Flutter Android 端啟動流程淺析

這篇文章主要是分析一下 Flutter Android 端的啟動流程，主要是上層程式碼的分析，不涉及底層 c/c++ 等的分析。同時，不同 Flutter 版本的 sdk ，程式碼也會有所不同，但是整體流程和原理不會有太大的不同。

1、FlutterApplication

Android 端 app 的啟動，一定會先初始化 Application，再去載入預設的第一個類 MainActivity。Flutter 專案對應的 Android 端應用程式，application 預設指定為 FlutterApplication。

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.flutter_share">
    <application
        android:name="io.flutter.app.FlutterApplication"
        android:label="flutter_share"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <meta-data
                android:name="io.flutter.app.android.SplashScreenUntilFirstFrame"
                android:value="true" />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
    </application>
</manifest>
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1.1 FlutterMain.startInitialization(this);

FlutterApplication 中 onCreate 核心初始化程式碼只有一行

        FlutterMain.startInitialization(this);
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FlutterMain 這個類在 flutter.jar 包中。不同的平臺對應不同的 flutter.jar 包，這個檔案在 flutter sdk 的路徑位置為： flutterSdkPath\flutter\bin\cache\artifacts\engine 。

接下來看 FlutterMain 中的程式碼

    public static void startInitialization(@NonNull Context applicationContext) {
        if (!isRunningInRobolectricTest) {
            startInitialization(applicationContext, new io.flutter.view.FlutterMain.Settings());
        }
    }

    public static void startInitialization(@NonNull Context applicationContext, @NonNull io.flutter.view.FlutterMain.Settings settings) {
        if (!isRunningInRobolectricTest) {
            if (Looper.myLooper() != Looper.getMainLooper()) {
                throw new IllegalStateException("startInitialization must be called on the main thread");
            } else if (sSettings == null) {
                sSettings = settings;
                long initStartTimestampMillis = SystemClock.uptimeMillis();
                initConfig(applicationContext);
                initResources(applicationContext);
                System.loadLibrary("flutter");
                VsyncWaiter.getInstance((WindowManager)applicationContext.getSystemService("window")).init();
                long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;
                FlutterJNI.nativeRecordStartTimestamp(initTimeMillis);
            }
        }
    }
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主要做了如下幾件事情：

• startInitialization 這個初始化函式必須在在主執行緒執行，否則丟擲異常。
• sSettings 這個變數只會初始化一次。
• initConfig 初始化配置資訊。
• initResources 初始化資源。
• System.loadLibrary("flutter") 載入 flutter 核心庫 libflutter.so。這個庫也在 flutter.jar 中，編譯 flutter 專案中的時候，這個庫會複製到 apk 中。
• FlutterJNI.nativeRecordStartTimestamp 主要是呼叫底層方法記錄初始化時間。

1.2 initConfig

這個方法如下：

    private static void initConfig(@NonNull Context applicationContext) {
        Bundle metadata = getApplicationInfo(applicationContext).metaData;
        if (metadata != null) {
            sAotSharedLibraryName = metadata.getString(PUBLIC_AOT_SHARED_LIBRARY_NAME, "libapp.so");
            sFlutterAssetsDir = metadata.getString(PUBLIC_FLUTTER_ASSETS_DIR_KEY, "flutter_assets");
            sVmSnapshotData = metadata.getString(PUBLIC_VM_SNAPSHOT_DATA_KEY, "vm_snapshot_data");
            sIsolateSnapshotData = metadata.getString(PUBLIC_ISOLATE_SNAPSHOT_DATA_KEY, "isolate_snapshot_data");
        }
    }
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主要是解析 metadata 初始化預設配置，如果沒有設定則使用預設值。

• sAotSharedLibraryName : libapp.so
• sFlutterAssetsDir: flutter_assets
• sVmSnapshotData:vm_snapshot_data
• sIsolateSnapshotData:isolate_snapshot_data

1.3 initResources

程式碼如下：

    private static void initResources(@NonNull Context applicationContext) {
        (new ResourceCleaner(applicationContext)).start();
        String dataDirPath = PathUtils.getDataDirectory(applicationContext);
        String packageName = applicationContext.getPackageName();
        PackageManager packageManager = applicationContext.getPackageManager();
        AssetManager assetManager = applicationContext.getResources().getAssets();
        sResourceExtractor = new ResourceExtractor(dataDirPath, packageName, packageManager, assetManager);
        sResourceExtractor.addResource(fromFlutterAssets(sVmSnapshotData)).addResource(fromFlutterAssets(sIsolateSnapshotData)).addResource(fromFlutterAssets("kernel_blob.bin"));
        sResourceExtractor.start();
    }

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主要的作用就兩個：

• 通過 ResourceCleaner 清理快取檔案
• 通過 sRecourceExtractor 載入指定目錄下的資原始檔，通過這些檔案將進行 flutter engine 和 Darv Vm 的初始化。

上面就是 Application 的初始化過程。接下來看 MainActivity 的執行。

2、MainActivity

Android 端預設啟動的第一個類就是 MainActivity，而建立的 Flutter 的專案中，MainActivity 這個類都是自動生成的，程式碼如下：

public class MainActivity extends FlutterActivity {
 @Override
 protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   GeneratedPluginRegistrant.registerWith(this);
 }
}

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可以看到，MainActivity 繼承自 Flutter 自己實現的 FlutterActivity。

2.1 GeneratedPluginRegistrant.registerWith(this)

MainActivity 中的核心程式碼只有一行

GeneratedPluginRegistrant.registerWith(this);
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GeneratedPluginRegistrant 這個類程式碼如下：

/**
 * Generated file. Do not edit.
 */
public final class GeneratedPluginRegistrant {
  public static void registerWith(PluginRegistry registry) {
    if (alreadyRegisteredWith(registry)) {
      return;
    }
  }

  private static boolean alreadyRegisteredWith(PluginRegistry registry) {
    final String key = GeneratedPluginRegistrant.class.getCanonicalName();
    if (registry.hasPlugin(key)) {
      return true;
    }
    registry.registrarFor(key);
    return false;
  }
}
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可以看到，這個類是自動生成的，並且會 執行 registry.registrarFor(key) 方法。 registry 是一個介面，並且 FlutterActivity 實現了這個接。而 MainActivity 也繼承了 FlutterActivity 。接下來分析 FlutterActivity 。

3、FlutterActivity

public class FlutterActivity extends Activity implements Provider, PluginRegistry, ViewFactory 
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可以看到 FlutterActivity 實現了 Provider、PluginRegistry,ViewFactory 三個介面。

    private final FlutterActivityDelegate delegate = new FlutterActivityDelegate(this, this);
    private final FlutterActivityEvents eventDelegate;
    private final Provider viewProvider;
    private final PluginRegistry pluginRegistry;

    public FlutterActivity() {
        this.eventDelegate = this.delegate;
        this.viewProvider = this.delegate;
        this.pluginRegistry = this.delegate;
    }
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可以看到，三個引用都指向了同一個物件，也就是 這個 delegate 物件將實現另外三個類宣告的功能。

接著看 FlutterActivity 中的主要方法:

    protected void onStart() {
        super.onStart();
        this.eventDelegate.onStart();
    }

    protected void onResume() {
        super.onResume();
        this.eventDelegate.onResume();
    }
   ...
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可以看到，在 FlutterActivity 中的核心方法中，只是簡單的回撥了 delegate 中對應的方法，也就是 FlutterActivity 這個類只是一個代理，真正實現功能的一定是 delegate 這個物件對應的類，也就是 FlutterActivityDelegate。

4、 FlutterActivityDelegate

public final class FlutterActivityDelegate implements FlutterActivityEvents, Provider, PluginRegistry 
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這個類通過 final 修飾，將不能被繼承，同時確實也實現了三個介面。

4.1 onCreate

    public void onCreate(Bundle savedInstanceState) {
        if (VERSION.SDK_INT >= 21) {
            Window window = this.activity.getWindow();
            window.addFlags(-2147483648);
            window.setStatusBarColor(1073741824);
            window.getDecorView().setSystemUiVisibility(1280);
        }

        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();
            }
        }

        if (!this.loadIntent(this.activity.getIntent())) {
            String appBundlePath = FlutterMain.findAppBundlePath();
            if (appBundlePath != null) {
                this.runBundle(appBundlePath);
            }

        }
    }

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onCreate 是這個類初始化之後執行的第一個函式，主要的作用是：

• 設定沉浸式狀態列
• 從 intent 中獲取引數
• 呼叫 FlutterMain.ensureInitializationComplete 確保資源初始化完成。
• 建立 FlutterNativeView 和 FlutterView 物件例項。
• 設定當前 activity 的檢視佈局為 flutterView。
• runBundle 這裡面將通過一些列方法的呼叫，載入 Dart 程式碼。

4.1 FlutterMain.ensureInitializationComplete

  public static void ensureInitializationComplete(@NonNull Context applicationContext, @Nullable String[] args) {
        if (!isRunningInRobolectricTest) {
            if (Looper.myLooper() != Looper.getMainLooper()) {
                throw new IllegalStateException("ensureInitializationComplete must be called on the main thread");
            } else if (sSettings == null) {
                throw new IllegalStateException("ensureInitializationComplete must be called after startInitialization");
            } else if (!sInitialized) {
                try {
                    if (sResourceExtractor != null) {
                        sResourceExtractor.waitForCompletion();
                    }

                    List<String> shellArgs = new ArrayList();
                    shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");
                    ApplicationInfo applicationInfo = getApplicationInfo(applicationContext);
                    shellArgs.add("--icu-native-lib-path=" + applicationInfo.nativeLibraryDir + File.separator + "libflutter.so");
                    if (args != null) {
                        Collections.addAll(shellArgs, args);
                    }

                    String kernelPath = null;
                    shellArgs.add("--aot-shared-library-name=" + sAotSharedLibraryName);
                    shellArgs.add("--aot-shared-library-name=" + applicationInfo.nativeLibraryDir + File.separator + sAotSharedLibraryName);
                    shellArgs.add("--cache-dir-path=" + PathUtils.getCacheDirectory(applicationContext));
                    if (sSettings.getLogTag() != null) {
                        shellArgs.add("--log-tag=" + sSettings.getLogTag());
                    }

                    String appStoragePath = PathUtils.getFilesDir(applicationContext);
                    String engineCachesPath = PathUtils.getCacheDirectory(applicationContext);
                    FlutterJNI.nativeInit(applicationContext, (String[])shellArgs.toArray(new String[0]), (String)kernelPath, appStoragePath, engineCachesPath);
                    sInitialized = true;
                } catch (Exception var7) {
                    Log.e("FlutterMain", "Flutter initialization failed.", var7);
                    throw new RuntimeException(var7);
                }
            }
        }
    }
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這裡分析的是 release 模式下的程式碼，主要的作用是：

• 主執行緒執行
• sSetting 不能為空，否則丟擲異常。
• 必須保證 sResourceExtractor 資源提取完成。
• 將所有的配置引數（如 flutter 核心庫的路徑、flutter APP 專案編譯之後生成的 so 庫的路徑）傳遞給 shellArgs，並且這個 List 陣列將會傳遞給底層進行 Dart Vm 的初始等。
• 通過 FlutterJNI 進行底層初始操作。

4.2 FlutterView 的初始化

public class FlutterView extends SurfaceView implements BinaryMessenger, TextureRegistry {
    ...
    
    public FlutterView(Context context, AttributeSet attrs, FlutterNativeView nativeView) {
        super(context, attrs);
        this.nextTextureId = new AtomicLong(0L);
        this.mIsSoftwareRenderingEnabled = false;
        this.didRenderFirstFrame = false;
        this.onAccessibilityChangeListener = new OnAccessibilityChangeListener() {
            public void onAccessibilityChanged(boolean isAccessibilityEnabled, boolean isTouchExplorationEnabled) {
                FlutterView.this.resetWillNotDraw(isAccessibilityEnabled, isTouchExplorationEnabled);
            }
        };
        Activity activity = getActivity(this.getContext());
        if (activity == null) {
            throw new IllegalArgumentException("Bad context");
        } else {
            if (nativeView == null) {
                this.mNativeView = new FlutterNativeView(activity.getApplicationContext());
            } else {
                this.mNativeView = nativeView;
            }

            this.dartExecutor = this.mNativeView.getDartExecutor();
            this.flutterRenderer = new FlutterRenderer(this.mNativeView.getFlutterJNI());
            this.mIsSoftwareRenderingEnabled = FlutterJNI.nativeGetIsSoftwareRenderingEnabled();
            this.mMetrics = new FlutterView.ViewportMetrics();
            this.mMetrics.devicePixelRatio = context.getResources().getDisplayMetrics().density;
            this.setFocusable(true);
            this.setFocusableInTouchMode(true);
            this.mNativeView.attachViewAndActivity(this, activity);
            this.mSurfaceCallback = new Callback() {
                public void surfaceCreated(SurfaceHolder holder) {
                    FlutterView.this.assertAttached();
                    FlutterView.this.mNativeView.getFlutterJNI().onSurfaceCreated(holder.getSurface());
                }

                public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
                    FlutterView.this.assertAttached();
                    FlutterView.this.mNativeView.getFlutterJNI().onSurfaceChanged(width, height);
                }

                public void surfaceDestroyed(SurfaceHolder holder) {
                    FlutterView.this.assertAttached();
                    FlutterView.this.mNativeView.getFlutterJNI().onSurfaceDestroyed();
                }
            };
            this.getHolder().addCallback(this.mSurfaceCallback);
            this.mActivityLifecycleListeners = new ArrayList();
            this.mFirstFrameListeners = new ArrayList();
            this.navigationChannel = new NavigationChannel(this.dartExecutor);
            this.keyEventChannel = new KeyEventChannel(this.dartExecutor);
            this.lifecycleChannel = new LifecycleChannel(this.dartExecutor);
            this.localizationChannel = new LocalizationChannel(this.dartExecutor);
            this.platformChannel = new PlatformChannel(this.dartExecutor);
            this.systemChannel = new SystemChannel(this.dartExecutor);
            this.settingsChannel = new SettingsChannel(this.dartExecutor);
            final PlatformPlugin platformPlugin = new PlatformPlugin(activity, this.platformChannel);
            this.addActivityLifecycleListener(new ActivityLifecycleListener() {
                public void onPostResume() {
                    platformPlugin.updateSystemUiOverlays();
                }
            });
            this.mImm = (InputMethodManager)this.getContext().getSystemService("input_method");
            PlatformViewsController platformViewsController = this.mNativeView.getPluginRegistry().getPlatformViewsController();
            this.mTextInputPlugin = new TextInputPlugin(this, this.dartExecutor, platformViewsController);
            this.androidKeyProcessor = new AndroidKeyProcessor(this.keyEventChannel, this.mTextInputPlugin);
            this.androidTouchProcessor = new AndroidTouchProcessor(this.flutterRenderer);
            this.mNativeView.getPluginRegistry().getPlatformViewsController().attachTextInputPlugin(this.mTextInputPlugin);
            this.sendLocalesToDart(this.getResources().getConfiguration());
            this.sendUserPlatformSettingsToDart();
        }
    }
    ...
}

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可以看到，FlutterView 繼承了 SurfaceView ，並且實現了 BinaryMessenger,TextureRegistry 兩個介面。

主要的作用：

• 建立 FlutterNativeView。
• 建立 CallBack 例項，當執行 Surface 回撥函式的時候，會通知底層。
• 建立平臺通道，並將平臺通過設定資訊傳遞給 Dart。

4.3 FlutterNativeView 初始化

public class FlutterNativeView implements BinaryMessenger {
    ...
    public FlutterNativeView(@NonNull Context context, boolean isBackgroundView) {
        this.flutterUiDisplayListener = new FlutterUiDisplayListener() {
            public void onFlutterUiDisplayed() {
                if (FlutterNativeView.this.mFlutterView != null) {
                    FlutterNativeView.this.mFlutterView.onFirstFrame();
                }
            }

            public void onFlutterUiNoLongerDisplayed() {
            }
        };
        this.mContext = context;
        this.mPluginRegistry = new FlutterPluginRegistry(this, context);
        this.mFlutterJNI = new FlutterJNI();
        this.mFlutterJNI.addIsDisplayingFlutterUiListener(this.flutterUiDisplayListener);
        this.dartExecutor = new DartExecutor(this.mFlutterJNI, context.getAssets());
        this.mFlutterJNI.addEngineLifecycleListener(new FlutterNativeView.EngineLifecycleListenerImpl());
        this.attach(this, isBackgroundView);
        this.assertAttached();
    }
    ...
}
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• 初始化物件FlutterPluginRegistry；
• 初始化物件FlutterJNI；
• 初始化物件RenderSurfaceImpl，並賦值給mFlutterJNI的成員變數renderSurface；
• 初始化物件DartExecutor；
• 設定引擎生命週期回撥監聽器；
• 並執行attach方法

    private void attach(FlutterNativeView view, boolean isBackgroundView) {
        this.mFlutterJNI.attachToNative(isBackgroundView);
        this.dartExecutor.onAttachedToJNI();
    }
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  @UiThread
    public void attachToNative(boolean isBackgroundView) {
        this.ensureRunningOnMainThread();
        this.ensureNotAttachedToNative();
        this.nativePlatformViewId = this.nativeAttach(this, isBackgroundView);
    }
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上面兩個函式執行之後，將執行 jni 層方法，進而將執行底層程式碼，進行 Flutter 引擎的初始化和啟動操作。

當 Flutter引擎和 Dart Vm 等初始化完成之後，程式碼將會執行到 FlutterActivityDelegate 中的 runBundle 方法。

4.4 runBundle

    private void runBundle(String appBundlePath) {
        if (!this.flutterView.getFlutterNativeView().isApplicationRunning()) {
            FlutterRunArguments args = new FlutterRunArguments();
            args.bundlePath = appBundlePath;
            args.entrypoint = "main";
            this.flutterView.runFromBundle(args);
        }

    }
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主要是建立 FlutterRunArguments 物件，指定入口函式名稱為 "main",接著執行 FlutterView 中的 runFromBundle 方法。

    public void runFromBundle(FlutterRunArguments args) {
        this.assertAttached();
        this.preRun();
        this.mNativeView.runFromBundle(args);
        this.postRun();
    }
    
        public void runFromBundle(FlutterRunArguments args) {
        if (args.entrypoint == null) {
            throw new AssertionError("An entrypoint must be specified");
        } else {
            this.assertAttached();
            if (this.applicationIsRunning) {
                throw new AssertionError("This Flutter engine instance is already running an application");
            } else {
                this.mFlutterJNI.runBundleAndSnapshotFromLibrary(args.bundlePath, args.entrypoint, args.libraryPath, this.mContext.getResources().getAssets());
                this.applicationIsRunning = true;
            }
        }
    }

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在 runFromBundle 中，主要是進一步呼叫了 jni 層的方法，呼叫的時候，指定了函式名稱，庫路徑，資原始檔等。

  @UiThread
    public void runBundleAndSnapshotFromLibrary(@NonNull String bundlePath, @Nullable String entrypointFunctionName, @Nullable String pathToEntrypointFunction, @NonNull AssetManager assetManager) {
        this.ensureRunningOnMainThread();
        this.ensureAttachedToNative();
        this.nativeRunBundleAndSnapshotFromLibrary(this.nativePlatformViewId, bundlePath, entrypointFunctionName, pathToEntrypointFunction, assetManager);
    }
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接著將呼叫底層方法：

    private native void nativeRunBundleAndSnapshotFromLibrary(long var1, @NonNull String var3, @Nullable String var4, @Nullable String var5, @NonNull AssetManager var6);
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底層方法經過一系列方法的呼叫，將會執行到 Dart 中的 main 函式，也就是將會執行到我們寫的 Dart 程式碼。

整個過程如下圖所示。

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