A simple rotate gesture detector with API simillar to android.view.ScaleGestureDetector

RotateGestureDetector.kt

class RotateGestureDetector(private val onRotateListener: OnRotateListener) {

    val rotationDeltaDegrees: Float
        get() {
            val diffRadians = atan2(prevDiffY, prevDiffX) - atan2(currDiffY, currDiffX)
            return Math.toDegrees(diffRadians.toDouble()).toFloat()
        }

    var focusX = 0f
        private set

    var focusY = 0f
        private set

    private var currDiffX = 0f
    private var currDiffY = 0f

    private var prevDiffX = 0f
    private var prevDiffY = 0f

    private var isInProgress = false

    fun onTouchEvent(event: MotionEvent): Boolean {
        val action = event.actionMasked
        val count = event.pointerCount

        val isStreamComplete =
            action == MotionEvent.ACTION_UP ||
                action == MotionEvent.ACTION_CANCEL ||
                action == MotionEvent.ACTION_POINTER_UP && count < 2

        if (isStreamComplete) {
            if (isInProgress) onRotateListener.onRotateEnd(this)
            isInProgress = false
            return true
        }

        if (count < 2) return false

        focusX = (event.getX(0) + event.getX(1)) / 2f
        focusY = (event.getY(0) + event.getY(1)) / 2f

        if (action == MotionEvent.ACTION_POINTER_DOWN && !isInProgress) {
            onRotateListener.onRotateBegin(this)
            calculateCurrDiff(event)
            isInProgress = true
            return true
        }

        prevDiffX = currDiffX
        prevDiffY = currDiffY
        
        calculateCurrDiff(event)

        onRotateListener.onRotate(this)

        return true
    }

    private fun calculateCurrDiff(event: MotionEvent) {
        currDiffX = event.getX(1) - event.getX(0)
        currDiffY = event.getY(1) - event.getY(0)
    }

    interface OnRotateListener {

        fun onRotateBegin(detector: RotateGestureDetector): Boolean
        fun onRotate(detector: RotateGestureDetector): Boolean
        fun onRotateEnd(detector: RotateGestureDetector)
    }
}