介绍
Flutter应用是由平台来创建、初始化并启动的，这里我们以android为例，对启动过程做一个走马观花式的了解,旨在对平台端的工作有个大致了解。
Android端的启动流程
启动流程实际上还涉及了很多native 层的工作，但是宥于篇幅，暂且只看Android端。FlutterApplication
flutter应用下，原生的启动流程并没有什么变化，我们来看Application的onCreate函数。
@Override
@CallSuper
public void onCreate() {
super.onCreate();
FlutterMain.startInitialization(this);
}很简单，继续往里走
public static void startInitialization(@NonNull Context applicationContext) {
if (isRunningInRobolectricTest) {
return;
}
FlutterLoader.getInstance().startInitialization(applicationContext);
}按上面方法的注释来看，是初始化 native system（即C++）的，最终会调用下面的方法：
我将说明以注释的形式写在下面public void startInitialization(@NonNull Context applicationContext, @NonNull Settings settings) {
if (this.settings != null) {
return;
}
///确保运行在 主线程
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
///等参数
initConfig(appContext);
///初始化VsyncWaiter，并设置回调
/// 当vsync信号到来时，就调用我们设置的回调，最终会触发页面的刷新
VsyncWaiter.getInstance((WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE))
.init();
// 子线程
///这里主要是抽取资源文件，
///加载 flutter（框架）代码
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 的相关流程就走完了。
另外，虽然上面的代码中使用了子线程，但是最终在这些任务没有完成前，是不会进入flutter侧的，我们接着走FlutterActivity。
FlutterActivity & onCreate
开始的地方依然是 onCreate()方法：
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
///切换主题
switchLaunchThemeForNormalTheme();
super.onCreate(savedInstanceState);
///通知生命周期
lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
///初始化delete，这个很重要，
///所有的工作都是由它来完成的
delegate = new FlutterActivityAndFragmentDelegate(this);
delegate.onAttach(this);
///是否需要恢复（包括通知插件）一些状态
delegate.onActivityCreated(savedInstanceState);
///配置窗口
configureWindowForTransparency();
///创建flutterView
setContentView(createFlutterView());
configureStatusBarForFullscreenFlutterExperience();
}这里面比较重的代码是这几行:
delegate = new FlutterActivityAndFragmentDelegate(this);
delegate.onAttach(this);
...
setContentView(createFlutterView());我们一步一步来,首先创建了FlutterActivityAndFragmentDelegate 并调用了它的attact(this)方法。
FlutterActivityAndFragmentDelegate
void onAttach(@NonNull Context context) {
ensureAlive();
///初始化engine
if (flutterEngine == null) {
///这里面会对已有的engine进行复用
setupFlutterEngine();
}
///初始化平台插件
///本质上，是将engine的 channel回调与平台的系统服务进行绑定
///如：震动、复制粘贴、声音播放等...
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
/// 并通知相关插件
/// PlatformViewsController 这个类你应该很熟悉（如果你接入过原生view的话）
flutterEngine
.getActivityControlSurface()
.attachToActivity(host.getActivity(), host.getLifecycle());
}
///注册插件
///通过反射调用 “io.flutter.plugins.GeneratedPluginRegistrant”
///的 “registerWith”方法，这个过程走完了，你的插件基本就能用了
host.configureFlutterEngine(flutterEngine);
}通过上面，我们大致了解了，在flutter端使用的平台功能是什么时候装配的了。
我们回到FlutterActivity，继续重要的第二步:
setContentView(createFlutterView());
@NonNull
private View createFlutterView() {
return delegate.onCreateView(
null /* inflater */, null /* container */, null /* savedInstanceState */);
}这里插一句，可以看一下这篇文章：Flutter&Android 启动页（闪屏页）的加载流程和优化方案
最终会调用 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应用是 RenderMode.surface，所以会进入到这里
///创建FlutterSurfaceView
FlutterSurfaceView flutterSurfaceView =
new FlutterSurfaceView(
host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);
// Allow our host to customize FlutterSurfaceView, if desired.
host.onFlutterSurfaceViewCreated(flutterSurfaceView);
// flutterView 创建完成后，便会调用addView
//将 flutterSurfaceView 显示出来，只不过啥都没有而已
flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
} else {
...省略代码...
}
// 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);
}
///这里显示闪屏页 默认是个白屏
///即，AndroidMainfest.xml 的<metadata>所设置
flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());
///将flutterview 绑定到 engine上
flutterView.attachToFlutterEngine(flutterEngine);
return flutterSplashView;
}flutterView 内部持有flutterSurfaceView （一个Surface），并最终通过attachToFlutterEngine绑定到engine上，我们来看一下其内部实现：
public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {
...省略部分代码...
this.flutterEngine = flutterEngine;
///通过engine的 getRenderer，
///可以将flutter的纹理绘制到android 上。
FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
renderSurface.attachToRenderer(flutterRenderer);
flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);
...省略部分代码...
///输入插件
textInputPlugin =
new TextInputPlugin(
this,
this.flutterEngine.getTextInputChannel(),
this.flutterEngine.getPlatformViewsController());
///国际化插件
localizationPlugin = this.flutterEngine.getLocalizationPlugin();
///与上面的textInputPlugin相关联
androidKeyProcessor =
new AndroidKeyProcessor(this.flutterEngine.getKeyEventChannel(), textInputPlugin);
/// 触摸事件的初始化
/// 相关触摸数据会发送到flutter端
androidTouchProcessor = new AndroidTouchProcessor(this.flutterEngine.getRenderer());
///辅助功能
accessibilityBridge =
new AccessibilityBridge(
this,
flutterEngine.getAccessibilityChannel(),
(AccessibilityManager) getContext().getSystemService(Context.ACCESSIBILITY_SERVICE),
getContext().getContentResolver(),
this.flutterEngine.getPlatformViewsController());
...省略部分代码...
///通过上面的初始化，将用户相关的设置发送到flutter端
sendUserSettingsToFlutter();
localizationPlugin.sendLocalesToFlutter(getResources().getConfiguration());
sendViewportMetricsToFlutter();
///将当前flutter view 绑定到 PlatformViewsController
flutterEngine.getPlatformViewsController().attachToView(this);
...省略部分代码...
}相关初始化工作完成，activity的生命周期也从onCreate来到了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 最终会调用doInitialFlutterViewRun（）方法：
private void doInitialFlutterViewRun() {
...省略部分代码...
// 这里就是获取我们打包所得的 libapp.so路径
// 即，我们所写的dart代码，并执行它
DartExecutor.DartEntrypoint entrypoint =
new DartExecutor.DartEntrypoint(
host.getAppBundlePath(), host.getDartEntrypointFunctionName());
flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
}至此整个android端的启动流程就走完了，这里再回顾总结一下。
总结
在flutterApplication中：
初始化一些资源路径，配置相关参数
抽取资源并加载(assets)
加载flutter.so关键库
- 最终走JNI_OnLoad 进入native进行相关工作
- 如绑定flutter jni
初始化vsyncWaiter在flutterActivity中：
会初始化重要类FlutterActivityAndFragmentDelegate
activity端的生命周期，也会触发delegate来对应回调
对平台的系统功能（震动、剪贴板）进行绑定
初始化platformViewController以及系统channel
创建flutterView（内部持有一个surfaceView）
- 会最终进入native进行engine的初始化工作
在onStart生命周期中加载咱们的dart代码，开始执行在这整个过程中，会穿插进行native层的工作，并最终通过native层的调用，转到flutter端的main()函数，由于这里的内容很多，将会在后面的文章中介绍。
最后，谢谢大家的阅读，如果有不对的地方，还请指出。
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