学习Flutter过程中，先撸了一遍Flutter，写了个仿boss直聘的demo， github地址：flutter_boss. 写完之后其实比较迷茫，android里到底干了啥，于是稍微看了一下源码，有种恍然大悟的感觉。

在创建完Flutter工程后，自动为我们生成了一个FlutterApplication和一个kotlin的Activity。在FlutterApplication里其实就做了一件事，通过调用FlutterMain里的startInitialization方法进行初始化。在生成的主的Activity里我们可以看见以下内容。

class MainActivity(): FlutterActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    GeneratedPluginRegistrant.registerWith(this)
  }
}

可以看到，这个MainActivity就是启动Activity，只不过是继承的FlutterActivity，于是进入FlutterActivity，发现是继承自Activity，而FlutterActivity里的生命周期是委托给另一个FlutterActivityDelegate管理的，还有一个类名字叫FlutterFragmentActivity共用了该类。看源码先抓重点，当然先是看FlutterActivityDelegate的onCreate里做了啥

String[] args = getArgsFromIntent(this.activity.getIntent());        

FlutterMain.ensureInitializationComplete(this.activity.getApplicationContext(), args);
        this.flutterView = this.viewFactory.createFlutterView(this.activity);
        if(this.flutterView == null) {
            FlutterNativeView nativeView = this.viewFactory.createFlutterNativeView();
            this.flutterView = new FlutterView(this.activity, (AttributeSet)null, nativeView);
            this.flutterView.setLayoutParams(matchParent);
            this.activity.setContentView(this.flutterView);
            this.launchView = this.createLaunchView();
            if(this.launchView != null) {
                this.addLaunchView();
            }
        }

前2行是看方法意思是关于确保了Flutter环境初始化完成，如果初始化失败，则会提示"Flutter initialization failed."并抛出RuntimeException，这块不用暂时不用太关心，Flutter工程IDE为我们创建好了，一般不会在这里出问题。

然后接下去看是怎么初始化的，实际项目里，我们是通过Dart来编写Flutter界面的，那么我们肯定最关心Flutter和activity里的界面是什么关系，怎么承载的。通过初始化我们可以看到flutterView是通过viewFactory接口里的2个方法createFlutterView和createFlutterNativeView里去创建，默认是直接返回null，这么写的目的是可以通过override由自己传入。接下去看，由于默认的flutterView是null，所以就通过new FlutterView创建。其实FlutterView继承自SurfaceView，这时候，Android自定义View的知识派上用处了。最后通过最熟悉的activity.setContentView(this.flutterView);设置完成。所以我们可以得出一个结论，Flutter开发出来的应用不管里面有多少个界面，都是一个继承自SurfaceView的FlutterView，既不是activity也不是fragment，只是一个view，必要时，我们可以重写FlutterActivityDelegate里的onCreate实现我们自己的需求。注意后面的createLaunchView方法，我们可以创建app的启动画面。

public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {
        super(context, attrs);
        ...
        Activity activity = (Activity)this.getContext();
        if(nativeView == null) {
            this.mNativeView = new FlutterNativeView(activity.getApplicationContext());
        } else {
            this.mNativeView = nativeView;
        }

        this.mNativeView.attachViewAndActivity(this, activity);

        ...

        this.mAccessibilityManager = (AccessibilityManager)this.getContext().getSystemService("accessibility");
        this.mActivityLifecycleListeners = new ArrayList();
        this.mFirstFrameListeners = new ArrayList();
        this.mFlutterLocalizationChannel = new MethodChannel(this, "flutter/localization", JSONMethodCodec.INSTANCE);
        this.mFlutterNavigationChannel = new MethodChannel(this, "flutter/navigation", JSONMethodCodec.INSTANCE);
        this.mFlutterKeyEventChannel = new BasicMessageChannel(this, "flutter/keyevent", JSONMessageCodec.INSTANCE);
        this.mFlutterLifecycleChannel = new BasicMessageChannel(this, "flutter/lifecycle", StringCodec.INSTANCE);
        this.mFlutterSystemChannel = new BasicMessageChannel(this, "flutter/system", JSONMessageCodec.INSTANCE);
        this.mFlutterSettingsChannel = new BasicMessageChannel(this, "flutter/settings", JSONMessageCodec.INSTANCE);
        PlatformPlugin platformPlugin = new PlatformPlugin(activity);
        MethodChannel flutterPlatformChannel = new MethodChannel(this, "flutter/platform", JSONMethodCodec.INSTANCE);
        flutterPlatformChannel.setMethodCallHandler(platformPlugin);
        this.addActivityLifecycleListener(platformPlugin);
        this.mTextInputPlugin = new TextInputPlugin(this);
        this.setLocale(this.getResources().getConfiguration().locale);
        this.setUserSettings();
        if((context.getApplicationInfo().flags & 2) != 0) {
            this.mDiscoveryReceiver = new FlutterView.DiscoveryReceiver(null);
            context.registerReceiver(this.mDiscoveryReceiver, new IntentFilter("io.flutter.view.DISCOVER"));
        } else {
            this.mDiscoveryReceiver = null;
        }

    }

代码太长，有的地方省略了，我认为比较重要的是做了以下几点：

真正创建了FlutterNativeView，里面真正工作的是Framework层的dart
载入系统默认的MethodChannel，至于MethodChannel是干什么的，官方有一张图可以说明。
我们也可以在Activity里做一些扩张，自定义自己的MethodChannel

       new MethodChannel(getFlutterView(), CHANNEL).setMethodCallHandler(
            new MethodCallHandler() {
                @Override
                public void onMethodCall(MethodCall call, Result result) {
                    // TODO
                }
            });

其他的activity里的几个生命周期方法，没什么好说的，简单说下onDestroy，

    public void destroy() {
        if(this.isAttached()) {
            if(this.mDiscoveryReceiver != null) {
                this.getContext().unregisterReceiver(this.mDiscoveryReceiver);
            }

            this.getHolder().removeCallback(this.mSurfaceCallback);
            this.mNativeView.destroy();
            this.mNativeView = null;
        }
    }

就是把FlutterNativeView清除。

时间关系，先了解个大概吧，后面有新的收获再来分享。