Flutter App啟動流程分析-有解無憂

首先手動創建一個hello_project的Flutter Project，再一步步跟著原始碼走

按照慣例我們首先查找是否有自定義的Application，發現Flutter Project沒有自定義Application，然后我們再來看MainActivity.kt

class MainActivity: FlutterActivity() {
}

MainActivity代碼無比簡單，我們繼續看FlutterActivity.java（Google官方都推Kotlin了，FlutterActivity居然還用Java…）

public class FlutterActivity extends Activity
    implements FlutterActivityAndFragmentDelegate.Host, LifecycleOwner

MainActivity直接繼承自Activity，同時實作了兩個介面：LifecycleOwner（暴露系結生命周期的方法），FlutterActivityAndFragmentDelegate.Host，這里的Host主要是FlutterActivity作為宿主提供暴露各種方法，Host介面方法命名都比較簡單直觀，這里不做詳細解釋，

再來看onCreate方法

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    switchLaunchThemeForNormalTheme();
    super.onCreate(savedInstanceState);
		// 創建delegate，重點后面分析
    delegate = new FlutterActivityAndFragmentDelegate(this);
    // 1、onAttach 重點，內部初始化引擎邏輯
    delegate.onAttach(this);
    //onRestoreInstanceState,類似Activity中的onRestoreInstanceState方法，可以用來恢復資料
    delegate.onRestoreInstanceState(savedInstanceState);

    //系結生命周期
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);

    configureWindowForTransparency();
    //2、創建FlutterView，顯示到Activity上
    setContentView(createFlutterView());
    configureStatusBarForFullscreenFlutterExperience();
  }

這里我們重點關注兩步

onAttach 內部創建初始化引擎
createFlutterView() 創建FlutterView顯示到Activity上

下面著重一步一步來分析

onAttach

  void onAttach(@NonNull Context context) {
    ensureAlive();

    // 是否已經初始化過引擎
    if (flutterEngine == null) {
      //下面重點分析引擎初始化
      setupFlutterEngine();
    }

    if (host.shouldAttachEngineToActivity()) {
      //engine 與 activity系結
      Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this delegate.");
      flutterEngine.getActivityControlSurface().attachToActivity(this, host.getLifecycle());
    }

    //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);

    //注冊插件,通過反射呼叫 “io.flutter.plugins.GeneratedPluginRegistrant.registerWith()”
    host.configureFlutterEngine(flutterEngine);
  }

setupFlutterEngine()

@VisibleForTesting
void setupFlutterEngine() {
    // 回傳FlutterEngine，如果為慷訓自動創建一個Engine
    flutterEngine = host.provideFlutterEngine(host.getContext());
    if (flutterEngine != null) {
      isFlutterEngineFromHost = true;
      return;
    }
    // 如果上面步驟都沒后獲取到flutterEngine的話，就創建一個FlutterEngine
    flutterEngine =
        new FlutterEngine(
            host.getContext(),
            host.getFlutterShellArgs().toArray(),
            /*automaticallyRegisterPlugins=*/ false,
            /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
    isFlutterEngineFromHost = false;
  }

我們先來看一下FlutterEngine的理解：

是一個獨立的Flutter執行環境
FlutterEngine 是一個容器，通過它可以在 Android 應用程式中運行 Dart 代碼，
FlutterEngine 中的 Dart 代碼可以在后臺執行，也可以使用附帶的 FlutterRenderer 和 Dart 代碼將 Dart 端 UI 效果渲染到螢屏上，渲染可以開始和停止，從而允許 FlutterEngine 從 UI 互動轉移到僅進行資料處理，然后又回傳到 UI 互動的能力，
可能同時存在多個FlutterEngine執行Dart代碼，并且在單個Android應用程式中繪制UI，為了更好的記憶體性能特性，通過FlutterEngineGroup構造多個FlutterEngine，而不是直接通過FlutterEngine的建構式，
使用 FlutterEngine 執行 Dart 或 Flutter 代碼需要先通過 FlutterEngine 獲取 DartExecutor 參考，然后呼叫 DartExecutor 的executeDartEntrypoint(DartExecutor.DartEntrypoint)執行 Dart 代碼即可，同一個 FlutterEngine 實體中獲取的 DartExecutor 的executeDartEntrypoint(DartExecutor.DartEntrypoint)方法只能被呼叫一次，
要開始將 Flutter 內容渲染到螢屏上，請使用 getRenderer() 獲取 FlutterRenderer，然后附加一個 RenderSurface，考慮使用 FlutterView 作為 RenderSurface，
App 每個行程中創建第一個 FlutterEngine 實體的時候會加載 Flutter 引擎的原生庫并啟動 Dart VM（VM 存活生命周期跟隨行程），隨后同行程中其他的 FlutterEngines 將在同一個 VM 實體上運行，但在運行 DartExecutor 時將擁有自己的 Dart Isolate，每個 Isolate 都是一個獨立的 Dart 環境，除非通過 Isolate 埠，否則無法相互通信，
App 每個行程中創建第一個 FlutterEngine 實體的時候會加載 Flutter 引擎的原生庫并啟動 Dart VM（VM 存活生命周期跟隨行程），隨后同行程中其他的 FlutterEngine 將在同一個 VM 實體上運行，但在運行 DartExecutor 時將擁有自己的 Dart Isolate，每個 Isolate 都是一個獨立的 Dart 環境，除非通過 Isolate 埠，否則無法相互通信，

最終整理對于一個多FlutterEngine的App來說，FlutterEngine，DartExecutor，Dart VM， Isloate關系如下：

在這里插入圖片描述

接下來我們繼續看FlutterEngine

public class FlutterEngine {
  //Flutter C/C++與平臺java層介面定義互動，Flutter 的引擎是用 C/C++ 構建的， Android Flutter 嵌入負責協調 Android 作業系統事件和應用程式用戶與 C/C++ 引擎的互動，
  @NonNull private final FlutterJNI flutterJNI;
  //表示 FlutterEngine 的渲染職責，
	//FlutterRenderer 與提供的 RenderSurface 協同作業，將 Flutter 像素繪制到 Android 視圖層次結構，
	//FlutterRenderer 管理用于渲染的紋理，并通過 JNI 將一些 Java 呼叫轉發到原生 Flutter 代碼， 相應的 RenderSurface 提供此渲染器繪制的 Android Surface，
	//io.flutter.embedding.android.FlutterSurfaceView 和 io.flutter.embedding.android.FlutterTextureView 是 RenderSurface 的實作
  @NonNull private final FlutterRenderer renderer;
  //Dart執行器，
  @NonNull private final DartExecutor dartExecutor;
  //用來管理安卓組件和Flutter plugins插件，
  @NonNull private final FlutterEngineConnectionRegistry pluginRegistry;
  //localization的安卓端實作插件，
  @NonNull private final LocalizationPlugin localizationPlugin;

  //一堆系統通道，負責與系統底層通信
  @NonNull private final AccessibilityChannel accessibilityChannel;
  @NonNull private final DeferredComponentChannel deferredComponentChannel;
  @NonNull private final KeyEventChannel keyEventChannel;
  @NonNull private final LifecycleChannel lifecycleChannel;
  @NonNull private final LocalizationChannel localizationChannel;
  @NonNull private final MouseCursorChannel mouseCursorChannel;
  @NonNull private final NavigationChannel navigationChannel;
  @NonNull private final RestorationChannel restorationChannel;
  @NonNull private final PlatformChannel platformChannel;
  @NonNull private final SettingsChannel settingsChannel;
  @NonNull private final SystemChannel systemChannel;
  @NonNull private final TextInputChannel textInputChannel;

  // Platform Views.
  // 管理平臺視圖，每個 io.flutter.app.FlutterPluginRegistry 都有一個平臺視圖控制器， 一個平臺視圖控制器最多可以附加到一個 Flutter 視圖
  @NonNull private final PlatformViewsController platformViewsController;
  // Engine Lifecycle.
  // 引擎宣告周期監聽
  @NonNull private final Set<EngineLifecycleListener> engineLifecycleListeners = new HashSet<>();
  //......

  //全引數的建構式，各種構造最終都走進這里
  public FlutterEngine(
      @NonNull Context context,
      @Nullable FlutterLoader flutterLoader,
      @NonNull FlutterJNI flutterJNI,
      @NonNull PlatformViewsController platformViewsController,
      @Nullable String[] dartVmArgs,
      boolean automaticallyRegisterPlugins,
      boolean waitForRestorationData) {
    //......
    //創建一個DartExecutor并將flutterJNI和安卓平臺的assetManager實體傳遞進去，
    this.dartExecutor = new DartExecutor(flutterJNI, assetManager);
    this.dartExecutor.onAttachedToJNI();
    //......
    //各種channel實體化
    accessibilityChannel = new AccessibilityChannel(dartExecutor, flutterJNI);
    deferredComponentChannel = new DeferredComponentChannel(dartExecutor);
    keyEventChannel = new KeyEventChannel(dartExecutor);
    lifecycleChannel = new LifecycleChannel(dartExecutor);
    localizationChannel = new LocalizationChannel(dartExecutor);
    mouseCursorChannel = new MouseCursorChannel(dartExecutor);
    navigationChannel = new NavigationChannel(dartExecutor);
    platformChannel = new PlatformChannel(dartExecutor);
    restorationChannel = new RestorationChannel(dartExecutor, waitForRestorationData);
    settingsChannel = new SettingsChannel(dartExecutor);
    systemChannel = new SystemChannel(dartExecutor);
    textInputChannel = new TextInputChannel(dartExecutor);
    //......
    //插件實體化，
    this.localizationPlugin = new LocalizationPlugin(context, localizationChannel);

    this.flutterJNI = flutterJNI;
    if (flutterLoader == null) {
      flutterLoader = FlutterInjector.instance().flutterLoader();
    }
    //......
		// 引擎與四大組件的橋梁
    this.pluginRegistry =
        new FlutterEngineConnectionRegistry(context.getApplicationContext(), this, flutterLoader);
	//默認就是自動注冊plugins的，可以通過清單檔案配置變更等，
    if (automaticallyRegisterPlugins && flutterLoader.automaticallyRegisterPlugins()) {
      registerPlugins();
    }
  }
  //......

  //注冊flutter專案根目錄下pubspec.yaml中依賴的所有flutter plugins，
  //Flutter tool會生成一個GeneratedPluginRegistrant的類，
  private void registerPlugins() {
    try {
      Class<?> generatedPluginRegistrant = Class.forName("io.flutter.plugins.GeneratedPluginRegistrant");
      Method registrationMethod = generatedPluginRegistrant.getDeclaredMethod("registerWith", FlutterEngine.class);
      registrationMethod.invoke(null, this);
    } catch (Exception e) {
      Log.w(TAG, "Tried to automatically register plugins with FlutterEngine ("
              + this + ") but could not find and invoke the GeneratedPluginRegistrant.");
    }
  }
  //......省略一堆屬性成員的get方法
}

總結下Flutter主要作用：

FlutterEngineConnectionRegistry：創建組件與四大引擎生命周期的橋梁，方便插件根據各個組件的生命周期處理
registerPlugins：通過反射注冊自定義插件
各種Channel的初始化，建立引擎與平臺系統之間的橋梁
DartExecutor：Dart執行環境創建，onAttachedToJNI之后，DartExecutor將開始處理與Dart背景關系之間的雙向通行，BinaryMessenger（向Dart發送訊息），PlatformMessageHandler，它接受來自Dart的訊息
FlutterRenderer：渲染環境的創建

到此FlutterEngine基本創建完成，內部細節不做詳解，下面再來分析下createFlutterView

createFlutterView

createFlutterView走到了FlutterActivityAndFragmentDelegate的onCreateView方法

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

    if (host.getRenderMode() == RenderMode.surface) {
      FlutterSurfaceView flutterSurfaceView =
          new FlutterSurfaceView(
              host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);

      // 允許自定義flutterSurfaceView
      host.onFlutterSurfaceViewCreated(flutterSurfaceView);

      // 創建擁有 FlutterSurfaceView 的 FlutterView，
      flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
    } else {
      FlutterTextureView flutterTextureView = new FlutterTextureView(host.getActivity());

      // 允許自定義flutterTextureView
      host.onFlutterTextureViewCreated(flutterTextureView);

      // 創建擁有 FlutterTextureView 的 FlutterView，
      flutterView = new FlutterView(host.getActivity(), flutterTextureView);
    }

    // 添加偵聽器以在 Flutter 呈現其第一幀時收到通知，
    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);
    }
    flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

    Log.v(TAG, "Attaching FlutterEngine to FlutterView.");
    flutterView.attachToFlutterEngine(flutterEngine);
    return flutterSplashView;
  }

該方法主要完成四個任務：

在 View 層次結構中創建一個新的 FlutterView
添加一個 FlutterUiDisplayListener
將 FlutterEngine 附加到新的 FlutterView
flutterSplashView（主要作用是在 FlutterView render 渲染出來之前顯示一個SplashScreen），類似于過度圖

我們在簡單看下FlutterView，FlutterView是一個Android 原生的控制元件，繼承自FrameLayout，它之所以顯示的是Flutter UI，完全是依賴于FlutterSurfaceView和FlutterTextureView，而FlutterSurfaceView和FlutterTextureView這兩個View的繪制內容又是來自于FlutterEngine提供，

FlutterView原始碼分析，我們后面再來單獨講…

結尾

? 講到這里，我們基本上了解了Flutter啟動的大致流程，其實無論是FlutterActivity還是FlutterFragment都是通過代理類FlutterActivityAndFragmentDelegate來封裝呼叫Flutter的各種行為，

? 下文我們在一起看看FlutterView是如何作業，Flutter UI是如何顯示到原生界面上的，，，，

轉載請註明出處，本文鏈接：https://www.uj5u.com/yidong/328036.html

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