Added onScroll(Begin|End)Drag Events

[EvanBacon]

• Added onScrollBeginDrag
• Added onScrollEndDrag

[EvanBacon]

Upon further review it seems that perhaps the onScrollEndDrag is implemented closer to onMomentumScrollEnd since a debounce is involved.
I can't find a very consistent way to observe if Momentum is enabled. Perhaps we should add that before implementing onMomentumScrollBegin, onMomentumScrollEnd.

[EvanBacon]

hmm #1091 🙁 perhaps that PR should be fixed instead. I need this and a few other features to get added to ScrollView to fix a couple of complex interactions.

[necolas]

Yeah this doesn't look quite right. I think these scroll events are going to be more complicated to implement as they will need to be aware of whether a touch is active in order to determine whether scrolling is momentum-based or not. No idea what the situation for trackpads is.

Ref #1021

[EvanBacon]

Perhaps we could create new props like onScrollBegin, onScrollEnd. It would be really helpful to access these values. The alternative of using onScroll with an animated value is a huge bottle neck on performance. Of course I would much rather have this done correctly, but the particular invocation this is currently surfacing is pretty valuable.

ex: onScroll & comparing every frame

[evie9z]

Hi, I'm working on a project that really needs this feature. May I ask any updates on this PR yet?

[shljessie]

How is this going ?
@EvanBacon @necolas

[KingAmo]

any update on this ? i just want do something when scroll begin

[lucaashrq]

Any update?

[EvanBacon]

I don't have time to investigate this further, please see @necolas's comment for reasons this cannot be merged in its current state.