Flutter之Android層面源碼分析（一）

學習Flutter過程當中，先擼了一遍Flutter，寫了個仿boss直聘的demo， github地址：flutter_boss. 寫完以後其實比較迷茫，android裏到底幹了啥，因而稍微看了一下源碼，有種恍然大悟的感受。

在建立完Flutter工程後，自動爲咱們生成了一個FlutterApplication和一個kotlin的Activity。 在FlutterApplication裏其實就作了一件事，經過調用FlutterMain裏的startInitialization方法進行初始化。 在生成的主的Activity裏咱們能夠看見如下內容。

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

能夠看到，這個MainActivity就是啓動Activity，只不過是繼承的FlutterActivity，因而進入FlutterActivity，發現是繼承自Activity，而FlutterActivity裏的生命週期是委託給另外一個FlutterActivityDelegate管理的，還有一個類名字叫FlutterFragmentActivity共用了該類。 看源碼先抓重點，固然先是看FlutterActivityDelegate的onCreate裏作了啥

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();
            }
        }
複製代碼

前2行是看方法意思是關於確保了Flutter環境初始化完成，若是初始化失敗，則會提示"Flutter initialization failed."並拋出RuntimeException，這塊不用暫時不用太關心，Flutter工程IDE爲咱們建立好了，通常不會在這裏出問題。

而後接下去看是怎麼初始化的， 實際項目裏，咱們是經過Dart來編寫Flutter界面的，那麼咱們確定最關心Flutter和activity裏的界面是什麼關係，怎麼承載的。經過初始化咱們能夠看到flutterView是經過viewFactory接口裏的2個方法createFlutterView和createFlutterNativeView裏去建立，默認是直接返回null，這麼寫的目的是能夠經過override由本身傳入。接下去看，因爲默認的flutterView是null，因此就經過new FlutterView建立。其實FlutterView繼承自SurfaceView，這時候，Android自定義View的知識派上用處了。 最後經過最熟悉的activity.setContentView(this.flutterView);設置完成。因此咱們能夠得出一個結論，Flutter開發出來的應用無論裏面有多少個界面，都是一個繼承自SurfaceView的FlutterView，既不是activity也不是fragment，只是一個view，必要時，咱們能夠重寫FlutterActivityDelegate裏的onCreate實現咱們本身的需求。 注意後面的createLaunchView方法，咱們能夠建立app的啓動畫面。

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;
        }

    }

複製代碼

代碼太長，有的地方省略了，我認爲比較重要的是作了如下幾點：

1. 真正建立了FlutterNativeView，裏面真正工做的是Framework層的dart
2. 載入系統默認的MethodChannel，至於MethodChannel是幹什麼的，官方有一張圖能夠說明。

   

   咱們也能夠在Activity裏作一些擴張，自定義本身的MethodChannel

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

其餘的activity裏的幾個生命週期方法，沒什麼好說的，簡單說下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;
        }
    }
複製代碼

就是把FlutterNativeView清除。

時間關係，先了解個大概吧，後面有新的收穫再來分享。