Consider exposing Animated.Value with transition progress (for shared element transitions)

It’d be great if the native stack navigator supported shared element transitions. One way of implementing that, without react-native-screens needing to know anything about shared element transitions, is by exposing an Animated.Value with the progress of the current transition running.

I explored a little how one might implement this.

In here, if you switched from [UIView animateWithDuration] to UIViewPropertyAnimator, you can get fractionComplete:

https://github.com/kmagiera/react-native-screens/blob/ed997ef4ecd4cc99383386cb016bc8d79bf70279/ios/RNSScreenStack.m#L354-L375

This is an example of using UIViewPropertyAnimator with a custom view controller transition: https://stackoverflow.com/questions/27638806/transitioncoordinator-return-nil-for-custom-container-view-controller-in-ios8

From there, you would have a CADisplayLink which sets the value for the animated node with the fractionComplete from the UIViewPropertyAnimator instance.

[WoLewicki]

I added a first look at exposing progress of transition between screens on both platforms in https://github.com/software-mansion/react-native-screens/pull/890. I would be thankful if you could test it and share your thoughts.

[mrousavy]

@Jarred-Sumner Any updates on this? 👀

cc @kmagiera …