Flutter的启动包括Embedder、Engine、Framework三部分，本文仅描述Android平台的Embdder模块的启动流程。Flutter一般经过启动一个FlutterActivity启动，纯Flutter App则经过FlutterApplication启动。本文之后一种状况为例分析。java
// io/flutter/app/FlutterApplication.java
@Override
@CallSuper
public void onCreate() {
super.onCreate();
FlutterInjector.instance().flutterLoader().startInitialization(this);
}
实际是调用FlutterLoader#startInitialization进行初始化。android
public void startInitialization(@NonNull Context applicationContext) {
startInitialization(applicationContext, new Settings());
}
public void startInitialization(@NonNull Context applicationContext, @NonNull Settings settings) {
if (this.settings != null) {
return;
}
if (Looper.myLooper() != Looper.getMainLooper()) {
throw new IllegalStateException("startInitialization must be called on the main thread");
}
final Context appContext = applicationContext.getApplicationContext();
this.settings = settings;
initStartTimestampMillis = SystemClock.uptimeMillis();
flutterApplicationInfo = ApplicationInfoLoader.load(appContext);
VsyncWaiter.getInstance((WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE))
.init();
// Use a background thread for initialization tasks that require disk access.
Callable<InitResult> initTask =
new Callable<InitResult>() {
@Override
public InitResult call() {
ResourceExtractor resourceExtractor = initResources(appContext);
if (FlutterInjector.instance().shouldLoadNative()) {
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);
}
主要作了如下几件事：shell
检查是否在主线程app
注册Vsync信号监听dom
启动一个异步任务，执行如下操做：异步
加载flutter engineide
异步初始化字体管理器oop
加载asset目录下的资源字体
以上是Application会执行的操做，接着通常会进入一个FlutterActivity，那么依次查看其回调。fetch
// io/flutter/embedding/android/FlutterActivity.java
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
switchLaunchThemeForNormalTheme();
super.onCreate(savedInstanceState);
lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
delegate = new FlutterActivityAndFragmentDelegate(this);
delegate.onAttach(this);
delegate.onActivityCreated(savedInstanceState);
configureWindowForTransparency();
setContentView(createFlutterView());
configureStatusBarForFullscreenFlutterExperience();
}
主要作了如下几件事：
从Splash切换成真正要显示的View
建立一个delegate，并触发对应回调
配置相关的窗口属性
建立一个新的FlutterView，做为Activity的ContentView
void onAttach(@NonNull Context context) {
ensureAlive();
if (flutterEngine == null) {
setupFlutterEngine();
}
if (host.shouldAttachEngineToActivity()) {
Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this delegate.");
flutterEngine.getActivityControlSurface().attachToActivity(this, host.getLifecycle());
}
platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);
host.configureFlutterEngine(flutterEngine);
}
主要作了如下几件事：
设置FlutterEngine，通常是新建一个
把Engine和容器Activity关联
新建一个PlatformPlugin
engine初始化完毕，触发对应回调
其中，FlutterActivityAndFragmentDelegate#setupFlutterEngine行为比较复杂
/* package */ void setupFlutterEngine() {
Log.v(TAG, "Setting up FlutterEngine.");
// First, check if the host wants to use a cached FlutterEngine.
String cachedEngineId = host.getCachedEngineId();
if (cachedEngineId != null) {
flutterEngine = FlutterEngineCache.getInstance().get(cachedEngineId);
isFlutterEngineFromHost = true;
if (flutterEngine == null) {
throw new IllegalStateException(
"The requested cached FlutterEngine did not exist in the FlutterEngineCache: '"
+ cachedEngineId
+ "'");
}
return;
}
// Second, defer to subclasses for a custom FlutterEngine.
flutterEngine = host.provideFlutterEngine(host.getContext());
if (flutterEngine != null) {
isFlutterEngineFromHost = true;
return;
}
Log.v(
TAG,
"No preferred FlutterEngine was provided. Creating a new FlutterEngine for"
+ " this FlutterFragment.");
flutterEngine =
new FlutterEngine(
host.getContext(),
host.getFlutterShellArgs().toArray(),
/*automaticallyRegisterPlugins=*/ false,
/*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
isFlutterEngineFromHost = false;
}
这里通常是第三种状况，直接新建一个Engine：
public FlutterEngine(
@NonNull Context context,
@Nullable FlutterLoader flutterLoader,
@NonNull FlutterJNI flutterJNI,
@NonNull PlatformViewsController platformViewsController,
@Nullable String[] dartVmArgs,
boolean automaticallyRegisterPlugins,
boolean waitForRestorationData) {
AssetManager assetManager;
try {
assetManager = context.createPackageContext(context.getPackageName(), 0).getAssets();
} catch (NameNotFoundException e) {
assetManager = context.getAssets();
}
this.dartExecutor = new DartExecutor(flutterJNI, assetManager);
this.dartExecutor.onAttachedToJNI();
accessibilityChannel = new AccessibilityChannel(dartExecutor, flutterJNI);
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);
this.localizationPlugin = new LocalizationPlugin(context, localizationChannel);
this.flutterJNI = flutterJNI;
if (flutterLoader == null) {
flutterLoader = FlutterInjector.instance().flutterLoader();
}
flutterLoader.startInitialization(context.getApplicationContext());
flutterLoader.ensureInitializationComplete(context, dartVmArgs);
flutterJNI.addEngineLifecycleListener(engineLifecycleListener);
flutterJNI.setPlatformViewsController(platformViewsController);
flutterJNI.setLocalizationPlugin(localizationPlugin);
attachToJni();
// TODO(mattcarroll): FlutterRenderer is temporally coupled to attach(). Remove that coupling if
// possible.
this.renderer = new FlutterRenderer(flutterJNI);
this.platformViewsController = platformViewsController;
this.platformViewsController.onAttachedToJNI();
this.pluginRegistry =
new FlutterEngineConnectionRegistry(context.getApplicationContext(), this, flutterLoader);
if (automaticallyRegisterPlugins) {
registerPlugins();
}
}
主要作了如下几件事：
新建Dart运行环境DartExecutor
建立各类用于Flutter和Android通讯的channel对象
确认engine初始化完成（调用Native侧的初始化方法，执行Flutter engine的启动逻辑）
插件注册
flutterLoader.startInitialization因为以前调用过，这里会直接返回。FlutterLoader#ensureInitializationComplete则负责engine最后的准备工做：
public void ensureInitializationComplete(
@NonNull Context applicationContext, @Nullable String[] args) {
if (initialized) {
return;
}
if (Looper.myLooper() != Looper.getMainLooper()) {
throw new IllegalStateException(
"ensureInitializationComplete must be called on the main thread");
}
if (settings == null) {
throw new IllegalStateException(
"ensureInitializationComplete must be called after startInitialization");
}
try {
InitResult result = initResultFuture.get();
List<String> shellArgs = new ArrayList<>();
shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");
shellArgs.add(
"--icu-native-lib-path="
+ flutterApplicationInfo.nativeLibraryDir
+ File.separator
+ DEFAULT_LIBRARY);
if (args != null) {
Collections.addAll(shellArgs, args);
}
String kernelPath = null;
if (BuildConfig.DEBUG || BuildConfig.JIT_RELEASE) {
String snapshotAssetPath =
result.dataDirPath + File.separator + flutterApplicationInfo.flutterAssetsDir;
kernelPath = snapshotAssetPath + File.separator + DEFAULT_KERNEL_BLOB;
shellArgs.add("--" + SNAPSHOT_ASSET_PATH_KEY + "=" + snapshotAssetPath);
shellArgs.add("--" + VM_SNAPSHOT_DATA_KEY + "=" + flutterApplicationInfo.vmSnapshotData);
shellArgs.add(
"--" + ISOLATE_SNAPSHOT_DATA_KEY + "=" + flutterApplicationInfo.isolateSnapshotData);
} else {
shellArgs.add(
"--" + AOT_SHARED_LIBRARY_NAME + "=" + flutterApplicationInfo.aotSharedLibraryName);
// Most devices can load the AOT shared library based on the library name
// with no directory path. Provide a fully qualified path to the library
// as a workaround for devices where that fails.
shellArgs.add(
"--"
+ AOT_SHARED_LIBRARY_NAME
+ "="
+ flutterApplicationInfo.nativeLibraryDir
+ File.separator
+ flutterApplicationInfo.aotSharedLibraryName);
}
shellArgs.add("--cache-dir-path=" + result.engineCachesPath);
if (!flutterApplicationInfo.clearTextPermitted) {
shellArgs.add("--disallow-insecure-connections");
}
if (flutterApplicationInfo.domainNetworkPolicy != null) {
shellArgs.add("--domain-network-policy=" + flutterApplicationInfo.domainNetworkPolicy);
}
if (settings.getLogTag() != null) {
shellArgs.add("--log-tag=" + settings.getLogTag());
}
long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;
if (FlutterInjector.instance().shouldLoadNative()) {
FlutterJNI.nativeInit(
applicationContext,
shellArgs.toArray(new String[0]),
kernelPath,
result.appStoragePath,
result.engineCachesPath,
initTimeMillis);
}
initialized = true;
} catch (Exception e) {
Log.e(TAG, "Flutter initialization failed.", e);
throw new RuntimeException(e);
}
}
主要是拼接各类参数，最终调用了一个native方法，让engine执行本身的初始化逻辑。
至此，FlutterActivityAndFragmentDelegate#onAttach的逻辑终于走完了。
void onActivityCreated(@Nullable Bundle bundle) {
Log.v(TAG, "onActivityCreated. Giving framework and plugins an opportunity to restore state.");
ensureAlive();
Bundle pluginState = null;
byte[] frameworkState = null;
if (bundle != null) {
pluginState = bundle.getBundle(PLUGINS_RESTORATION_BUNDLE_KEY);
frameworkState = bundle.getByteArray(FRAMEWORK_RESTORATION_BUNDLE_KEY);
}
if (host.shouldRestoreAndSaveState()) {
flutterEngine.getRestorationChannel().setRestorationData(frameworkState);
}
if (host.shouldAttachEngineToActivity()) {
flutterEngine.getActivityControlSurface().onRestoreInstanceState(pluginState);
}
}
状态恢复相关，通常无需关注。
@NonNull
private View createFlutterView() {
return delegate.onCreateView(
null /* inflater */, null /* container */, null /* savedInstanceState */);
}
继续：
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);
host.onFlutterSurfaceViewCreated(flutterSurfaceView);
flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
} else {
FlutterTextureView flutterTextureView = new FlutterTextureView(host.getActivity());
host.onFlutterTextureViewCreated(flutterTextureView);
flutterView = new FlutterView(host.getActivity(), flutterTextureView);
}
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;
}
这里会根据不一样mode，建立不一样类型的FlutterView，这个View承载了Flutter的UI在Android端的显示。
此外，会建立一个FlutterSplashView做为启动阶段的闪屏。
最后，FlutterView#attachToFlutterEngine会把FlutterView和FlutterEngine关联起来：
public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {
Log.v(TAG, "Attaching to a FlutterEngine: " + flutterEngine);
if (isAttachedToFlutterEngine()) {
if (flutterEngine == this.flutterEngine) {
Log.v(TAG, "Already attached to this engine. Doing nothing.");
return;
}
Log.v(
TAG,
"Currently attached to a different engine. Detaching and then attaching"
+ " to new engine.");
detachFromFlutterEngine();
}
this.flutterEngine = flutterEngine;
FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
renderSurface.attachToRenderer(flutterRenderer);
flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);
// in a way that Flutter understands.
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
mouseCursorPlugin = new MouseCursorPlugin(this, this.flutterEngine.getMouseCursorChannel());
}
textInputPlugin =
new TextInputPlugin(
this,
this.flutterEngine.getTextInputChannel(),
this.flutterEngine.getPlatformViewsController());
localizationPlugin = this.flutterEngine.getLocalizationPlugin();
androidKeyProcessor =
new AndroidKeyProcessor(this, this.flutterEngine.getKeyEventChannel(), textInputPlugin);
androidTouchProcessor =
new AndroidTouchProcessor(this.flutterEngine.getRenderer(), /*trackMotionEvents=*/ false);
accessibilityBridge =
new AccessibilityBridge(
this,
flutterEngine.getAccessibilityChannel(),
(AccessibilityManager) getContext().getSystemService(Context.ACCESSIBILITY_SERVICE),
getContext().getContentResolver(),
this.flutterEngine.getPlatformViewsController());
accessibilityBridge.setOnAccessibilityChangeListener(onAccessibilityChangeListener);
resetWillNotDraw(
accessibilityBridge.isAccessibilityEnabled(),
accessibilityBridge.isTouchExplorationEnabled());
this.flutterEngine.getPlatformViewsController().attachAccessibilityBridge(accessibilityBridge);
this.flutterEngine
.getPlatformViewsController()
.attachToFlutterRenderer(this.flutterEngine.getRenderer());
textInputPlugin.getInputMethodManager().restartInput(this);
// Push View and Context related information from Android to Flutter.
sendUserSettingsToFlutter();
localizationPlugin.sendLocalesToFlutter(getResources().getConfiguration());
sendViewportMetricsToFlutter();
flutterEngine.getPlatformViewsController().attachToView(this);
// Notify engine attachment listeners of the attachment.
for (FlutterEngineAttachmentListener listener : flutterEngineAttachmentListeners) {
listener.onFlutterEngineAttachedToFlutterView(flutterEngine);
}
if (isFlutterUiDisplayed) {
flutterUiDisplayListener.onFlutterUiDisplayed();
}
}
这里主要是关联两个对象的字段，同时对一些必要的Plugin进行初始化。
@Override
protected void onStart() {
super.onStart();
lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START);
delegate.onStart();
}
核心逻辑在delegate里面：
void onStart() {
Log.v(TAG, "onStart()");
ensureAlive();
doInitialFlutterViewRun();
}
private void doInitialFlutterViewRun() {
if (host.getCachedEngineId() != null) {
return;
}
if (flutterEngine.getDartExecutor().isExecutingDart()) {
return;
}
String initialRoute = host.getInitialRoute();
if (initialRoute == null) {
initialRoute = getInitialRouteFromIntent(host.getActivity().getIntent());
}
Log.v(
TAG,
"Executing Dart entrypoint: "
+ host.getDartEntrypointFunctionName()
+ ", and sending initial route: "
+ initialRoute);
if (initialRoute != null) {
flutterEngine.getNavigationChannel().setInitialRoute(initialRoute);
}
String appBundlePathOverride = host.getAppBundlePath();
if (appBundlePathOverride == null || appBundlePathOverride.isEmpty()) {
appBundlePathOverride = FlutterInjector.instance().flutterLoader().findAppBundlePath();
}
DartExecutor.DartEntrypoint entrypoint =
new DartExecutor.DartEntrypoint(
appBundlePathOverride, host.getDartEntrypointFunctionName());
flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
}
可见其核心行为是启动dart虚拟机。
@Override
protected void onResume() {
super.onResume();
lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
delegate.onResume();
}
一样看delegate的代码：
// io/flutter/embedding/android/FlutterActivityAndFragmentDelegate.java
void onResume() {
Log.v(TAG, "onResume()");
ensureAlive();
flutterEngine.getLifecycleChannel().appIsResumed();
}
这里向Flutter发送了一个通知，宿主Activity处于onResume状态了。
以上，就是Android Embedder层的启动流程。