flutter单引擎方案
- 2020 年 1 月 8 日
- 筆記
假设有两个模块，FlutterA，FlutterB，我们利用io.flutter.embedding.android.FlutterFragment下面的接入方式来接入flutter的话，下图，展示的是FlutterA模块，拉起一个独立的FLutterB模块，此时，会依照顺序发生下面的生命周期函数。
这里需要注意的点有：
FlutterA页面在拉起FlutterB页面之后，没有执行onDestoryView方法，也就是说View还在。- FLutterA页面拉起FlutterB之后，一直到FlutterB完全可见之后，才执行了onStop方法。
- FlutterB回退到FlutterA之后，最终走到了onDesctoryVIew
FLutterA完全拉起FlutterB之后，为什么内存中只有一个引擎了。
首先，我们知道FLutterA一定会走到其生命周期函数onStop，进而会触发FlutterActivityAndFragmentDelegate的onStop方法。
//FlutterActivityAndFragmentDelegate void onStop() { Log.v("FlutterActivityAndFragmentDelegate", "onStop()"); this.ensureAlive(); this.flutterEngine.getLifecycleChannel().appIsPaused(); this.flutterView.detachFromFlutterEngine(); }
我们注意到，这里调用了flutterView的detachFromFlutterEngine方法。
public void detachFromFlutterEngine() { Log.d("FlutterView", "Detaching from a FlutterEngine: " + this.flutterEngine); if (!this.isAttachedToFlutterEngine()) { Log.d("FlutterView", "Not attached to an engine. Doing nothing."); } else { Iterator var1 = this.flutterEngineAttachmentListeners.iterator(); while(var1.hasNext()) { FlutterView.FlutterEngineAttachmentListener listener = (FlutterView.FlutterEngineAttachmentListener)var1.next(); listener.onFlutterEngineDetachedFromFlutterView(); } this.flutterEngine.getPlatformViewsController().detachAccessibiltyBridge(); this.accessibilityBridge.release(); this.accessibilityBridge = null; this.textInputPlugin.getInputMethodManager().restartInput(this); this.textInputPlugin.destroy(); FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer(); this.didRenderFirstFrame = false; flutterRenderer.removeOnFirstFrameRenderedListener(this.onFirstFrameRenderedListener); flutterRenderer.detachFromRenderSurface(); this.flutterEngine = null; } }
我们最终看到，调用了this.flutterEngine = null;，引擎被释放了，所以，只要是这种模式，你无论开多少个Flutter模块，最后都只会有一个引擎。
FlutterB回退到FlutterA，FlutterA的状态为什么可以继续保存
我们注意到生命FlutterA会来时会执行周期函数onStart,它又会走到FlutterActivityAndFragmentDelegate的onStart。
void onStart() { Log.v("FlutterActivityAndFragmentDelegate", "onStart()"); this.ensureAlive(); (new Handler()).post(new Runnable() { public void run() { Log.v("FlutterActivityAndFragmentDelegate", "Attaching FlutterEngine to FlutterView."); FlutterActivityAndFragmentDelegate.this.flutterView.attachToFlutterEngine(FlutterActivityAndFragmentDelegate.this.flutterEngine); FlutterActivityAndFragmentDelegate.this.doInitialFlutterViewRun(); } }); }
我们注意到这里又一个attachToFlutterEngine的方法，那么，它绑定的是FlutterActivityAndFragmentDelegate.this.flutterEngine，这个引擎是我们实现的一个EngineProvider每次需要的时候new出来的。
public class TipEngineProvider { public static FlutterEngine obtain() { return new FlutterEngine(IGameApplication.getIGameApplicationContext()); } }
那既然是new出来的一个引擎，我们不禁要问了，为什么回退回来，FlutterA的页面状态还可以保存，比如，列表滑动到一定的位置，打开FlutterB，在回退回来，FlutterA还保存在列表滑动到的最后位置，没变化。
带着这个问题，我们只能去看看，onStart中绑定引擎，及初始化引起的过程中，做了一些什么？
首先看看Flutter引擎的初始化
public FlutterEngine(@NonNull Context context) { this.flutterJNI.addEngineLifecycleListener(this.engineLifecycleListener); this.attachToJni(); this.dartExecutor = new DartExecutor(this.flutterJNI, context.getAssets()); this.dartExecutor.onAttachedToJNI(); this.renderer = new FlutterRenderer(this.flutterJNI); }
以上代码有省略，只保留至关重要的部分。这里我们看到，在FlutterEngine的初始化中，flutterJNI绑定了Native，就在此时拿到了nativePlatformViewId，这里我们先埋下一个伏笔，继续走，然后，初始化了DartExecutor，DartExecutor绑定到了flutterJNI，然后初始化了FlutterRenderer。好，这里先看到这一步，总结一下，FlutterEngine融合了很多关键的成员。
在FlutterSurfaceView中，这个类继承至SurfaceView，实现了RenderSurface接口，在通过调用FlutterRenderer的surfaceCreated方法，最终会调用到flutterJNI的surfaceCreated，最终通过调用nativeSurfaceCreated方法和nativePlatformViewId产生了关联，以下是源码部分。
//FlutterSurfaceView 中的connectSurfaceToRenderer方法 private void connectSurfaceToRenderer() { if (this.flutterRenderer != null && this.getHolder() != null) { this.flutterRenderer.surfaceCreated(this.getHolder().getSurface()); } else { throw new IllegalStateException("connectSurfaceToRenderer() should only be called when flutterRenderer and getHolder() are non-null."); } }
//FlutterRenderer 中的surfaceCreated方法 public void surfaceCreated(@NonNull Surface surface) { this.flutterJNI.onSurfaceCreated(surface); }
//FlutterJNI中的onSurfaceCreated方法 @UiThread public void onSurfaceCreated(@NonNull Surface surface) { this.ensureRunningOnMainThread(); this.ensureAttachedToNative(); this.nativeSurfaceCreated(this.nativePlatformViewId, surface); }
换句话说，最终flutter底层通过nativePlatformViewId将数据绘制到了原生的surface上。
我们注意到，connectSurfaceToRenderer方法有两次调用的时机，第一次是在surface初次create的时候，还有一次是主动去掉，看代码
private FlutterSurfaceView(@NonNull Context context, @Nullable AttributeSet attrs, boolean renderTransparently) { super(context, attrs); this.isSurfaceAvailableForRendering = false; this.isAttachedToFlutterRenderer = false; this.onFirstFrameRenderedListeners = new HashSet(); this.surfaceCallback = new Callback() { public void surfaceCreated(@NonNull SurfaceHolder holder) { Log.v("FlutterSurfaceView", "SurfaceHolder.Callback.surfaceCreated()"); FlutterSurfaceView.this.isSurfaceAvailableForRendering = true; if (FlutterSurfaceView.this.isAttachedToFlutterRenderer) { FlutterSurfaceView.this.connectSurfaceToRenderer(); } }
public void attachToRenderer(@NonNull FlutterRenderer flutterRenderer) { Log.v("FlutterSurfaceView", "Attaching to FlutterRenderer."); if (this.flutterRenderer != null) { Log.v("FlutterSurfaceView", "Already connected to a FlutterRenderer. Detaching from old one and attaching to new one."); this.flutterRenderer.detachFromRenderSurface(); } this.flutterRenderer = flutterRenderer; this.isAttachedToFlutterRenderer = true; if (this.isSurfaceAvailableForRendering) { Log.v("FlutterSurfaceView", "Surface is available for rendering. Connecting FlutterRenderer to Android surface."); this.connectSurfaceToRenderer(); } }
很显然，从FlutterB回到FlutterA的时候，因为FlutterA的页面还在，所以，surface还在，所以，只可能调用attachToRenderer，才能够将nativePlatformViewId与surface进行关联。那么，attachToRenderer是谁在什么时候调用的呢？
答案就是FlutterView的attachToFlutterEngine方法，它就是在什么周期函数onStart时调用的，那么，问题来了，surface没有变化，时决定FlutterA回来不变的唯一原因吗？别忘记了，我们还有一个nativePlatformViewId呢。
当我们重新给一个FlutterEngine给FlutterView绑定时，发生了很多的过程，我们把焦点放到nativePlatformViewId时如何重新拿到的。上面我们已经知道是在FlutterEngine的初始化中，flutterJNI绑定了Native，就在此时拿到了nativePlatformViewId。
@UiThread public void attachToNative(boolean isBackgroundView) { this.ensureRunningOnMainThread(); this.ensureNotAttachedToNative(); this.nativePlatformViewId = this.nativeAttach(this, isBackgroundView); }
这是一个native的方法
// Called By Java static jlong AttachJNI(JNIEnv* env, jclass clazz, jobject flutterJNI, jboolean is_background_view) { fml::jni::JavaObjectWeakGlobalRef java_object(env, flutterJNI); auto shell_holder = std::make_unique<AndroidShellHolder>( FlutterMain::Get().GetSettings(), java_object, is_background_view); if (shell_holder->IsValid()) { return reinterpret_cast<jlong>(shell_holder.release()); } else { return 0; } }
根据我打的日志来观察，nativePlatformViewId在FlutterB返回FlutterA之后，此时拿到的这个nativePlatformViewId与之前的那个是一个long。因此，现在的情况是。
nativePlatformViewId和surface都没有变化，还是然来的那个，自然而然FlutterA的页面渲染出来的内容没有发生变化。
至此，我们知道FlutterEngine，其实本身没有保存数据，它只是管理着原生与flutter通讯的众多成员，起到了一个中心控制者的作用，一旦它不再了，surface与nativePlatformViewId就断开了，surface将不会在刷新，一旦它被安装到了FlutterView上，surface与nativePlatformViewId重新建立起了关联，surface又可以刷新了。
