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flutter：跨平台（Android/iOS），图形引擎，Dart语言 这些都有什么关系呢
我们先看下在安卓平台下的初始化：
入口：manifest
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.flutter_app">
<!-- io.flutter.app.FlutterApplication is an android.app.Application that
calls FlutterMain.startInitialization(this); in its onCreate method.
In most cases you can leave this as-is, but you if you want to provide
additional functionality it is fine to subclass or reimplement
FlutterApplication and put your custom class here. -->
<application
android:name="io.flutter.app.FlutterApplication"
android:label="flutter_app"
android:icon="@mipmap/ic_launcher">
<activity
android:name=".MainActivity"
android:launchMode="singleTop"
android:theme="@style/LaunchTheme"
android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
android:hardwareAccelerated="true"
android:windowSoftInputMode="adjustResize">
<!-- Specifies an Android theme to apply to this Activity as soon as
the Android process has started. This theme is visible to the user
while the Flutter UI initializes. After that, this theme continues
to determine the Window background behind the Flutter UI. -->
<meta-data
android:name="io.flutter.embedding.android.NormalTheme"
android:resource="@style/NormalTheme"
/>
<!-- Displays an Android View that continues showing the launch screen
Drawable until Flutter paints its first frame, then this splash
screen fades out. A splash screen is useful to avoid any visual
gap between the end of Android's launch screen and the painting of
Flutter's first frame. -->
<meta-data
android:name="io.flutter.embedding.android.SplashScreenDrawable"
android:resource="@drawable/launch_background"
/>
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<!-- Don't delete the meta-data below.
This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
<meta-data
android:name="flutterEmbedding"
android:value="2" />
</application>
</manifest>
我们看到没有 Application，只有一个 MainActivity，还有一堆 meta-data
接下来看下 MainActivity 做了什么
class MainActivity: FlutterActivity() {}
发现只是继承了 io.flutter 包下的FlutterActivity
public class FlutterActivity extends Activity implements Host, LifecycleOwner {
继承了 android.app.Activity 实现了LifecycleOwner 通过 LifecycleRegistry(this); 实现了一套 Lifecycler ，还有个 Host 接口 来扩展 Flutter 相关的功能，那么Flutter 引擎如何初始化的呢？发现 FlutterActivity 的 onCreate 执行了一些相关逻辑
protected void onCreate(@Nullable Bundle savedInstanceState) {
this.switchLaunchThemeForNormalTheme();
super.onCreate(savedInstanceState);
this.lifecycle.handleLifecycleEvent(Event.ON_CREATE);
this.delegate = new FlutterActivityAndFragmentDelegate(this);
this.delegate.onAttach(this);
this.delegate.onActivityCreated(savedInstanceState);
this.configureWindowForTransparency();
this.setContentView(this.createFlutterView());
this.configureStatusBarForFullscreenFlutterExperience();
}
1: 创建了 FlutterActivityAndFragmentDelegate 并 onAttach
2: 创建 FlutterView 通过 setContentView 添加到 Window
先看下第一步
void onAttach(@NonNull Context context) {
this.ensureAlive();
if (this.flutterEngine == null) {
this.setupFlutterEngine();
}
this.platformPlugin = this.host.providePlatformPlugin(this.host.getActivity(), this.flutterEngine);
if (this.host.shouldAttachEngineToActivity()) {
Log.v("FlutterActivityAndFragmentDelegate", "Attaching FlutterEngine to the Activity that owns this Fragment.");
this.flutterEngine.getActivityControlSurface().attachToActivity(this.host.getActivity(), this.host.getLifecycle());
}
this.host.configureFlutterEngine(this.flutterEngine);
}
void setupFlutterEngine() {
Log.v("FlutterActivityAndFragmentDelegate", "Setting up FlutterEngine.");
String cachedEngineId = this.host.getCachedEngineId();
if (cachedEngineId != null) {
this.flutterEngine = FlutterEngineCache.getInstance().get(cachedEngineId);
this.isFlutterEngineFromHost = true;
if (this.flutterEngine == null) {
throw new IllegalStateException("The requested cached FlutterEngine did not exist in the FlutterEngineCache: '" + cachedEngineId + "'");
}
} else {
this.flutterEngine = this.host.provideFlutterEngine(this.host.getContext());
if (this.flutterEngine != null) {
this.isFlutterEngineFromHost = true;
} else {
Log.v("FlutterActivityAndFragmentDelegate", "No preferred FlutterEngine was provided. Creating a new FlutterEngine for this FlutterFragment.");
this.flutterEngine = new FlutterEngine(this.host.getContext(), this.host.getFlutterShellArgs().toArray(), false, this.host.shouldRestoreAndSaveState());
this.isFlutterEngineFromHost = false;
}
}
}
setupFlutterEngin 方法去创建了 FlutterEngine 对象，这个对象是做什么到呢，我们看它到构造
public FlutterEngine(@NonNull Context context, @NonNull FlutterLoader flutterLoader, @NonNull FlutterJNI flutterJNI, @NonNull PlatformViewsController platformViewsController, @Nullable String[] dartVmArgs, boolean automaticallyRegisterPlugins, boolean waitForRestorationData) {
this.engineLifecycleListeners = new HashSet();
this.engineLifecycleListener = new FlutterEngine.EngineLifecycleListener() {
public void onPreEngineRestart() {
Log.v("FlutterEngine", "onPreEngineRestart()");
Iterator var1 = FlutterEngine.this.engineLifecycleListeners.iterator();
while(var1.hasNext()) {
FlutterEngine.EngineLifecycleListener lifecycleListener = (FlutterEngine.EngineLifecycleListener)var1.next();
lifecycleListener.onPreEngineRestart();
}
FlutterEngine.this.platformViewsController.onPreEngineRestart();
FlutterEngine.this.restorationChannel.clearData();
}
};
this.dartExecutor = new DartExecutor(flutterJNI, context.getAssets());
this.dartExecutor.onAttachedToJNI();
this.accessibilityChannel = new AccessibilityChannel(this.dartExecutor, flutterJNI);
this.keyEventChannel = new KeyEventChannel(this.dartExecutor);
this.lifecycleChannel = new LifecycleChannel(this.dartExecutor);
this.localizationChannel = new LocalizationChannel(this.dartExecutor);
this.mouseCursorChannel = new MouseCursorChannel(this.dartExecutor);
this.navigationChannel = new NavigationChannel(this.dartExecutor);
this.platformChannel = new PlatformChannel(this.dartExecutor);
this.restorationChannel = new RestorationChannel(this.dartExecutor, waitForRestorationData);
this.settingsChannel = new SettingsChannel(this.dartExecutor);
this.systemChannel = new SystemChannel(this.dartExecutor);
this.textInputChannel = new TextInputChannel(this.dartExecutor);
this.localizationPlugin = new LocalizationPlugin(context, this.localizationChannel);
this.flutterJNI = flutterJNI;
flutterLoader.startInitialization(context.getApplicationContext());
flutterLoader.ensureInitializationComplete(context, dartVmArgs);
flutterJNI.addEngineLifecycleListener(this.engineLifecycleListener);
flutterJNI.setPlatformViewsController(platformViewsController);
flutterJNI.setLocalizationPlugin(this.localizationPlugin);
this.attachToJni();
this.renderer = new FlutterRenderer(flutterJNI);
this.platformViewsController = platformViewsController;
this.platformViewsController.onAttachedToJNI();
this.pluginRegistry = new FlutterEnginePluginRegistry(context.getApplicationContext(), this, flutterLoader);
if (automaticallyRegisterPlugins) {
this.registerPlugins();
}
}
监听了引擎到变化，初始化了一些系统基础 Channel（Dart通道） ， FlutterLoader 对象 start ，flutterJNI 对象初始化，FlutterRender及FlutterView初始化，registerPlugins(插件初始化)
先看下 FlutterLoader 的 startInitialization 方法吧 ensureInitializationComplete，这个是整个 Flutter引擎 初始化的基础：
startInitialization 方法
public void startInitialization(@NonNull Context applicationContext, @NonNull FlutterLoader.Settings settings) {
if (this.settings == null) {
if (Looper.myLooper() != Looper.getMainLooper()) {
throw new IllegalStateException("startInitialization must be called on the main thread");
} else {
final Context appContext = applicationContext.getApplicationContext();
this.settings = settings;
this.initStartTimestampMillis = SystemClock.uptimeMillis();
this.initConfig(appContext);
VsyncWaiter.getInstance((WindowManager)appContext.getSystemService("window")).init();
Callable<FlutterLoader.InitResult> initTask = new Callable<FlutterLoader.InitResult>() {
public FlutterLoader.InitResult call() {
ResourceExtractor resourceExtractor = FlutterLoader.this.initResources(appContext);
System.loadLibrary("flutter");
Executors.newSingleThreadExecutor().execute(new Runnable() {
public void run() {
FlutterJNI.nativePrefetchDefaultFontManager();
}
});
if (resourceExtractor != null) {
resourceExtractor.waitForCompletion();
}
return new FlutterLoader.InitResult(PathUtils.getFilesDir(appContext), PathUtils.getCacheDirectory(appContext), PathUtils.getDataDirectory(appContext));
}
};
this.initResultFuture = Executors.newSingleThreadExecutor().submit(initTask);
}
}
}
初始化了 flutter 的 c 库，并将 安卓系统的 vsync 同步到 flutter
public void init() {
FlutterJNI.setAsyncWaitForVsyncDelegate(this.asyncWaitForVsyncDelegate);
float fps = this.windowManager.getDefaultDisplay().getRefreshRate();
FlutterJNI.setRefreshRateFPS(fps);
}
private final AsyncWaitForVsyncDelegate asyncWaitForVsyncDelegate = new AsyncWaitForVsyncDelegate() {
public void asyncWaitForVsync(final long cookie) {
Choreographer.getInstance().postFrameCallback(new FrameCallback() {
public void doFrame(long frameTimeNanos) {
float fps = VsyncWaiter.this.windowManager.getDefaultDisplay().getRefreshRate();
long refreshPeriodNanos = (long)(1.0E9D / (double)fps);
FlutterJNI.nativeOnVsync(frameTimeNanos, frameTimeNanos + refreshPeriodNanos, cookie);
}
});
}
};
ensureInitializationComplete方法
public void ensureInitializationComplete(@NonNull Context applicationContext, @Nullable String[] args) {
if (!this.initialized) {
if (Looper.myLooper() != Looper.getMainLooper()) {
throw new IllegalStateException("ensureInitializationComplete must be called on the main thread");
} else if (this.settings == null) {
throw new IllegalStateException("ensureInitializationComplete must be called after startInitialization");
} else {
try {
FlutterLoader.InitResult result = (FlutterLoader.InitResult)this.initResultFuture.get();
List<String> shellArgs = new ArrayList();
shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");
ApplicationInfo applicationInfo = this.getApplicationInfo(applicationContext);
shellArgs.add("--icu-native-lib-path=" + applicationInfo.nativeLibraryDir + File.separator + "libflutter.so");
if (args != null) {
Collections.addAll(shellArgs, args);
}
String kernelPath = null;
String snapshotAssetPath = result.dataDirPath + File.separator + this.flutterAssetsDir;
kernelPath = snapshotAssetPath + File.separator + "kernel_blob.bin";
shellArgs.add("--snapshot-asset-path=" + snapshotAssetPath);
shellArgs.add("--vm-snapshot-data=" + this.vmSnapshotData);
shellArgs.add("--isolate-snapshot-data=" + this.isolateSnapshotData);
shellArgs.add("--cache-dir-path=" + result.engineCachesPath);
if (this.settings.getLogTag() != null) {
shellArgs.add("--log-tag=" + this.settings.getLogTag());
}
long initTimeMillis = SystemClock.uptimeMillis() - this.initStartTimestampMillis;
Bundle bundle = applicationInfo.metaData;
if (bundle != null) {
boolean use_embedded_view = bundle.getBoolean("io.flutter.embedded_views_preview");
if (use_embedded_view) {
shellArgs.add("--use-embedded-view");
}
}
FlutterJNI.nativeInit(applicationContext, (String[])shellArgs.toArray(new String[0]), kernelPath, result.appStoragePath, result.engineCachesPath, initTimeMillis);
this.initialized = true;
} catch (Exception var11) {
Log.e("FlutterLoader", "Flutter initialization failed.", var11);
throw new RuntimeException(var11);
}
}
}
}
加载 assets 下的资源， vm_snapshot_data 和 isolate_snapshot_data 并调用Nactive 方法 FlutterJNI.nativeInit 交给 Flutter 引擎
再回到 FlutterActivityAndFragmentDelegate 我们看下在 setContentView FlutterView 的创建：
View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
Log.v("FlutterActivityAndFragmentDelegate", "Creating FlutterView.");
this.ensureAlive();
if (this.host.getRenderMode() == RenderMode.surface) {
FlutterSurfaceView flutterSurfaceView = new FlutterSurfaceView(this.host.getActivity(), this.host.getTransparencyMode() == TransparencyMode.transparent);
this.host.onFlutterSurfaceViewCreated(flutterSurfaceView);
this.flutterView = new FlutterView(this.host.getActivity(), flutterSurfaceView);
} else {
FlutterTextureView flutterTextureView = new FlutterTextureView(this.host.getActivity());
this.host.onFlutterTextureViewCreated(flutterTextureView);
this.flutterView = new FlutterView(this.host.getActivity(), flutterTextureView);
}
this.flutterView.addOnFirstFrameRenderedListener(this.flutterUiDisplayListener);
this.flutterSplashView = new FlutterSplashView(this.host.getContext());
if (VERSION.SDK_INT >= 17) {
this.flutterSplashView.setId(View.generateViewId());
} else {
this.flutterSplashView.setId(486947586);
}
this.flutterSplashView.displayFlutterViewWithSplash(this.flutterView, this.host.provideSplashScreen());
Log.v("FlutterActivityAndFragmentDelegate", "Attaching FlutterEngine to FlutterView.");
this.flutterView.attachToFlutterEngine(this.flutterEngine);
return this.flutterSplashView;
}
我们看到 contentView 是 FlutterSplashView 并调用了 displayFlutterViewWithSplash 方法
final class FlutterSplashView extends FrameLayout {
public FlutterSplashView(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
this.flutterEngineAttachmentListener = new FlutterEngineAttachmentListener() {
public void onFlutterEngineAttachedToFlutterView(@NonNull FlutterEngine engine) {
FlutterSplashView.this.flutterView.removeFlutterEngineAttachmentListener(this);
FlutterSplashView.this.displayFlutterViewWithSplash(FlutterSplashView.this.flutterView, FlutterSplashView.this.splashScreen);
}
public void onFlutterEngineDetachedFromFlutterView() {
}
};
......
public void displayFlutterViewWithSplash(@NonNull FlutterView flutterView, @Nullable SplashScreen splashScreen) {
if (this.flutterView != null) {
this.flutterView.removeOnFirstFrameRenderedListener(this.flutterUiDisplayListener);
this.removeView(this.flutterView);
}
if (this.splashScreenView != null) {
this.removeView(this.splashScreenView);
}
this.flutterView = flutterView;
this.addView(flutterView);
this.splashScreen = splashScreen;
if (splashScreen != null) {
if (this.isSplashScreenNeededNow()) {
Log.v(TAG, "Showing splash screen UI.");
this.splashScreenView = splashScreen.createSplashView(this.getContext(), this.splashScreenState);
this.addView(this.splashScreenView);
flutterView.addOnFirstFrameRenderedListener(this.flutterUiDisplayListener);
} else if (this.isSplashScreenTransitionNeededNow()) {
Log.v(TAG, "Showing an immediate splash transition to Flutter due to previously interrupted transition.");
this.splashScreenView = splashScreen.createSplashView(this.getContext(), this.splashScreenState);
this.addView(this.splashScreenView);
this.transitionToFlutter();
} else if (!flutterView.isAttachedToFlutterEngine()) {
Log.v(TAG, "FlutterView is not yet attached to a FlutterEngine. Showing nothing until a FlutterEngine is attached.");
flutterView.addFlutterEngineAttachmentListener(this.flutterEngineAttachmentListener);
}
}
}
将 flutterView 和 SplashScreen（一个metadata 生成的 DrawableView）先后添加到 frameLayout 里，FlutterView 在 attachToFlutterEngine 时候回调将 SplashScreen 这个 View transitionToFlutter ,也就是 Alpha 设置为 0
public void transitionToFlutter(@NonNull final Runnable onTransitionComplete) {
if (this.splashView == null) {
onTransitionComplete.run();
} else {
this.splashView.animate().alpha(0.0F).setDuration(this.crossfadeDurationInMillis).setListener(new AnimatorListener() {
public void onAnimationStart(Animator animation) {
}
public void onAnimationEnd(Animator animation) {
onTransitionComplete.run();
}
public void onAnimationCancel(Animator animation) {
onTransitionComplete.run();
}
public void onAnimationRepeat(Animator animation) {
}
});
}
}
看了上面到源码感觉 flutter 引擎很像 WebView：
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