How to add GestureDetector and ScaleGestureDetector on a ImageView with setOnTouchListener

Question

In my MainActivity i have:

class MainActivity : AppCompatActivity() {
    lateinit var imageView: ImageView

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

        setContentView(R.layout.activity_main)

        val imageViewGestureDetector = GestureDetector( this, object : GestureDetector.SimpleOnGestureListener() {
            override fun onDoubleTap(e: MotionEvent?): Boolean {
                return true
            }

            override fun onFling(
                e1: MotionEvent?,
                e2: MotionEvent?,
                velocityX: Float,
                velocityY: Float
            ): Boolean {
                Log.i("MainActivity", "Fling!")
                return super.onFling(e1, e2, velocityX, velocityY)
            }
        })

        imageView.setOnTouchListener { _, event -> imageViewGestureDetector.onTouchEvent(event) }
    }

}

Now I thought it might be nice to define a behaviour for onScale but I noticed Scale Gestures are detected by the ScaleGestureDetector.

I'd rather not sub-class the ImageView and override fun onTouchEvent() - or should I? That's what all of the solutions in the net seem to suggest. I'd assume there might be a convenient way to combine these common gestures in one view.

Answer 1

I found a quite simple way myself.

I wrote a ScaleGestureDetector like that

val imageViewScaleGestureDetector = ScaleGestureDetector(this, object: ScaleGestureDetector.SimpleOnScaleGestureListener() {
    override fun onScale(detector: ScaleGestureDetector?): Boolean {
        Log.i(“MainActivity”, “scale”)
        return super.onScale(detector)
    }
})

and changed the line

imageView.setOnTouchListener { _, event -> imageViewGestureDetector.onTouchEvent(event) }

to

imageView.setOnTouchListener { _, event ->
    imageViewScaleGestureDetector.onTouchEvent(event)
    imageViewGestureDetector.onTouchEvent(event)
}

My ImageView is detecting both fling and scale gesture. Let me know if that's bad practice or there are better solutions.