背景

上一篇《Flutter Android 工程結構及應用層編譯源碼深刻分析》咱們分析了 Flutter Android 相關的應用層主要編譯流程，其中分析到底層本質命令工具【Flutter SDK 下bin/flutter編譯命令分析】小節時只提到，咱們執行任何 flutter 命令的本質都是把參數傳遞到了FLUTTER_SDK_DIR/packages/flutter_tools/bin/flutter_tools.dart源碼的 main 方法中，沒有對這裏面進行深刻分析。本文要作的事就是層層遞進揭開這裏的本質，並與上篇呼應解釋編譯產物的由來。java

flutter_tools 介紹

經過flutter -h命令咱們能夠直觀全局感覺都支持哪些參數，有些參數還有子參數。咱們所執行的全部參數本質都走進了下面模塊的源碼入口中。所以咱們若是直接想從源碼方式使用 flutter tools，則能夠直接當前目錄中以下命令：android

# ARGS 就是一堆參數，譬如咱們上篇的 build apk
dart bin/flutter_tools.dart ARGS
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若是想從新生成 Flutter Tools snapshot，能夠直接當前目錄中執行以下命令：shell

rm ../../bin/cache/flutter_tools.stamp ../../bin/cache/flutter_tools.snapshot
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這樣就成功刪除了上篇中 shell 腳本調用的 Flutter Tools snapshot，而後在執行時會自動從新生成一個。緩存

源碼分析

上面既然交代了整個背景，那麼咱們接下來就基於 Flutter SDK 入口packages/flutter_tools/bin/flutter_tools.dart開始分析，整個分析繼續承接上篇flutter build apk命令，以下：bash

//一、導入packages/flutter_tools/lib/executable.dart文件
import 'package:flutter_tools/executable.dart' as executable;
//二、入口重點，執行executable.main方法，並將咱們`build apk`參數傳入
void main(List<String> args) {
  executable.main(args);
}
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接下來咱們去packages/flutter_tools/lib/executable.dart看看他的 main 方法，以下：markdown

Future<void> main(List<String> args) async {
  //一堆參數解析判斷啥的，譬如解析flutter doctor的doctor參數啥的
  ......
  //一、重點！runner的本質是import 'runner.dart' as runner;
  // 本質就是調用run方法的各類參數傳遞，重點關注第一個和第二個參數便可
  await runner.run(
    args,
    () => generateCommands(
      verboseHelp: verboseHelp,
      verbose: verbose,
    ),
    ......,
  );
}
//二、步驟1中runner.run的第二個核心參數方法定義
//FlutterCommand爲packages/flutter_tools/lib/src/runner/flutter_command.dart中定義的抽象類
//這個方法本質就是把flutter執行的命令參數列表所有加入列表，相似命令模式
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(),
];
......
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讓咱們把目光先移動到runner.dart文件的 run 方法，而後回過頭來看上面代碼中的步驟1如何調用步驟2，以下：app

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 {
  ......
  //一、FlutterCommandRunner位於packages/flutter_tools/lib/src/runner/flutter_command_runner.dart
  return runInContext<int>(() async {
    reportCrashes ??= !await globals.isRunningOnBot;
    //二、建立runner對象實例，並把上一片斷代碼中步驟2方法返回的FlutterCommand列表追加進runner中
    final FlutterCommandRunner runner = FlutterCommandRunner(verboseHelp: verboseHelp);
    commands().forEach(runner.addCommand);
    ......
    return runZoned<Future<int>>(() async {
      try {
      	//三、依據args參數執行runner實例的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);
}
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能夠看到，首先實例化了一個 FlutterCommandRunner 對象，接着把全部支持的 FlutterCommand 列表加入 runner 對象中，而後調用了 runner 的 run 方法，因此咱們如今查看packages/flutter_tools/lib/src/runner/flutter_command_runner.dart文件的 run 方法，以下：async

......
@override
Future<void> run(Iterable<String> args) {
  ......
  //本質調用了父類CommandRunner的run方法，run方法調用了子類FlutterCommandRunner的runCommand方法
  //子類FlutterCommandRunner的runCommand最終又調用了父類CommandRunner的runCommand方法
  return super.run(args);
}
......
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因此咱們接下來看父類 CommandRunner 的 runCommand 方法，以下：ide

Future<T?> runCommand(ArgResults topLevelResults) async {
  	//一、flutter命令後面傳遞進來參數，譬如build apk
    var argResults = topLevelResults;
    //二、前面分析過的，runner中添加的支持命令列表
    var commands = _commands;
    //三、定義一個Command變量，用來最終依據參數賦值爲對應的Command對象實例
    Command? command;
    var commandString = executableName;
	//四、while條件爲真，由於commands爲支持的參數列表
    while (commands.isNotEmpty) {
      ......
      //五、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}';
	  ......
    }
    ......
    //六、執行對應命令的run方法
    return (await command.run()) as T?;
  }
  ......
}
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能夠看到，這就是一個標準的命令模式設計，先把支持的命令添加到列表，而後依據參數遍歷匹配對應命令進行執行。下面咱們以flutter build apk命令爲例來看其對應的 BuildCommand 命令（packages/flutter_tools/lib/src/commands/build.dart）實現，以下：工具

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));
  }
  //上一小段代碼中command = commands[argResults.name]就是這麼獲得的
  //name=build就是執行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;
}
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能夠看到，任意一個命令基本都繼承自 FlutterCommand 實現，命令的執行都是調用了 FlutterCommand 的 run 方法，以下：

abstract class FlutterCommand extends Command<void> {
  ......
  //runner對象中最終執行調用的方法是這個
  @override
  Future<void> run() {
    ......
    return context.run<void>(
      name: 'command',
      overrides: <Type, Generator>{FlutterCommand: () => this},
      body: () async {
        ......
        try {
          //見名知意，先校驗再運行命令
          commandResult = await verifyThenRunCommand(commandPath);
        } finally {
          ......
        }
      },
    );
  }
  ......
    @mustCallSuper
  Future<FlutterCommandResult> verifyThenRunCommand(String commandPath) async {
    //一、若是須要更新緩存就先更新緩存
    if (shouldUpdateCache) {
      await globals.cache.updateAll(<DevelopmentArtifact>{DevelopmentArtifact.universal});
      await globals.cache.updateAll(await requiredArtifacts);
    }
    globals.cache.releaseLock();
	//二、校驗命令
    await validateCommand();
	//三、若是須要先執行pub就先執行，譬如pub get下載依賴
    if (shouldRunPub) {
      ......
	  //四、執行pub get下載依賴，即下載pubspec.yaml裏配置的依賴
      await pub.get(
        context: PubContext.getVerifyContext(name),
        generateSyntheticPackage: project.manifest.generateSyntheticPackage,
        checkUpToDate: cachePubGet,
      );
      await project.regeneratePlatformSpecificTooling();
      if (reportNullSafety) {
        await _sendNullSafetyAnalyticsEvents(project);
      }
    }

    setupApplicationPackages();
	......
	//五、真正開始執行命令
    return runCommand();
  }
}
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繞一圈最終咱們又回到 BuildCommand 類，能夠發現其 runCommand 方法重寫爲空實現，而其構造時經過 addSubcommand 方法追加了不少子命令，譬如執行flutter build aar編譯 aar 的 BuildAarCommand 命令、執行flutter build apk編譯 apk 的 BuildApkCommand 命令。整個 sub command 與其宿主又算是一個責任鏈，因此上面一樣的套路順序對於 sub command 一樣適用，所以咱們去看下編譯 apk 產物的 BuildApkCommand 源碼（packages/flutter_tools/lib/src/commands/build_apk.dart），以下：

class BuildApkCommand extends BuildSubCommand {
  BuildApkCommand({bool verboseHelp = false}) {
    ......
    //一堆參數的確認
  }
  //對應flutter build apk裏面子命令字符串apk
  @override
  final String name = 'apk';
  ......
  //本質命令執行方法
  @override
  Future<FlutterCommandResult> runCommand() async {
    ......
    //調用androidBuilder的buildApk方法進行真正的編譯，目測裏面的產物也就是上一篇文章分析的那些
    //androidBuilder位於packages/flutter_tools/lib/src/android/android_builder.dart
    await androidBuilder.buildApk(
      project: FlutterProject.current(),
      target: targetFile,
      androidBuildInfo: androidBuildInfo,
    );
    return FlutterCommandResult.success();
  }
}
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順着這條路咱們繼續跟進位於packages/flutter_tools/lib/src/android/android_builder.dart的 androidBuilder 屬性的 buildApk 方法，以下：

//本質是packages/flutter_tools/lib/src/context_runner.dart中context.run方法中的AndroidGradleBuilder實例
AndroidBuilder get androidBuilder {
  return context.get<AndroidBuilder>();
}
//抽象類定義，AndroidBuilder
abstract class AndroidBuilder {
  const AndroidBuilder();
  // 定義編譯aar的方法
  Future<void> buildAar({
    @required FlutterProject project,
    @required Set<AndroidBuildInfo> androidBuildInfo,
    @required String target,
    @required String outputDirectoryPath,
    @required String buildNumber,
  });

  // 定義編譯apk的方法
  Future<void> buildApk({
    @required FlutterProject project,
    @required AndroidBuildInfo androidBuildInfo,
    @required String target,
  });

  // 定義編譯aab的方法
  Future<void> buildAab({
    @required FlutterProject project,
    @required AndroidBuildInfo androidBuildInfo,
    @required String target,
    bool validateDeferredComponents = true,
    bool deferredComponentsEnabled = false,
  });
}
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因此咱們繼續去看 AndroidGradleBuilder 實現類（packages/flutter_tools/lib/src/android/gradle.dart）的 buildApk 方法，以下：

class AndroidGradleBuilder implements AndroidBuilder {
  AndroidGradleBuilder({
    ......
  }) : ......;
  ......
  //一、編譯 apk 的方法
  @override
  Future<void> buildApk({
    @required FlutterProject project,
    @required AndroidBuildInfo androidBuildInfo,
    @required String target,
  }) async {
  	//二、調用
    await buildGradleApp(
      project: project,
      androidBuildInfo: androidBuildInfo,
      target: target,
      isBuildingBundle: false,
      localGradleErrors: gradleErrors,
    );
  }
  ......
  //三、真的編譯
  Future<void> buildGradleApp({
    @required FlutterProject project, //FlutterProject.current()
    @required AndroidBuildInfo androidBuildInfo, //build configuration
    @required String target, //dart代碼入口，缺省lib/main.dart
    @required bool isBuildingBundle, //是aab仍是apk，默認false則apk
    @required List<GradleHandledError> localGradleErrors,
    bool shouldBuildPluginAsAar = false, //是否是將插件編譯爲aar
    bool validateDeferredComponents = true,
    bool deferredComponentsEnabled = false,
    int retries = 1,
  }) async {
    //四、檢查支持的android版本，獲取android編譯產物目錄，即gradle中配置的build產物目錄，默認爲項目根目錄下的build目錄
    if (!project.android.isSupportedVersion) {
      _exitWithUnsupportedProjectMessage(_usage, _logger.terminal);
    }
    final Directory buildDirectory = project.android.buildDirectory;
	//五、讀取安卓相關屬性文件判斷是否使用androidx，而後發送編譯事件參數
    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();
      ......
    }
    //六、更新安卓項目中local.properties中的versionName和versionCode值，值來自於public.yaml文件配置
    updateLocalProperties(project: project, buildInfo: androidBuildInfo.buildInfo);
	//七、編譯aar的話就走buildPluginsAsAar方法進行
    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'),
      );
    }
	//八、獲取編譯apk或者aab對應的標準安卓task name，構建參數等信息，也就是gradle命令後面一堆的參數構造
    final BuildInfo buildInfo = androidBuildInfo.buildInfo;
    final String assembleTask = isBuildingBundle
        ? getBundleTaskFor(buildInfo)
        : getAssembleTaskFor(buildInfo);
	......
    final List<String> command = <String>[
      _gradleUtils.getExecutable(project),
    ];
    ......
    //九、依據條件追加command的一堆參數，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(); } ...... } ...... } 複製代碼

哈哈，真相了，這下配合《Flutter Android 工程結構及應用層編譯源碼深刻分析》一文首尾呼應後你應該完全明白 Flutter android apk 是怎麼編譯的流程！

總結

如今咱們結合《Flutter Android 工程結構及應用層編譯源碼深刻分析》和這篇進行關聯總結，能夠總結出執行flutter build apk命令背後的大體主流程以下：既然執行flutter build apk命令你都搞明白了，那麼其餘 flutter 相關的任何命令你是否也能夠本身觸類旁通進行分析學習，本質都同樣哈。因爲我這裏時間有限，因此對於flutter pub get、flutter doctor等其餘命令再也不作詳細分析。

Flutter 命令本質之 Flutter tools 機制源碼深刻分析

背景

flutter_tools 介紹

源碼分析

總結