How to detect the pinch-zoom event with OnGestureListener in Android?

Question

How to detect the pinch-zoom event with OnGestureListener in Android?

Answer 1

Take a look at this post on the Android Developers blog: http://android-developers.blogspot.com/2010/06/making-sense-of-multitouch.html

The later sections talk about using ScaleGestureDetector with code examples.

Answer 2

You can achieve this in 3 easy steps:

1. Create SimpleOnScaleGestureListener class like this:

    class CustomOnScaleGestureListener : 
      ScaleGestureDetector.SimpleOnScaleGestureListener() {
      override fun onScale(detector: ScaleGestureDetector): Boolean {
        val scaleFactor = detector.scaleFactor
        if (scaleFactor > 1) {
            // Zooming Out
        } else {
           // Zooming In
        }
        return true
    }
   
    override fun onScaleBegin(detector: ScaleGestureDetector): Boolean {
        return true
    }
   
    override fun onScaleEnd(detector: ScaleGestureDetector) { }
   }
   

2. Create ScaleGestureDetector object

    scaleGestureDetector = ScaleGestureDetector(this, 
      CustomOnScaleGestureListener())
   

3. Override onTouchEvent in your activity

    override fun onTouchEvent(event: MotionEvent): Boolean {
        scaleGestureDetector?.onTouchEvent(event)
        return true
    }
   

Full Example:

class MainActivity : AppCompatActivity(){

private lateinit var mBinding: ActivityMainBinding

var scaleGestureDetector: ScaleGestureDetector? = null

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

    mBinding = ActivityMainBinding.inflate(layoutInflater)

    setContentView(mBinding.root)

    scaleGestureDetector = ScaleGestureDetector(this, CustomOnScaleGestureListener())

}

override fun onTouchEvent(event: MotionEvent): Boolean {
    scaleGestureDetector?.onTouchEvent(event)
    return true
}

inner class CustomOnScaleGestureListener : ScaleGestureDetector.SimpleOnScaleGestureListener() {
    override fun onScale(detector: ScaleGestureDetector): Boolean {
        val scaleFactor = detector.scaleFactor
        if (scaleFactor > 1) {
           // Zooming Out
        } else {
           // Zooming In
        }
        return true
    }

    override fun onScaleBegin(detector: ScaleGestureDetector): Boolean {
        return true
    }

    override fun onScaleEnd(detector: ScaleGestureDetector) { }
}


}