=============================================================================
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。
================================================================
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
《Android学习笔记总结+最新移动架构视频+大厂安卓面试真题+项目实战源码讲义》
【docs.qq.com/doc/DSkNLaERkbnFoS0ZF】 完整内容开源分享
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 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 main(List 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 generateCommands({
@required bool verboseHelp,
@required bool verbose,
}) => [
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 run(
List args,
List 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(() 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>(() 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 run(Iterable 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>;
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 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 {
…
//runner对象中最终执行调用的方法是这个
@override
Future run() {
…
return context.run(
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 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.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 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();
}
//抽象类定义，AndroidBuilder
abstract class AndroidBuilder {
const AndroidBuilder();
// 定义编译aar的方法
Future buildAar({
@required FlutterProject project,
@required Set androidBuildInfo,
@required String target,
@required String outputDirectoryPath,
