Basically, Android start the transition with your pre-defined View and transitionName and automatically use the same properties for the return transition. When you change your focused View in ViewPager, Android doesn't know about that and keep the transition on the previous one on its way back. So you need to inform Android about the changes:
- Remap the transition properties: Use
setEnterSharedElementCallback to change the transitionName and View to the new one before returning from Activity2.
- Wait for the
Activity1 to finish rendering addOnPreDrawListener.
It's a bit complex in the final implementation. But you can look at my sample code https://github.com/tamhuynhit/PhotoGallery. I try to implement the shared-element-transition from many simple to complex sections.
Your problem appeared from Level 3 and solved in Level 4.
I am writing a tutorial about this but it's not in English so hope the code can help
UPDATE 1: Work flow
Here is how I implement it in my code:
Override finishAfterTransition in Activity2 and call setEnterSharedElementCallback method to re-map the current selected item in ViewPager. Also, call setResult to pass the new selected index back to previous activity here.
@Override
@TargetApi(Build.VERSION_CODES.LOLLIPOP)
public void finishAfterTransition() {
setEnterSharedElementCallback(new SharedElementCallback() {
@Override
public void onMapSharedElements(List<String> names, Map<String, View> sharedElements) {
View selectedView = getSelectedView();
if (selectedView == null)
return;
// Clear all current shared views and names
names.clear();
sharedElements.clear();
// Store new selected view and name
String transitionName = ViewCompat.getTransitionName(selectedView);
names.add(transitionName);
sharedElements.put(transitionName, selectedView);
setExitSharedElementCallback((SharedElementCallback) null);
}
});
Intent intent = new Intent();
intent.putExtra(PHOTO_FOCUSED_INDEX, mCurrentIndex);
setResult(RESULT_PHOTO_CLOSED, intent);
super.finishAfterTransition();
}
Write a custom ShareElementCallback so I can set the callback before knowing which View is going to be used.
@TargetApi(Build.VERSION_CODES.LOLLIPOP)
private static class CustomSharedElementCallback extends SharedElementCallback {
private View mView;
/**
* Set the transtion View to the callback, this should be called before starting the transition so the View is not null
*/
public void setView(View view) {
mView = view;
}
@Override
public void onMapSharedElements(List<String> names, Map<String, View> sharedElements) {
// Clear all current shared views and names
names.clear();
sharedElements.clear();
// Store new selected view and name
String transitionName = ViewCompat.getTransitionName(mView);
names.add(transitionName);
sharedElements.put(transitionName, mView);
}
}
Override onActivityReenter in Activity1, get the selected index from the result Intent. Set setExitSharedElementCallback to re-map new selected View when the transition begins.Call supportPostponeEnterTransition to delay a bit because your new View may not be rendered at this point. Use getViewTreeObserver().addOnPreDrawListener to listen for the layout changes, find the right View by the selected index and continue the transition supportStartPostponedEnterTransition.
@Override
@TargetApi(Build.VERSION_CODES.LOLLIPOP)
public void onActivityReenter(int resultCode, Intent data) {
if (resultCode != LevelFourFullPhotoActivity.RESULT_PHOTO_CLOSED || data == null)
return;
final int selectedIndex = data.getIntExtra(LevelFourFullPhotoActivity.PHOTO_FOCUSED_INDEX, -1);
if (selectedIndex == -1)
return;
// Scroll to the new selected view in case it's not currently visible on the screen
mPhotoList.scrollToPosition(selectedIndex);
final CustomSharedElementCallback callback = new CustomSharedElementCallback();
getActivity().setExitSharedElementCallback(callback);
// Listen for the transition end and clear all registered callback
getActivity().getWindow().getSharedElementExitTransition().addListener(new Transition.TransitionListener() {
@Override
public void onTransitionStart(Transition transition) {}
@Override
public void onTransitionPause(Transition transition) {}
@Override
public void onTransitionResume(Transition transition) {}
@Override
public void onTransitionEnd(Transition transition) {
removeCallback();
}
@Override
public void onTransitionCancel(Transition transition) {
removeCallback();
}
private void removeCallback() {
if (getActivity() != null) {
getActivity().getWindow().getSharedElementExitTransition().removeListener(this);
getActivity().setExitSharedElementCallback((SharedElementCallback) null);
}
}
});
// Pause transition until the selected view is fully drawn
getActivity().supportPostponeEnterTransition();
// Listen for the RecyclerView pre draw to make sure the selected view is visible,
// and findViewHolderForAdapterPosition will return a non null ViewHolder
mPhotoList.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
@Override
public boolean onPreDraw() {
mPhotoList.getViewTreeObserver().removeOnPreDrawListener(this);
RecyclerView.ViewHolder holder = mPhotoList.findViewHolderForAdapterPosition(selectedIndex);
if (holder instanceof ViewHolder) {
callback.setView(((ViewHolder) holder).mPhotoImg);
}
// Continue the transition
getActivity().supportStartPostponedEnterTransition();
return true;
}
});
}
UPDATE 2: getSelectedItem
To get selected View from the ViewPager, don't use getChildAt or you get the wrong View, use findViewWithTag instead
In the PagerAdapter.instantiateItem, use position as tag for each View:
@Override
public View instantiateItem(ViewGroup container, int position) {
// Create the View
view.setTag(position)
// ...
}
Listen to onPageSelected event to get the selected index:
mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
@Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
}
@Override
public void onPageSelected(int position) {
mSelectedIndex = position;
}
@Override
public void onPageScrollStateChanged(int state) {
}
});
Call getSelectedView to get the current view by the selected index
private View getSelectedView() {
try {
return mPhotoViewPager.findViewWithTag(mSelectedIndex);
} catch (IndexOutOfBoundsException | NullPointerException ex) {
return null;
}
}
