原文連結：cloud.tencent.com/developer/a…
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週期函式。
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一定會走到其生命週期函式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();
}複製程式碼我們注意到，這裡呼叫了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;
}
}複製程式碼我們最終看到，呼叫了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儲存
我們注意到生命FlutterA會來時會執行周期函式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();
}
});
}複製程式碼我們注意到這裡又一個attachToFlutterEngine的方法，那麼，它繫結的是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());
}
}複製程式碼那既然是new出來的一個引擎，我們不禁要問了，為什麼回退回來，FlutterA的頁面狀態還可以儲存，比如，列表滑動到一定的位置，開啟FlutterB，在回退回來，FlutterA還儲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中繫結引擎，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);
}複製程式碼以上程式碼有省略，只保留至關重要的部分。這裡我們看到，在FlutterEngine的初始化中，flutterJNI繫結了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繫結到了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，在通過呼叫FlutterRenderer的surfaceCreated方法，最終會呼叫到flutterJNI的surfaceCreated，最終通過呼叫nativeSurfaceCreated方法和nativePlatformViewId產生了關聯，以下是原始碼部分。
//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.");
}
}複製程式碼//FlutterRenderer 中的surfaceCreated方法
public void surfaceCreated(@NonNull Surface surface) {
this.flutterJNI.onSurfaceCreated(surface);
}複製程式碼//FlutterJNI中的onSurfaceCreated方法
@UiThread
public void onSurfaceCreated(@NonNull Surface surface) {
this.ensureRunningOnMainThread();
this.ensureAttachedToNative();
this.nativeSurfaceCreated(this.nativePlatformViewId, surface);
}複製程式碼換句話說，最終flutter底層通過nativePlatformViewId將資料繪製到了原生的surface上。
我們注意到，connectSurfaceToRenderer方法有兩次呼叫的時機，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();
}
}複製程式碼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();
}
}複製程式碼很顯然，從FlutterB回到FlutterA的時候，因為FlutterA的頁面還在，所以，surface還在，所以，只可能呼叫attachToRenderer，才能夠將nativePlatformViewId與surface進行關聯。那麼，attachToRenderer是誰在什麼時候呼叫的呢？
答案就是FlutterView的attachToFlutterEngine方法，它就是在什麼周期函式onStart時呼叫的，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繫結時，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繫結了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);
}複製程式碼這是一個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;
}
}複製程式碼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，其實本身沒有儲存資料，它只是管理著原生與flutter通訊的眾多成員，起到了一箇中心控制者的作用，一旦它不再了，surface與nativePlatformViewId就斷開了，surface將不會在重新整理，一旦它被安裝到了FlutterView上，surface與nativePlatformViewId重新建立起了關聯，surface又可以重新整理了。