Simple drag gesture detector with API simillar to android.view.ScaleGestureDetector

DragGestureDetector.kt

class DragGestureDetector(private val onDragListener: OnDragListener) {

    var posX = 0f
        private set
    var posY = 0f
        private set

    private var activePointerId = -1

    fun onTouchEvent(event: MotionEvent): Boolean {
        when (event.actionMasked) {
            MotionEvent.ACTION_DOWN -> {
                saveEventPosition(event, event.actionIndex)
                activePointerId = event.getPointerId(0)
                onDragListener.onDragBegin(this)
            }
            MotionEvent.ACTION_MOVE -> {
                saveEventPosition(event, event.findPointerIndex(activePointerId))
                onDragListener.onDrag(this)
            }
            MotionEvent.ACTION_UP, MotionEvent.ACTION_CANCEL -> {
                activePointerId = -1
                onDragListener.onDragEnd(this)
            }
            MotionEvent.ACTION_POINTER_UP -> {
                val pointerIndex = event.actionIndex
                event.getPointerId(pointerIndex).takeIf { it == activePointerId }?.run {
                    val newPointerIndex = if (pointerIndex == 0) 1 else 0
                    saveEventPosition(event, newPointerIndex)
                    activePointerId = event.getPointerId(newPointerIndex)
                }
            }
            else -> return false
        }
        return true
    }

    private fun saveEventPosition(event: MotionEvent, pointerIndex: Int) {
        posX = event.getX(pointerIndex)
        posY = event.getY(pointerIndex)
    }

    interface OnDragListener {

        fun onDragBegin(detector: DragGestureDetector): Boolean
        fun onDrag(detector: DragGestureDetector)
        fun onDragEnd(detector: DragGestureDetector)
    }
}