o4oren

    private fun calculateAngle(x: Float, y: Float): Float {
        val px = (x / width.toFloat()) - 0.5
        val py = ( 1 - y / height.toFloat()) - 0.5
        var angle = -(Math.toDegrees(atan2(py, px)))
            .toFloat() + 90
        if (angle > 180) angle -= 360
        return angle
    }

o4oren

    private fun setKnobPosition(angle: Float) {
        val matrix = Matrix()
        knobImageView.scaleType = ScaleType.MATRIX
        matrix.postRotate(angle, width.toFloat() / 2, height.toFloat() / 2)
        knobImageView.imageMatrix = matrix
    }

o4oren

    <geva.oren.rotaryknobdemo.RotaryKnobView
        android:id="@+id/knob"
        class="geva.oren.rotaryknobdemo.RotaryKnobView"
        android:layout_width="@dimen/knob_width"
        android:layout_height="@dimen/knob_height"
        android:layout_marginBottom="312dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"

o4oren

package geva.oren.rotaryknobdemo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity(), RotaryKnobView.RotaryKnobListener {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

o4oren

    override fun onTouchEvent(event: MotionEvent): Boolean {
        return if (gestureDetector.onTouchEvent(event))
            true
        else
            super.onTouchEvent(event)
    }

    override fun onDown(event: MotionEvent): Boolean {
        return true
    }