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Flutter應用是由平臺來創建、初始化并啟動的，這里我們以android為例，對啟動程序做一個走馬觀花式的了解,旨在對平臺端的作業有個大致了解，

Android端的啟動流程

啟動流程實際上還涉及了很多native 層的作業，但是宥于篇幅，暫且只看Android端，

FlutterApplication

flutter應用下，原生的啟動流程并沒有什么變化，我們來看Application的onCreate函式，

  @Override
  @CallSuper
  public void onCreate() {
    super.onCreate();
    FlutterMain.startInitialization(this);
  }

很簡單，繼續往里走

  public static void startInitialization(@NonNull Context applicationContext) {
    if (isRunningInRobolectricTest) {
      return;
    }
    FlutterLoader.getInstance().startInitialization(applicationContext);
  }

按上面方法的注釋來看，是初始化 native system（即C++）的，最侄訓呼叫下面的方法：

我將說明以注釋的形式寫在下面

  public void startInitialization(@NonNull Context applicationContext, @NonNull Settings settings) {
    
    if (this.settings != null) {
      return;
    }
    ///確保運行在 主執行緒
    if (Looper.myLooper() != Looper.getMainLooper()) {
      throw new IllegalStateException("startInitialization must be called on the main thread");
    }

    // Ensure that the context is actually the application context.
    final Context appContext = applicationContext.getApplicationContext();

    this.settings = settings;

    initStartTimestampMillis = SystemClock.uptimeMillis();
    
    ///配置 aotSharedLibraryName、flutterAssetsDir、
    ///      vmSnapshotData、isolateSnapshotData
    ///等引數
    
    initConfig(appContext);
    ///初始化VsyncWaiter，并設定回呼
    /// 當vsync信號到來時，就呼叫我們設定的回呼，最侄訓觸發頁面的重繪
    VsyncWaiter.getInstance((WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE))
        .init();

    // 子執行緒
    
    ///這里主要是抽取資源檔案，
    ///加載 flutter（框架）代碼
    Callable<InitResult> initTask =
        new Callable<InitResult>() {
          @Override
          public InitResult call() {
            ResourceExtractor resourceExtractor = initResources(appContext);

            System.loadLibrary("flutter");

            // Prefetch the default font manager as soon as possible on a background thread.
            // It helps to reduce time cost of engine setup that blocks the platform thread.
            Executors.newSingleThreadExecutor()
                .execute(
                    new Runnable() {
                      @Override
                      public void run() {
                        FlutterJNI.nativePrefetchDefaultFontManager();
                      }
                    });

            if (resourceExtractor != null) {
              resourceExtractor.waitForCompletion();
            }

            return new InitResult(
                PathUtils.getFilesDir(appContext),
                PathUtils.getCacheDirectory(appContext),
                PathUtils.getDataDirectory(appContext));
          }
        };
    initResultFuture = Executors.newSingleThreadExecutor().submit(initTask);
  }

至此FlutterApplication 的相關流程就走完了，

另外，雖然上面的代碼中使用了子執行緒，但是最終在這些任務沒有完成前，是不會進入flutter側的，我們接著走FlutterActivity，

FlutterActivity & onCreate

開始的地方依然是 onCreate()方法：

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
  	///切換主題
    switchLaunchThemeForNormalTheme();

    super.onCreate(savedInstanceState);
	///通知生命周期
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
	
    ///初始化delete，這個很重要，
    ///所有的作業都是由它來完成的
    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    ///是否需要恢復（包括通知插件）一些狀態
    delegate.onActivityCreated(savedInstanceState);
	///配置視窗
    configureWindowForTransparency();
    ///創建flutterView
    setContentView(createFlutterView());
    configureStatusBarForFullscreenFlutterExperience();
  }

這里面比較重的代碼是這幾行:

    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    ...
    setContentView(createFlutterView());

我們一步一步來,首先創建了FlutterActivityAndFragmentDelegate 并呼叫了它的attact(this)方法，

FlutterActivityAndFragmentDelegate

void onAttach(@NonNull Context context) {
    ensureAlive();
	///初始化engine
    if (flutterEngine == null) {
    ///這里面會對已有的engine進行復用
      setupFlutterEngine();
    }
	
    ///初始化平臺插件
    ///本質上，是將engine的 channel回呼與平臺的系統服務進行系結
    ///如：震動、復制粘貼、聲音播放等...
    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);

    if (host.shouldAttachEngineToActivity()) {

      Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this Fragment.");
      /// 激活 原生viewController
      /// 并通知相關插件
      /// PlatformViewsController 這個類你應該很熟悉（如果你接入過原生view的話）
      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
    }
	///注冊插件
    ///通過反射呼叫 “io.flutter.plugins.GeneratedPluginRegistrant”
    ///的 “registerWith”方法，這個程序走完了，你的插件基本就能用了
    host.configureFlutterEngine(flutterEngine);
  }

通過上面，我們大致了解了，在flutter端使用的平臺功能是什么時候裝配的了，

我們回到FlutterActivity，繼續重要的第二步:

setContentView(createFlutterView());

  @NonNull
  private View createFlutterView() {
    return delegate.onCreateView(
        null /* inflater */, null /* container */, null /* savedInstanceState */);
  }

這里插一句，可以看一下這篇文章：

Flutter&Android 啟動頁（閃屏頁）的加載流程和優化方案

最侄訓呼叫 delete的onCreateView :

  @NonNull
  View onCreateView(
      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    Log.v(TAG, "Creating FlutterView.");
    ensureAlive();

    if (host.getRenderMode() == RenderMode.surface) {
    
    	///一般flutter應用是 RenderMode.surface，所以會進入到這里
        ///創建FlutterSurfaceView
      FlutterSurfaceView flutterSurfaceView =
          new FlutterSurfaceView(
              host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);

      // Allow our host to customize FlutterSurfaceView, if desired.
      host.onFlutterSurfaceViewCreated(flutterSurfaceView);

      // flutterView 創建完成后，便會呼叫addView
      //將 flutterSurfaceView 顯示出來，只不過啥都沒有而已
      flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
    } else {
      ...省略代碼...
    }

    // Add listener to be notified when Flutter renders its first frame.
    flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);

    flutterSplashView = new FlutterSplashView(host.getContext());
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
      flutterSplashView.setId(View.generateViewId());
    } else {

      flutterSplashView.setId(486947586);
    }
    ///這里顯示閃屏頁 默認是個白屏
    ///即，AndroidMainfest.xml 的<metadata>所設定
    flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

    ///將flutterview 系結到 engine上
    flutterView.attachToFlutterEngine(flutterEngine);

    return flutterSplashView;
  }

flutterView 內部持有flutterSurfaceView （一個Surface），并最終通過attachToFlutterEngine系結到engine上，我們來看一下其內部實作：

  public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {
  
   ...省略部分代碼...

    this.flutterEngine = flutterEngine;

    ///通過engine的 getRenderer，
    ///可以將flutter的紋理繪制到android 上，
    FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
    isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
    renderSurface.attachToRenderer(flutterRenderer);
    flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);

    ...省略部分代碼...
    
    ///輸入插件
    textInputPlugin =
        new TextInputPlugin(
            this,
            this.flutterEngine.getTextInputChannel(),
            this.flutterEngine.getPlatformViewsController());
    ///國際化插件
    localizationPlugin = this.flutterEngine.getLocalizationPlugin();
    ///與上面的textInputPlugin相關聯
    androidKeyProcessor =
        new AndroidKeyProcessor(this.flutterEngine.getKeyEventChannel(), textInputPlugin);
        
     /// 觸摸事件的初始化
     /// 相關觸摸資料會發送到flutter端
    androidTouchProcessor = new AndroidTouchProcessor(this.flutterEngine.getRenderer());
    ///輔助功能
    accessibilityBridge =
        new AccessibilityBridge(
            this,
            flutterEngine.getAccessibilityChannel(),
            (AccessibilityManager) getContext().getSystemService(Context.ACCESSIBILITY_SERVICE),
            getContext().getContentResolver(),
            this.flutterEngine.getPlatformViewsController());
   
   ...省略部分代碼...

    ///通過上面的初始化，將用戶相關的設定發送到flutter端
    sendUserSettingsToFlutter();
    localizationPlugin.sendLocalesToFlutter(getResources().getConfiguration());
    sendViewportMetricsToFlutter();
	
    ///將當前flutter view 系結到 PlatformViewsController
    flutterEngine.getPlatformViewsController().attachToView(this);

    ...省略部分代碼...
  }

相關初始化作業完成，activity的生命周期也從onCreate來到了onStart()

FlutterActivity & onStart()

  @Override
  protected void onStart() {
    super.onStart();
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START);
    ///重要入口
    delegate.onStart();
  }

delegate.onStart()此方法最侄訓呼叫doInitialFlutterViewRun（）方法：

  private void doInitialFlutterViewRun() {
    
	...省略部分代碼...
	
    // 這里就是獲取我們打包所得的 libapp.so路徑
    // 即，我們所寫的dart代碼，并執行它
    DartExecutor.DartEntrypoint entrypoint =
        new DartExecutor.DartEntrypoint(
            host.getAppBundlePath(), host.getDartEntrypointFunctionName());
    flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
  }

至此整個android端的啟動流程就走完了，這里再回顧總結一下，

總結

在flutterApplication中：

初始化一些資源路徑，配置相關引數
抽取資源并加載(assets)
加載flutter.so關鍵庫
    - 最終走JNI_OnLoad 進入native進行相關作業
    - 如系結flutter jni
初始化vsyncWaiter

在flutterActivity中：

會初始化重要類FlutterActivityAndFragmentDelegate
activity端的生命周期，也會觸發delegate來對應回呼
對平臺的系統功能（震動、剪貼板）進行系結
初始化platformViewController以及系統channel
創建flutterView（內部持有一個surfaceView）
    - 會最終進入native進行engine的初始化作業
在onStart生命周期中加載咱們的dart代碼，開始執行

在這整個程序中，會穿插進行native層的作業，并最終通過native層的呼叫，轉到flutter端的main()函式，由于這里的內容很多，將會在后面的文章中介紹，

最后，謝謝大家的閱讀，如果有不對的地方，還請指出，

系列文章

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Flutter&Android 啟動頁（閃屏頁）的加載流程和優化方案

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Flutter——實作網易云音樂的漸進式卡片切換

Flutter 仿同花順自選股串列

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