCommand(verbose: verbose),
DowngradeCommand(verboseHelp: verboseHelp),
DriveCommand(verboseHelp: verboseHelp,
......
),
EmulatorsCommand(),
FormatCommand(),
GenerateCommand(),
GenerateLocalizationsCommand(
......
),
InstallCommand(),
LogsCommand(),
MakeHostAppEditableCommand(),
PackagesCommand(),
PrecacheCommand(
......
),
RunCommand(verboseHelp: verboseHelp),
ScreenshotCommand(),
ShellCompletionCommand(),
TestCommand(verboseHelp: verboseHelp),
UpgradeCommand(verboseHelp: verboseHelp),
SymbolizeCommand(
......
),
// Development-only commands. These are always hidden,
IdeConfigCommand(),
UpdatePackagesCommand(),
];
......
让我们把目光先移动到runner.dart文件的 run 方法，然后回过头来看上面代码中的步骤1如何调用步骤2，如下：
Future<int> run(
List<String> args,
List<FlutterCommand> Function() commands,
bool muteCommandLogging = false,
bool verbose = false,
bool verboseHelp = false,
bool reportCrashes,
String flutterVersion,
Map<Type, Generator> overrides,
) async
......
//1、FlutterCommandRunner位于packages/flutter_tools/lib/src/runner/flutter_command_runner.dart
return runInContext<int>(() async
reportCrashes ??= !await globals.isRunningOnBot;
//2、创建runner对象实例，并把上一片段代码中步骤2方法返回的FlutterCommand列表追加进runner中
final FlutterCommandRunner runner = FlutterCommandRunner(verboseHelp: verboseHelp);
commands().forEach(runner.addCommand);
......
return runZoned<Future<int>>(() async
try
//3、依据args参数执行runner实例的run方法
await runner.run(args);
......
catch (error, stackTrace) // ignore: avoid_catches_without_on_clauses
......
, onError: (Object error, StackTrace stackTrace) async // ignore: deprecated_member_use
......
);
, overrides: overrides);
可以看到，首先实例化了一个 FlutterCommandRunner 对象，接着把所有支持的 FlutterCommand 列表加入 runner 对象中，然后调用了 runner 的 run 方法，所以我们现在查看packages/flutter_tools/lib/src/runner/flutter_command_runner.dart文件的 run 方法，如下：
......
@override
Future<void> run(Iterable<String> args)
......
//本质调用了父类CommandRunner的run方法，run方法调用了子类FlutterCommandRunner的runCommand方法
//子类FlutterCommandRunner的runCommand最终又调用了父类CommandRunner的runCommand方法
return super.run(args);
......
所以我们接下来看父类 CommandRunner 的 runCommand 方法，如下：
Future<T?> runCommand(ArgResults topLevelResults) async
//1、flutter命令后面传递进来参数，譬如build apk
var argResults = topLevelResults;
//2、前面分析过的，runner中添加的支持命令列表
var commands = _commands;
//3、定义一个Command变量，用来最终依据参数赋值为对应的Command对象实例
Command? command;
var commandString = executableName;
//4、while条件为真，因为commands为支持的参数列表
while (commands.isNotEmpty)
......
//5、填充指令
argResults = argResults.command!;
command = commands[argResults.name]!;
command._globalResults = topLevelResults;
command._argResults = argResults;
commands = command._subcommands as Map<String, Command<T>>;
commandString += ' $argResults.name';
......
......
//6、执行对应命令的run方法
return (await command.run()) as T?;
......
可以看到，这就是一个标准的命令模式设计，先把支持的命令添加到列表，然后依据参数遍历匹配对应命令进行执行。下面我们以flutter build apk命令为例来看其对应的 BuildCommand 命令（packages/flutter_tools/lib/src/commands/build.dart）实现，如下：
class BuildCommand extends FlutterCommand
BuildCommand( bool verboseHelp = false )
addSubcommand(BuildAarCommand(verboseHelp: verboseHelp));
addSubcommand(BuildApkCommand(verboseHelp: verboseHelp));
addSubcommand(BuildAppBundleCommand(verboseHelp: verboseHelp));
addSubcommand(BuildiosCommand(verboseHelp: verboseHelp));
addSubcommand(BuildIOSFrameworkCommand(
buildSystem: globals.buildSystem,
verboseHelp: verboseHelp,
));
addSubcommand(BuildIOSArchiveCommand(verboseHelp: verboseHelp));
addSubcommand(BuildBundleCommand(verboseHelp: verboseHelp));
addSubcommand(BuildWebCommand(verboseHelp: verboseHelp));
addSubcommand(BuildMacosCommand(verboseHelp: verboseHelp));
addSubcommand(BuildLinuxCommand(
operatingSystemUtils: globals.os,
verboseHelp: verboseHelp
));
addSubcommand(BuildWindowsCommand(verboseHelp: verboseHelp));
addSubcommand(BuildWindowsUwpCommand(verboseHelp: verboseHelp));
addSubcommand(BuildFuchsiaCommand(verboseHelp: verboseHelp));
//上一小段代码中command = commands[argResults.name]就是这么得到的
//name=build就是执行flutter build apk中的build字符串
@override
final String name = 'build';
@override
final String description = 'Build an executable app or install bundle.';
@override
Future<FlutterCommandResult> runCommand() async => null;
可以看到，任意一个命令基本都继承自 FlutterCommand 实现，命令的执行都是调用了 FlutterCommand 的 run 方法，如下：
abstract class FlutterCommand extends Command<void>
......
//runner对象中最终执行调用的方法是这个
@override
Future<void> run()
......
return context.run<void>(
name: 'command',
overrides: <Type, Generator>FlutterCommand: () => this,
body: () async
......
try
//见名知意，先校验再运行命令
commandResult = await verifyThenRunCommand(commandPath);
finally
......
,
);
......
@mustCallSuper
Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async
//1、如果需要更新缓存就先更新缓存
if (shouldUpdateCache)
await globals.cache.updateAll(<DevelopmentArtifact>DevelopmentArtifact.universal);
await globals.cache.updateAll(await requiredArtifacts);
globals.cache.releaseLock();
//2、校验命令
await validateCommand();
//3、如果需要先执行pub就先执行，譬如pub get下载依赖
if (shouldRunPub)
......
//4、执行pub get下载依赖，即下载pubspec.yaml里配置的依赖
await pub.get(
context: PubContext.getVerifyContext(name),
generateSyntheticPackage: project.manifest.generateSyntheticPackage,
checkUpToDate: cachePubGet,
);
await project.regeneratePlatformSpecificTooling();
if (reportNullSafety)
await _sendNullSafetyAnalyticsEvents(project);
setupApplicationPackages();
......
//5、真正开始执行命令
return runCommand();
绕一圈最终我们又回到 BuildCommand 类，可以发现其 runCommand 方法重写为空实现，而其构造时通过 addSubcommand 方法追加了很多子命令，譬如执行flutter build aar编译 aar 的 BuildAarCommand 命令、执行flutter build apk编译 apk 的 BuildApkCommand 命令。整个 sub command 与其宿主又算是一个责任链，所以上面同样的套路顺序对于 sub command 同样适用，因此我们去看下编译 apk 产物的 BuildApkCommand 源码（packages/flutter_tools/lib/src/commands/build_apk.dart），如下：
class BuildApkCommand extends BuildSubCommand
BuildApkCommand(bool verboseHelp = false)
......
//一堆参数的确认
//对应flutter build apk里面子命令字符串apk
@override
final String name = 'apk';
......
//本质命令执行方法
@override
Future<FlutterCommandResult> runCommand() async
......
//调用androidBuilder的buildApk方法进行真正的编译，目测里面的产物也就是上一篇文章分析的那些
//androidBuilder位于packages/flutter_tools/lib/src/android/android_builder.dart
await androidBuilder.buildApk(
project: FlutterProject.current(),
target: targetFile,
androidBuildInfo: androidBuildInfo,
);
return FlutterCommandResult.success();
顺着这条路我们继续跟进位于packages/flutter_tools/lib/src/android/android_builder.dart的 androidBuilder 属性的 buildApk 方法，如下：
//本质是packages/flutter_tools/lib/src/context_runner.dart中context.run方法中的AndroidGradleBuilder实例
AndroidBuilder get androidBuilder
return context.get<AndroidBuilder>();
//抽象类定义，AndroidBuilder
abstract class AndroidBuilder
const AndroidBuilder();
// 定义编译aar的方法
Future<void> buildAar(
@required FlutterProject project,
@required Set<AndroidBuildInfo> androidBuildInfo,
@required String target,
@required String outputDirectoryPath,
@required String buildNumber,
);
// 定义编译apk的方法
Future<void> buildApk(
@required FlutterProject project,
@required AndroidBuildInfo androidBuildInfo,
@required String target,
);
// 定义编译aab的方法
Future<void> buildAab(
@required FlutterProject project,
@required AndroidBuildInfo androidBuildInfo,
@required String target,
bool validateDeferredComponents = true,
bool deferredComponentsEnabled = false,
);
所以我们继续去看 AndroidGradleBuilder 实现类（packages/flutter_tools/lib/src/android/gradle.dart）的 buildApk 方法，如下：
class AndroidGradleBuilder implements AndroidBuilder
AndroidGradleBuilder(
......
) : ......;
......
//1、编译 apk 的方法
@override
Future<void> buildApk(
@required FlutterProject project,
@required AndroidBuildInfo androidBuildInfo,
@required String target,
) async
//2、调用
await buildGradleApp(
project: project,
androidBuildInfo: androidBuildInfo,
target: target,
isBuildingBundle: false,
localGradleErrors: gradleErrors,
);
......
//3、真的编译
Future<void> buildGradleApp(
@required FlutterProject project, //FlutterProject.current()
@required AndroidBuildInfo androidBuildInfo, //build configuration
@required String target, //dart代码入口，缺省lib/main.dart
@required bool isBuildingBundle, //是aab还是apk，默认false则apk
@required List<GradleHandledError> localGradleErrors,
bool shouldBuildPluginAsAar = false, //是不是将插件编译为aar
bool validateDeferredComponents = true,
bool deferredComponentsEnabled = false,
int retries = 1,
) async
//4、检查支持的android版本，获取android编译产物目录，即gradle中配置的build产物目录，默认为项目根目录下的build目录
if (!project.android.isSupportedVersion)
_exitWithUnsupportedProjectMessage(_usage, _logger.terminal);
final Directory buildDirectory = project.android.buildDirectory;
//5、读取安卓相关属性文件判断是否使用androidx，然后发送编译事件参数
final bool usesAndroidX = isAppUsingAndroidX(project.android.hostAppGradleRoot);
if (usesAndroidX)
BuildEvent('app-using-android-x', flutterUsage: _usage).send();
else if (!usesAndroidX)
BuildEvent('app-not-using-android-x', flutterUsage: _usage).send();
......
//6、更新安卓项目中local.properties中的versionName和versionCode值，值来自于public.yaml文件配置
updateLocalProperties(project: project, buildInfo: androidBuildInfo.buildInfo);
//7、编译aar的话就走buildPluginsAsAar方法进行
if (shouldBuildPluginAsAar)
// Create a settings.gradle that doesn't import the plugins as subprojects.
createSettingsAarGradle(project.android.hostAppGradleRoot, _logger);
await buildPluginsAsAar(
project,
androidBuildInfo,
buildDirectory: buildDirectory.childDirectory('app'),
);
//8、获取编译apk或者aab对应的标准安卓task name，构建参数等信息，也就是gradle命令后面一堆的参数构造
final BuildInfo buildInfo = androidBuildInfo.buildInfo;
final String assembleTask = isBuildingBundle
? getBundleTaskFor(buildInfo)
: getAssembleTaskFor(buildInfo);
......
final List<String> command = <String>[
_gradleUtils.getExecutable(project),
];
......
//9、依据条件追加command的一堆参数，譬如-Psplit-per-abi=true、-Pverbose=true、--no-daemon等
......
try
exitCode = await _processUtils.stream(
command,
workingDirectory: project.android.hostAppGradleRoot.path,
allowReentrantFlutter: true,
environment: <String, String>
if (javaPath != null)
'JAVA_HOME': javaPath,
,
mapFunction: consumeLog,
);
on ProcessException catch (exception)
......
finally
status.stop();
......
......
哈哈，真相了，这下配合《Flutter Android 工程结构及应用层编译源码深入分析》一文首尾呼应后你应该彻底明白 Flutter android apk 是怎么编译的流程！
总结
现在我们结合《Flutter Android 工程结构及应用层编译源码深入分析》和这篇进行关联总结，可以总结出执行flutter build apk命令背后的大致主流程如下：
既然执行flutter build apk命令你都搞明白了，那么其他 flutter 相关的任何命令你是否也可以自己举一反三进行分析学习
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