Curious about when a composable function should return a `St

Question

Curious about when a composable function should return a `State lt T gt ` or just `T` An example from Crane the official android samples inside thread thread

Stylianos Gakis · Answer

One example <https github com android compose samples blob 3ef01b5b02e463daf2fd22b380ee5a588d19a81f Crane app src main java androidx compose samples crane home SearchUserInput kt L118|function> defined in the Crane app looks like this ``` Composable private fun tintPeopleUserInput transitionState MutableTransitionState lt PeopleUserInputAnimationState gt State lt Color gt val validColor = MaterialTheme colors onSurface val invalidColor = MaterialTheme colors secondary val transition = updateTransition transitionState return transition animateColor transitionSpec = tween durationMillis = 300 if it == Valid validColor else invalidColor ``` Now as I understand it Compose is smart enough to just handle it perfectly fine if the function looked like this instead ``` Composable private fun tintPeopleUserInput transitionState MutableTransitionState lt PeopleUserInputAnimationState gt Color val validColor = MaterialTheme colors onSurface val invalidColor = MaterialTheme colors secondary val transition = updateTransition transitionState label = val color by transition animateColor transitionSpec = tween durationMillis = 300 label = if it == Valid validColor else invalidColor return color ``` So why would one ever chose one over the other If they are functionally equivalent it feels like the simple non state return type should be preferable Are there any guidelines on this

Zach Klippenstein (he/him) [MOD] · Answer

Returning the raw value means it will cause the caller s recompose scope to recompose on every animation frame because the function itself reads the animation value By returning a `State` this function allows the caller to determine where to read the animated state and thus how much what to recompose reexecute on each frame

Zach Klippenstein (he/him) [MOD] · Answer

e g the caller might only read from the returned `State` inside a `drawBehind` call which means that each animation frame of this transition would only trigger a new draw pass not a whole recomposition

corneil · Answer

I think since the input the the function is derived from State the function is going to be invoked on a change and the rest of the related functions man shrugging

Stylianos Gakis · Answer

Right I think I understand it you have explained this <https dev to zachklipp scoped recomposition jetpack compose what happens when state changes l78|here> as well right If the value is read directly on the top of the calling function that entire composable will be re composed whenever the color value changes Am I getting this right

Zach Klippenstein (he/him) [MOD] · Answer

gt directly on the top of the calling function I m not sure what you mean by this but I think you ve probably got the right idea

Zach Klippenstein (he/him) [MOD] · Answer

Also note that it s not necessarily bad to recompose the whole thing and if that s what you need to do then it s fine But by returning a `State` the function doesn t force that to be the only option it leaves it up to the caller

Stylianos Gakis · Answer

I meant like if it s read as the first line into the calling composable for example Meaning in would be in the outer most recompose scope therefore everything under it would be re composed Like <https github com android compose samples blob 3ef01b5b02e463daf2fd22b380ee5a588d19a81f Crane app src main java androidx compose samples crane home SearchUserInput kt L72|this> does ```Column val transitionState = remember peopleState animationState val tint Color = tintPeopleUserInput transitionState here val people = peopleState people CraneUserInput text = vectorImageId = tint = tint onClick = if transitionState targetState == Invalid Text ``` The whole CraneUserInput and whatever else was included inside this Column and outside of it since it s inline would recompose instead of only whatever uses tint inside the CraneUserInput Would this be the case

Zach Klippenstein (he/him) [MOD] · Answer

yes if `tintPeopleUserInput` returned just `T` instead of `State lt T gt ` correct

Zach Klippenstein (he/him) [MOD] · Answer

unless i m misunderstanding it doesn t matter what line it s on

Stylianos Gakis · Answer

Amazing thank you for helping me out And also I would like to say that your posts that I linked before have been invaluable to me I would never be able to understand any of this without it Thank you so much

Zach Klippenstein (he/him) [MOD] · Answer

glad they helped