Is it possible to snap an animation, `Animatable` or `updateTransition` , during the same compositio...

# compose

[Dominaezzz]

Is it possible to snap an animation, Animatable or updateTransition , during the same composition? When I update the state or call snapTo , the update happens in the next frame or so.

[Doris Liu]

Could you share a bit more about your use case?

[Dominaezzz]

Basically, I'm implementing LazyColumn drag and drop, and I want the items in the list to animate into their new positions.
To do this, when an item moves to the next/previous position, I use graphicsLayer(translationY = ....) to draw the item in the previous position, then I animate the item into it's new position.
When I detect the change in position, I want to snap the translation to the old position immediately. Right now, the item would be rendered in the new position first, then snap to the old position, then animate back to the new position.

[Dominaezzz]

I had a brief chat about this in https://kotlinlang.slack.com/archives/CJLTWPH7S/p1626566677385200 .

[Doris Liu]

The only way to achieve that is create a new Animatable when you need to snap (in the current composition pass):
var translation by remember { mutableStateOf(Animatable(...)) } and mutate that translation as needed. The LaunchedEffect that triggers animations for Animatable would now need to have translation added to the keys.

[Dominaezzz]

Ah thanks! Is that the "correct" way or a workaround?

[Dominaezzz]

Also, is the mutableStateOf necessary? Could I just do val translation by remember(changed) { Animatable(...) }?

[Doris Liu]

That'd be a workaround. But it's unclear to me what a correct and non-confusing API for that would be at the moment. We are trying to avoid one-off APIs. If a similar usage pattern emerges in the future, we would be able to create an API based on the pattern.

[Doris Liu]

Also, is the mutableStateOf necessary? Could I just do val translation by remember(changed) { Animatable(...) }? 

Yes, you could do that. mutableStateOf would be necessary if you need to mutate translation in a coroutine. 🙂

[Dominaezzz]

Ahhh, got it.

[Dominaezzz]

My use case is arguably a workaround for LazyColumn not having drag and drop (or the necessary primitives I guess), so it may not be worth considering just yet. Depends on what the LazyColumn devs think I guess.