Component used: activity-compose - PredictiveBackHandler

Right now, when getting the progress events, you can tell the touchX and touchY and the progress, but you’d need to manually track the velocity to understand the velocity with which a gesture is performed.

I’d like this velocity to be able to use animateDecay, or animateTo() on Animatable, to be able to smoothly finish the animation from where it left off. To do this, I’ve added VelocityTracker, but the BackEvent doesn’t contain uptimeMillis, leading me to use System.currentTimeMillis, which could be inaccurate if an event comes in a different order.

val translationX = remember {
            Animatable(0f)
        }

        val drawerWidth = with(LocalDensity.current) {
            DrawerWidth.toPx()
        }
        translationX.updateBounds(0f, drawerWidth)
        val velocityTracker = remember {
            VelocityTracker()
        }
        PredictiveBackHandler(drawerState == DrawerState.Closed) { progress: Flow<BackEventCompat> ->
            // code for gesture back started
            try {
                progress.collect { backEvent ->
                    // code for progress
                    if (drawerState == DrawerState.Closed) {
                        translationX.snapTo(backEvent.progress * drawerWidth)
// I'd like to not need to use System time but rather the gesture event time.
                        velocityTracker.addPosition(System.currentTimeMillis(), Offset(backEvent.touchX, backEvent.touchY))
                    }
                }
                // code for completion
                val endProgress = if (translationX.value > drawerWidth * 0.5f) {
                    drawerWidth
                } else {
                    0f
                }
                translationX.animateTo(endProgress, initialVelocity = velocityTracker.calculateVelocity().x)

                if (endProgress == 0f){
                    drawerState = DrawerState.Closed
                } else {
                    drawerState = DrawerState.Open
                }
            } catch (e: CancellationException) {
                // code for cancellation
                Log.d("!!!", "predictive back gesture cancelled")
            }
        }