Annotazione di codice per il pinch zoom

Ecco un codice che sembra funzionare, per il pinch zoom di una bitmap:

public class MainActivity extends Activity {
 
  RelativeLayout mainLayout;
  ImageView immagine;
  Bitmap bitmap;
  ScaleGestureDetector detector;
  float scala=1.f;
  int bmpWidth, bmpHeight;
  TextView textView;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        mainLayout=(RelativeLayout) findViewById(R.id.mainLayout);
        
        
        detector=new ScaleGestureDetector(this,new listener());
        immagine=(ImageView)findViewById(R.id.imageView1);
        textView=(TextView)findViewById(R.id.textView1);
        
        
        
        bitmap=BitmapFactory.decodeResource(getResources(), R.drawable.calcestruzzo);
        bmpWidth=bitmap.getWidth();
        bmpHeight=bitmap.getHeight();
        drawMatrix();
        
    }
    
    public void drawMatrix(){
     bmpWidth=(int) (bmpWidth*scala);
     bmpHeight=(int) (bmpWidth*scala);
     Bitmap resizedBitmap=Bitmap.createScaledBitmap(bitmap, bmpWidth, bmpHeight, false);
     immagine.setImageBitmap(resizedBitmap);
     textView.setText(bmpWidth+" - "+bmpHeight);
    }
    
    @Override
    public boolean onTouchEvent(MotionEvent event){
     detector.onTouchEvent(event);
  return true;
     
    }
    
    class listener extends ScaleGestureDetector.SimpleOnScaleGestureListener{
     
     @Override
     public boolean onScale(ScaleGestureDetector detector){
      scala=detector.getScaleFactor();
      
      drawMatrix();
   return true;
      
     }
    }
    
}

Me lo annoto prima di giocherellarci sopra...