源码解读Flutter tools机制

一、Flutter tools命令

1.1 概述

开发Flutter应用过程，经常会用过Flutter命令，比如flutter run可用于安装并运行Flutter应用，flutter build可用于构建产物，相信有不少人会好奇flutter命令背后的原理。对于flutter命令的起点位于flutter sdk中路径/flutter/bin/目录中的flutter命令，该命令最终会调用到flutter/packages/flutter_tools工程。

1.2 flutter命令参数表

列举Flutter命令、对应类以及说明，实现见下文[小节2.3]。

名称	对应类	说明
create	CreateCommand	创建新的Flutter项目
build	BuildCommand	Flutter构建命令
install	InstallCommand	安装Flutter应用到已连接设备
run	RunCommand	运行Flutter应用于已连接设备
packages	PackagesCommand	管理Flutter包的命令
devices	DevicesCommand	列出所有已连接的设备
emulators	EmulatorsCommand	列出，启动，创建模拟器
attach	AttachCommand	附加到正在运行的应用程序
trace	TraceCommand	开始和停止跟踪正在运行的Flutter应用程序
logs	LogsCommand	显示Flutter应用运行中的log
doctor	DoctorCommand	显示关于已安装工具的信息
upgrade	UpgradeCommand	升级Flutter
clean	CleanCommand	删除build/和.dart_tool/ 目录
analyze	AnalyzeCommand	分析项目Dart代码
format	FormatCommand	格式化一个或多个dart文件
config	ConfigCommand	配置Flutter settings
drive	DriveCommand	为当前项目运行Flutter Driver测试
test	TestCommand	为当前项目运行Flutter 单元测试

另外，对于flutter build有子命令，其子命令的对应类及说明如下：

命令	对应类	说明
build aot	BuildAotCommand	构建AOT编译产物
build apk	BuildApkCommand	构建Android APK
build ios	BuildIOSCommand	构建iOS应用bundle
build appbundle	BuildAppBundleCommand	构建Android应用bundle
build bundle	BuildBundleCommand	构建Flutter assets

比如flutter run则执行RunCommand.runCommand()，flutter install则执行InstallCommand.runCommand()。

二、深入源码flutter命令

2.1 flutter命令起点

[-> /flutter/bin/flutter]

...
FLUTTER_TOOLS_DIR="$FLUTTER_ROOT/packages/flutter_tools"
SNAPSHOT_PATH="$FLUTTER_ROOT/bin/cache/flutter_tools.snapshot"
STAMP_PATH="$FLUTTER_ROOT/bin/cache/flutter_tools.stamp"
SCRIPT_PATH="$FLUTTER_TOOLS_DIR/bin/flutter_tools.dart"
DART_SDK_PATH="$FLUTTER_ROOT/bin/cache/dart-sdk"

DART="$DART_SDK_PATH/bin/dart"
PUB="$DART_SDK_PATH/bin/pub"

//真正的执行逻辑
"$DART" $FLUTTER_TOOL_ARGS "$SNAPSHOT_PATH" "$@"

该方法功能：

$DART：是指$ FLUTTER_ROOT/bin/cache/dart-sdk/bin/dart；
$SNAPSHOT\_PATH：是指$ FLUTTER_ROOT/bin/cache/flutter_tools.snapshot，这是由packages/flutter_tools项目编译所生成的产物文件。

那么flutter命令等价于如下：

/bin/cache/dart-sdk/bin/dart $FLUTTER_TOOL_ARGS "bin/cache/flutter_tools.snapshot" "$@"

dart执行flutter_tools.snapshot，其实也就是执行flutter_tools.dart的main()方法，也就是说将上述命令改为如下语句，则运行flutter命令可以执行本地flutter_tools的项目代码，可用于本地调试分析。

/bin/cache/dart-sdk/bin/dart $FLUTTER_TOOL_ARGS "$FLUTTER_ROOT/packages/flutter_tools/bin/flutter_tools.dart" "$@"

接下来，执行流程进入flutter/packages/flutter_tools/目录。

2.2 flutter_tools.main

[-> flutter/packages/flutter_tools/bin/flutter_tools.dart]

import 'package:flutter_tools/executable.dart' as executable;

void main(List<String> args) {
  executable.main(args);  //[见小节2.3]
}

2.3 executable.main

[-> lib/executable.dart]

import 'runner.dart' as runner;

Future<void> main(List<String> args) async {
  ...
  //[见小节2.4]
  await runner.run(args, <FlutterCommand>[
    AnalyzeCommand(verboseHelp: verboseHelp),
    AttachCommand(verboseHelp: verboseHelp),
    BuildCommand(verboseHelp: verboseHelp),
    ChannelCommand(verboseHelp: verboseHelp),
    CleanCommand(),
    ConfigCommand(verboseHelp: verboseHelp),
    CreateCommand(),
    DaemonCommand(hidden: !verboseHelp),
    DevicesCommand(),
    DoctorCommand(verbose: verbose),
    DriveCommand(),
    EmulatorsCommand(),
    FormatCommand(),
    GenerateCommand(),
    IdeConfigCommand(hidden: !verboseHelp),
    InjectPluginsCommand(hidden: !verboseHelp),
    InstallCommand(),
    LogsCommand(),
    MakeHostAppEditableCommand(),
    PackagesCommand(),
    PrecacheCommand(),
    RunCommand(verboseHelp: verboseHelp),
    ScreenshotCommand(),
    ShellCompletionCommand(),
    StopCommand(),
    TestCommand(verboseHelp: verboseHelp),
    TraceCommand(),
    TrainingCommand(),
    UpdatePackagesCommand(hidden: !verboseHelp),
    UpgradeCommand(),
    VersionCommand(),
  ], verbose: verbose,
     muteCommandLogging: muteCommandLogging,
     verboseHelp: verboseHelp,
     overrides: <Type, Generator>{
       CodeGenerator: () => const BuildRunner(),
     });
}

2.4 runner.run

[-> lib/runner.dart]

Future<int> run(
  List<String> args,
  List<FlutterCommand> commands, {
  bool muteCommandLogging = false,
  bool verbose = false,
  bool verboseHelp = false,
  bool reportCrashes,
  String flutterVersion,
  Map<Type, Generator> overrides,
}) {
  ...
  //创建FlutterCommandRunner对象
  final FlutterCommandRunner runner = FlutterCommandRunner(verboseHelp: verboseHelp);
  //[见小节2.4.1] 将创建的命令对象都加入到_commands
  commands.forEach(runner.addCommand);

  return runInContext<int>(() async {
    ...
    // [见小节2.5]
    await runner.run(args);
    ...
  }, overrides: overrides);
}

2.4.1 addCommand

[-> package:args/command_runner.dart]

void addCommand(Command<T> command) {
  var names = [command.name]..addAll(command.aliases);
  for (var name in names) {
    _commands[name] = command;
    argParser.addCommand(name, command.argParser);
  }
  command._runner = this;
}

所有命令都加入到_commands。比如flutter run对应的命令对象为RunCommand，flutter build对应的命令对象为buildCommand。

2.5 FlutterCommandRunner.run

[-> lib/src/runner/flutter_command_runner.dart]

class FlutterCommandRunner extends CommandRunner<void> {

  Future<void> run(Iterable<String> args) {
    return super.run(args); // [见小节2.6]
  }
}

2.6 CommandRunner.run

[-> package:args/command_runner.dart]

class CommandRunner<T> {

  Future<T> run(Iterable<String> args) =>
      new Future.sync(() => runCommand(parse(args))); //见下文

  Future<T> runCommand(ArgResults topLevelResults) async {
    var argResults = topLevelResults;
    var commands = _commands;
    Command command;
    var commandString = executableName;

    while (commands.isNotEmpty) {
      ...
      argResults = argResults.command;
      //根据命令名从命令列表中找到相应的命令
      command = commands[argResults.name];
      command._globalResults = topLevelResults;
      command._argResults = argResults;
      commands = command._subcommands;  //查找到子命令
      commandString += " ${argResults.name}";
    }
    // 执行真正对应命令的run()方法 [见小节2.7]
    return (await command.run()) as T;
  }
}

该方法会根据命令后通过循环遍历查找子命令，直到找到最后的命令为止。但这些命令都直接或者间接继承于FlutterCommand命令

2.7 FlutterCommand.run

[-> lib/src/runner/flutter_command.dart]

abstract class FlutterCommand extends Command<void> {

  Future<void> run() {
    final DateTime startTime = systemClock.now();

    return context.run<void>(
      name: 'command',
      overrides: <Type, Generator>{FlutterCommand: () => this},
      body: () async {
        ...
        try {
          // [见小节2.8]
          commandResult = await verifyThenRunCommand(commandPath);
        } on ToolExit {
          commandResult = const FlutterCommandResult(ExitStatus.fail);
          rethrow;
        } finally {
          ...
        }
      },
    );
  }
}

2.8 FlutterCommand.verifyThenRunCommand

[-> lib/src/runner/flutter_command.dart]

abstract class FlutterCommand extends Command<void> {

  Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async {
    await validateCommand();
    if (shouldUpdateCache) {
      await cache.updateAll(await requiredArtifacts);
    }
    if (shouldRunPub) {
      //获取pub
      await pubGet(context: PubContext.getVerifyContext(name));
      final FlutterProject project = await FlutterProject.current();
      await project.ensureReadyForPlatformSpecificTooling();
    }
    setupApplicationPackages();
    ...

    // 执行真正对应的命令类
    return await runCommand();
  }
}

该方法先执行pubGet()用于下载pubspec.yaml里配置的依赖，该pub对应执行命令为：

$flutterRoot/bin/cache/dart-sdk/bin/pub --verbosity=warning get --no-precompile

本文转自 http://gityuan.com/2019/09/01/flutter_tool/，如有侵权，请联系删除。

源码解读Flutter tools机制