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I don't know if this is something that can be changed, but I'll look into it.

Hmm... it is supposed to work like this, but you are seeing a bug.

It cou8ld be tricky to get this right because the onAnimationEnd() and onTransitionEnd() often modify the state. We must execute the Runnable only after the state has been changed to its final value.

I think it could be dangerous to make a change, but it is possible to run all the internal listeners first, then run all of the external ones with the Runnable executed between. Again, it wouldn't be safe because we can't guarantee that only the internal listeners change the system state.

Passing to Jeremy to get his thoughts

What we really need is a signal to say when all transitions are done and with it running early, we don't have that. For fragments this means that we could end up moving the fragment to RESUMED before all the transitions are actually completed and execute some code that is normally fine when the system is settled, but dangerous when it is not.

If moving onTransitionEnd() might be too dangerous are there other options? If you are in the animateToStart case should you even get an onTransitionEnd(). Maybe all external listeners get is an onTransitionCancel() since you are reversing a transition you started in the middle of it. Is that a safer signal to send back?

I think it is okay to separate the signals as long as they are explicit, but we for sure need a way to know that the Transition system is completely done doing any work and our Views are back in the original state that we passed them in.

Maybe you could notify at the end of your animateToStart lambda?

Per offline discussion, we can move ahead with the solution suggested in comment#4 and run external listeners last.

This will change the current order:

start transition > animateToStart > onTransitionEnd() > Runnable > finish up transition work

to

start transition > animateToStart > Runnable > finish up transition work > onTransitionEnd()


This has the implications that if developers are doing something with a specific result in their onTransitionEnd() callback, they will also need to handle the cancellation case by overriding the new onTransitionEnd(Transition, boolean).

For example, if there is a transition from View_A to View_B and the onTransitionEnd() callback has logic that expects View_B to be the final state, it will now need to add some additional logic based on the boolean in the case that View_A is the final state.

The following release(s) address this bug.It is possible this bug has only been partially addressed:

• androidx.transition:transition:1.5.0-alpha06