日常爬坑-Android 过渡动画无效

最近在我的应用KeepassA中碰到了一个诡异的过渡动画问题

正常状态应该如下：

当我从一级设置界面，进入二级设置界面后，并从二级设置界面返回时，一级界面当回主页的过渡动画神器消失了

原因分析

阅读源码发现，返回时调用的finishAfterTransition()最终会调用ActivityTransitionState的startExitBackTransition方法，但是当我从二级界面返回到一级界面，并从一级界面返回主页时，pendingExitNames变为了空，导致直接走了finish，而没有走过渡动画的逻辑。

public void finishAfterTransition() {
       if (!mActivityTransitionState.startExitBackTransition(this)) {
           finish();
       }
}

public boolean startExitBackTransition(final Activity activity) {
       ArrayList<String> pendingExitNames = getPendingExitNames();
       if (pendingExitNames == null || mCalledExitCoordinator != null) {
           return false;
       } else {
        ...
       }
       ....
 }

为什么会出现pendingExitNames为空的情况呢，继续阅读源代码，通过Activity的生命周期可以知道，每当activity开始活动时（从二级界面返回一级界面会回调onStart），导致重新调用了onNewActivityOptions方法。

/** @hide */
public void onNewActivityOptions(ActivityOptions options) {
    mActivityTransitionState.setEnterActivityOptions(this, options); // 重新设置了option
    if (!mStopped) {
        mActivityTransitionState.enterReady(this);
    }
}

在 mActivityTransitionState.setEnterActivityOptions(this, options);中会重新设置共享元素，如果当前activity已经停止（启动了二级页面，并从二级界面返回）则会调用 mActivityTransitionState.enterReady重新构建过渡动画。而在这一步debug发现ActivityOptions，设置的过渡元素为null了。

public void setEnterActivityOptions(Activity activity, ActivityOptions options) {
       ...
       if (window.hasFeature(Window.FEATURE_ACTIVITY_TRANSITIONS)
               && options != null && mEnterActivityOptions == null
               && mEnterTransitionCoordinator == null
               && options.getAnimationType() == ActivityOptions.ANIM_SCENE_TRANSITION) {
           mEnterActivityOptions = options; // 重新更新了option，但是这时option的共享元素列表被清空了
           ...
       }
   }

public void enterReady(Activity activity) {
      ...
     
       // 重新创建过渡场景，但是该场景的共享元素列表 sharedElementNames 没有数据
       mEnterTransitionCoordinator = new EnterTransitionCoordinator(activity,
               resultReceiver, sharedElementNames, mEnterActivityOptions.isReturning(),
               mEnterActivityOptions.isCrossTask());
      
   ...

       if (!mIsEnterPostponed) {
           startEnter();
       }
   }

这个时候，只是共享元素的列表大小为0，并没有为null，还达不到那个条件，继续阅读代码，看到EnterTransitionCoordinator找到了一个处理共享元素状态的方法onReceiveResult，在这里面看到，只有接收到的消息类型为MSG_ALLOW_RETURN_TRANSITION才会给mPendingExitNames赋值。但是这个消息接收又是从那个地方回调的呢？

@Override
   protected void onReceiveResult(int resultCode, Bundle resultData) {
       switch (resultCode) {
           case MSG_TAKE_SHARED_ELEMENTS:
               if (!mIsCanceled) {
                   mSharedElementsBundle = resultData;
                   onTakeSharedElements();
               }
               break;
           case MSG_EXIT_TRANSITION_COMPLETE:
               if (!mIsCanceled) {
                   mIsExitTransitionComplete = true;
                   if (mSharedElementTransitionStarted) {
                       onRemoteExitTransitionComplete();
                   }
               }
               break;
           case MSG_CANCEL:
               cancel();
               break;
           case MSG_ALLOW_RETURN_TRANSITION:
               if (!mIsCanceled) {
                   mPendingExitNames = mAllSharedElementNames;
               }
               break;
       }
   }

根据其父类ActivityTransitionCoordinator,发现其本质上是个handler的处理函数。

public ActivityTransitionCoordinator(Window window,
        ArrayList<String> allSharedElementNames,
        SharedElementCallback listener, boolean isReturning) {
    super(new Handler());
    mWindow = window;
    mListener = listener;
    mAllSharedElementNames = allSharedElementNames;
    mIsReturning = isReturning;
}

因此只要找到发送消息的地方，就能知道启动的各种条件。继续阅读源码，该方法在onresume中被调用，将会取消所有待处理的过渡动画！！！！

/**
 * This is called onResume. If an Activity is resuming and the transitions
 * haven't started yet, force the views to appear. This is likely to be
 * caused by the top Activity finishing before the transitions started.
 * In that case, we can finish any transition that was started, but we
 * should cancel any pending transition and just bring those Views visible.
 */
public void forceViewsToAppear() {
    if (!mIsReturning) {
        return;
    }
    if (!mIsReadyForTransition) {
        mIsReadyForTransition = true;
        final ViewGroup decor = getDecor();
        if (decor != null && mViewsReadyListener != null) {
            mViewsReadyListener.removeListener();
            mViewsReadyListener = null;
        }
        showViews(mTransitioningViews, true);
        setTransitioningViewsVisiblity(View.VISIBLE, true);
        mSharedElements.clear();
        mAllSharedElementNames.clear();
        mTransitioningViews.clear();
        mIsReadyForTransition = true;
        viewsTransitionComplete();
        sharedElementTransitionComplete();
    } else {
        if (!mSharedElementTransitionStarted) {
            moveSharedElementsFromOverlay();
            mSharedElementTransitionStarted = true;
            showViews(mSharedElements, true);
            mSharedElements.clear();
            sharedElementTransitionComplete();
        }
        if (!mIsViewsTransitionStarted) {
            mIsViewsTransitionStarted = true;
            showViews(mTransitioningViews, true);
            setTransitioningViewsVisiblity(View.VISIBLE, true);
            mTransitioningViews.clear();
            viewsTransitionComplete();
        }
        cancelPendingTransitions();
    }
    mAreViewsReady = true;
    if (mResultReceiver != null) {
        mResultReceiver.send(MSG_CANCEL, null);
        mResultReceiver = null;
    }
}