flutter單引擎方案
- 2020 年 1 月 8 日
- 筆記
假設有兩個模組，FlutterA，FlutterB，我們利用io.flutter.embedding.android.FlutterFragment下面的接入方式來接入flutter的話，下圖，展示的是FlutterA模組，拉起一個獨立的FLutterB模組，此時，會依照順序發生下面的生命周期函數。
這裡需要注意的點有：
FlutterA頁面在拉起FlutterB頁面之後，沒有執行onDestoryView方法，也就是說View還在。- FLutterA頁面拉起FlutterB之後，一直到FlutterB完全可見之後，才執行了onStop方法。
- FlutterB回退到FlutterA之後，最終走到了onDesctoryVIew
FLutterA完全拉起FlutterB之後，為什麼記憶體中只有一個引擎了。
首先，我們知道FLutterA一定會走到其生命周期函數onStop，進而會觸發FlutterActivityAndFragmentDelegate的onStop方法。
//FlutterActivityAndFragmentDelegate void onStop() { Log.v("FlutterActivityAndFragmentDelegate", "onStop()"); this.ensureAlive(); this.flutterEngine.getLifecycleChannel().appIsPaused(); this.flutterView.detachFromFlutterEngine(); }
我們注意到，這裡調用了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; } }
我們最終看到，調用了this.flutterEngine = null;，引擎被釋放了，所以，只要是這種模式，你無論開多少個Flutter模組，最後都只會有一個引擎。
FlutterB回退到FlutterA，FlutterA的狀態為什麼可以繼續保存
我們注意到生命FlutterA會來時會執行周期函數onStart,它又會走到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(); } }); }
我們注意到這裡又一個attachToFlutterEngine的方法，那麼，它綁定的是FlutterActivityAndFragmentDelegate.this.flutterEngine，這個引擎是我們實現的一個EngineProvider每次需要的時候new出來的。
public class TipEngineProvider { public static FlutterEngine obtain() { return new FlutterEngine(IGameApplication.getIGameApplicationContext()); } }
那既然是new出來的一個引擎，我們不禁要問了，為什麼回退回來，FlutterA的頁面狀態還可以保存，比如，列表滑動到一定的位置，打開FlutterB，在回退回來，FlutterA還保存在列表滑動到的最後位置，沒變化。
帶著這個問題，我們只能去看看，onStart中綁定引擎，及初始化引起的過程中，做了一些什麼？
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); }
以上程式碼有省略，只保留至關重要的部分。這裡我們看到，在FlutterEngine的初始化中，flutterJNI綁定了Native，就在此時拿到了nativePlatformViewId，這裡我們先埋下一個伏筆，繼續走，然後，初始化了DartExecutor，DartExecutor綁定到了flutterJNI，然後初始化了FlutterRenderer。好，這裡先看到這一步，總結一下，FlutterEngine融合了很多關鍵的成員。
在FlutterSurfaceView中，這個類繼承至SurfaceView，實現了RenderSurface介面，在通過調用FlutterRenderer的surfaceCreated方法，最終會調用到flutterJNI的surfaceCreated，最終通過調用nativeSurfaceCreated方法和nativePlatformViewId產生了關聯，以下是源碼部分。
//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); }
換句話說，最終flutter底層通過nativePlatformViewId將數據繪製到了原生的surface上。
我們注意到，connectSurfaceToRenderer方法有兩次調用的時機，第一次是在surface初次create的時候，還有一次是主動去掉，看程式碼
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(); } }
很顯然，從FlutterB回到FlutterA的時候，因為FlutterA的頁面還在，所以，surface還在，所以，只可能調用attachToRenderer，才能夠將nativePlatformViewId與surface進行關聯。那麼，attachToRenderer是誰在什麼時候調用的呢？
答案就是FlutterView的attachToFlutterEngine方法，它就是在什麼周期函數onStart時調用的，那麼，問題來了，surface沒有變化，時決定FlutterA回來不變的唯一原因嗎？別忘記了，我們還有一個nativePlatformViewId呢。
當我們重新給一個FlutterEngine給FlutterView綁定時，發生了很多的過程，我們把焦點放到nativePlatformViewId時如何重新拿到的。上面我們已經知道是在FlutterEngine的初始化中，flutterJNI綁定了Native，就在此時拿到了nativePlatformViewId。
@UiThread public void attachToNative(boolean isBackgroundView) { this.ensureRunningOnMainThread(); this.ensureNotAttachedToNative(); this.nativePlatformViewId = this.nativeAttach(this, isBackgroundView); }
這是一個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; } }
根據我打的日誌來觀察，nativePlatformViewId在FlutterB返回FlutterA之後，此時拿到的這個nativePlatformViewId與之前的那個是一個long。因此，現在的情況是。
nativePlatformViewId和surface都沒有變化，還是然來的那個，自然而然FlutterA的頁面渲染出來的內容沒有發生變化。
至此，我們知道FlutterEngine，其實本身沒有保存數據，它只是管理著原生與flutter通訊的眾多成員，起到了一個中心控制者的作用，一旦它不再了，surface與nativePlatformViewId就斷開了，surface將不會在刷新，一旦它被安裝到了FlutterView上，surface與nativePlatformViewId重新建立起了關聯，surface又可以刷新了。
