[Code in thread :point_down:] I have this code whe...

# compose

[Se7eN]

[Code in thread 👇] I have this code where I'm using the pointerInput modifier to allow dragging inside my Box. The problem is when the locked parameter (and other parameters) change, onPositionChange and onDragStateChange still executes with the old value of the parameters. Like if locked was true by default and the state changes to false, the if statement in onPositionChange will still run. I don't know it might be related to some kotlin magic but I was using the now deprecated dragGestureFilter modifier before alpha12 and it was working fine with the same code. I've also tried using detectDragGestures inside pointerInput but no luck.

[Se7eN]

fun Parent() {
    val locked by remember { mutableStateOf(true) }
    MyComposable(locked, ...)
}

fun MyComposable(locked: Boolean, ...) {
    val inputModifier = Modifier.simpleDragInput(
        onDragStateChange = { dragging -> ... },
        onPositionChange = { dragPosition ->
            if(locked) {
                ...
            }
        }
    )

    Box(modifier = inputModifier)
}

fun Modifier.simpleDragInput(
    onPositionChange: (Offset) -> Unit,
    onDragStateChange: (Boolean) -> Unit,
): Modifier = pointerInput(Unit) {
    forEachGesture {
        awaitPointerEventScope {
            val down = awaitFirstDown()
            onDragStateChange(true)
            onPositionChange(down.position)
            drag(down.id) { change ->
                change.consumePositionChange(
                    change.position.x - change.previousPosition.x,
                    change.position.y - change.previousPosition.y
                )
                onPositionChange(change.position)
            }
            onDragStateChange(false)
        }
    }
}

[Vsevolod Ganin]

This is because pointerInput(Unit) is remembering your first passed lambda for composition lifetime. Your should pass appropriate keys to make it update lambda

[Vsevolod Ganin]

Every instance of lambda captures local variables in turn. So your first `pointerInput`’s lambda remembers first onPositionChange and onDragStateChange and they remember first seen locked value

[Se7eN]

Ah I see. So I should pass all my parameters to pointerInput I'm using inside my lambdas? I have like 5-6 parameters.

[Vsevolod Ganin]

I guess so, yes

[Se7eN]

Alright thanks

[Vsevolod Ganin]

It’s the same story with DisposableEffect and LaunchedEffect. Moreover this behavior is due to the fact that pointerInput relies on LaunchedEffect in its implementation

[Se7eN]

Okay I added all my parameters but now the drag doesn't work properly. It only moves the drag position when I start the drag but after that the drag just stops working

[Vsevolod Ganin]

I experienced similar problems, yes. It was because now my lambda was updating on every recomposition, cancelling previous efforts. Turned out, it was because of keys that I forgot to wrap with remember in the first place, so they were always new on every recomposition. So try to inspect each key if it is different on every recomposition

[manueldidonna]

Try to pass your properties to rememberUpdatedState. It's an API to publish data from recomposition to ongoing or long-lived processes such as Disposable/LaunchedEffects

[Se7eN]

Still the same thing after passing keys with and without rememberUpdatedState. I do have a list key that is updated inside onPositionChanged using the drag position. I'll try making a minimal reproducible example

[Adam Powell]

The parameters to pointerInput determine when your detector state machine will reset, and rememberUpdatedState lets you update things like these callbacks out from under the running pointerInput state machine without resetting it. The pattern @manueldidonna is describing above is:

fun Modifier.simpleGesture(
  onGesture: () -> Unit
): Modifier {
  val currentOnGesture by rememberUpdatedState(onGesture)

  // Unit because we do not want to restart
  return pointerInput(Unit) {
    // ...
    // when detected, call currentOnGesture, not onGesture.
    // currentOnGesture is kept up to date by composition.
    currentOnGesture()
  }
}

[Se7eN]

Got it, this works. Thanks everyone!

[Se7eN]

One more thing: When should the state machine reset?

[Adam Powell]

It depends on the use case. Possibly never, possibly when something deeply significant to the gesture changes such that old input in progress is invalid and things should be cancelled.

[Adam Powell]

A little hand-wavey, I know, sorry, I'm only on my first cup of coffee for the day 😅

[Se7eN]

Haha it's okay I get the general idea