This repo will show you how to support shared elements animations among multi activities (>= 3) using reflection.

Still no idea (2021.3) and here is my opinion.

If you happen to speak Chinese, you can also refer to 中文版.

Why?

1. Assuming there are 3 activities, A => B => C, (A in the bottom), calling fromsupportFinishAfterTransition() in C, on API 29 platform.

2. Results in calling mActivityTransitionState.startExitBackTransition(this), it will construct an ExitTransitionCoordinator which isReturning is TRUE , then field mIsBackgroundReady will be false.

   mReturnExitCoordinator = new ExitTransitionCoordinator(activity,
                           activity.getWindow(), activity.mEnterTransitionListener, pendingExitNames,
                           null, null, true);
   
   
   public ExitTransitionCoordinator(Activity activity, Window window,
           SharedElementCallback listener, ArrayList<String> names,
           ArrayList<String> accepted, ArrayList<View> mapped, boolean isReturning) {
       super(window, names, listener, isReturning);
       viewsReady(mapSharedElements(accepted, mapped));
       stripOffscreenViews();
       mIsBackgroundReady = !isReturning; // here
       mActivity = activity;
   }

3. In B's performStart() will call mActivityTransitionState.enterReady(this), at that time sharedElementNames is still containing the shared elements' name. But sharedElementNames will just assign to mAllSharedElementNames in EnterTransitionCoordinator, mPendingExitNames won't get the value from mAllSharedElementNames without receiving message MSG_ALLOW_RETURN_TRANSITION.

4. In ExitTransitionCoordinator, only one piece will send such message:

   protected void notifyComplete() {
       if (isReadyToNotify()) {
           if (!mSharedElementNotified) {
               mSharedElementNotified = true;
               delayCancel();
   
               if (!mActivity.isTopOfTask()) {
                   // here
                   mResultReceiver.send(MSG_ALLOW_RETURN_TRANSITION, null);
               }
   
               if (mListener == null) {
                   mResultReceiver.send(MSG_TAKE_SHARED_ELEMENTS, mSharedElementBundle);
                   notifyExitComplete();
               } else {
                   final ResultReceiver resultReceiver = mResultReceiver;
                   final Bundle sharedElementBundle = mSharedElementBundle;
                   mListener.onSharedElementsArrived(mSharedElementNames, mSharedElements,
                           new OnSharedElementsReadyListener() {
                               @Override
                               public void onSharedElementsReady() {
                                   resultReceiver.send(MSG_TAKE_SHARED_ELEMENTS,
                                           sharedElementBundle);
                                   notifyExitComplete();
                               }
                           });
               }
           } else {
               notifyExitComplete();
           }
       }
   }

5. But isReadyToNotify() won't return true since mIsBackgroundReady is false while isReturning is true.

   protected boolean isReadyToNotify() {
       return mSharedElementBundle != null && mResultReceiver != null && mIsBackgroundReady;
   }

6. Therefore, no way to let B's mEnterTransitionCoordinator.getPendingExitSharedElementNames() return a non-null value without reflection. Since mActivityTransitionState is hidden in reflection we also need to bypass it, finally lead to such 'ugly' solution provided in repo.

Really need to share elements among multi 'pages'?

Through sharing elements among activities seems to be tricky, but everything turns good if we use Fragment instead of Activity. I had added another sample for fragment scenario. Here are also some docs and posts written by Google.

https://developer.android.com/guide/fragments/animate#kotlin

https://android-developers.googleblog.com/2018/02/continuous-shared-element-transitions.html

Good luck!