Can we use scale gesture detector for pinch zoom in Android?

Question

Can we use scale gesture detector for pinch zoom in Android?

Answer 1

You can create a reusable class that implements OnTouchListener to accomplish this.

public class MyScaleGestures implements OnTouchListener, OnScaleGestureListener {       
    private View view;
    private ScaleGestureDetector gestureScale;
    private float scaleFactor = 1;  
    private boolean inScale = false;

    public MyScaleGestures (Context c){ gestureScale = new ScaleGestureDetector(c, this); }

    @Override
    public boolean onTouch(View view, MotionEvent event) {
        this.view = view; 
        gestureScale.onTouchEvent(event);
        return true;
    }   

    @Override
    public boolean onScale(ScaleGestureDetector detector) {
        scaleFactor *= detector.getScaleFactor();
        scaleFactor = (scaleFactor < 1 ? 1 : scaleFactor); // prevent our view from becoming too small //
        scaleFactor = ((float)((int)(scaleFactor * 100))) / 100; // Change precision to help with jitter when user just rests their fingers //
        view.setScaleX(scaleFactor);
        view.setScaleY(scaleFactor);
        return true;
    }

    @Override
    public boolean onScaleBegin(ScaleGestureDetector detector) {
        inScale = true;
        return true;
    }

    @Override
    public void onScaleEnd(ScaleGestureDetector detector) { inScale = false; }
}

Then assign it as your View's OnTouchListener like so.

myView.setOnTouchListener(new MyScaleGestures(context));

If you want to add a scrolling ability to the View you will need to implement onScroll from the OnGestureListener interface. You can add this override to the MyScaleGestures class to accomplish this.

@Override
public boolean onScroll(MotionEvent event1, MotionEvent event2, float x, float y) {
    float newX = view.getX();
    float newY = view.getY();
    if(!inScale){
        newX -= x;
        newY -= y;
    }
    WindowManager wm = (WindowManager) view.getContext().getSystemService(Context.WINDOW_SERVICE);
    Display d = wm.getDefaultDisplay();
    Point p = new Point();
    d.getSize(p);

    if (newX > (view.getWidth() * scaleFactor - p.x) / 2){
        newX = (view.getWidth() * scaleFactor - p.x) / 2;
    } else if (newX < -((view.getWidth() * scaleFactor - p.x) / 2)){
        newX = -((view.getWidth() * scaleFactor - p.x) / 2);
    }

    if (newY > (view.getHeight() * scaleFactor - p.y) / 2){
        newY = (view.getHeight() * scaleFactor - p.y) / 2;
    } else if (newY < -((view.getHeight() * scaleFactor - p.y) / 2)){
        newY = -((view.getHeight() * scaleFactor - p.y) / 2);
    }

    view.setX(newX);
    view.setY(newY);

    return true;
}

The end result of doing all of the above will give you class like this one:

public class StandardGestures implements OnTouchListener, OnGestureListener, OnDoubleTapListener, OnScaleGestureListener {
    private View view;
    private GestureDetector gesture;
    private ScaleGestureDetector gestureScale;
    private float scaleFactor = 1;
    private boolean inScale;

    public StandardGestures(Context c){
        gesture = new GestureDetector(c, this);
        gestureScale = new ScaleGestureDetector(c, this);
    }

    @Override
    public boolean onTouch(View view, MotionEvent event) {
        this.view = view;
        gesture.onTouchEvent(event);
        gestureScale.onTouchEvent(event);
        return true;
    }

    @Override
    public boolean onDown(MotionEvent event) {
        return true;
    }

    @Override
    public boolean onFling(MotionEvent event1, MotionEvent event2, float x, float y) {
        return true;
    }

    @Override
    public void onLongPress(MotionEvent event) {
    }

    @Override
    public boolean onScroll(MotionEvent event1, MotionEvent event2, float x, float y) {
        float newX = view.getX();
        float newY = view.getY();
        if(!inScale){
            newX -= x;
            newY -= y;
        }
        WindowManager wm = (WindowManager) view.getContext().getSystemService(Context.WINDOW_SERVICE);
        Display d = wm.getDefaultDisplay();
        Point p = new Point();
        d.getSize(p);

        if (newX > (view.getWidth() * scaleFactor - p.x) / 2){
            newX = (view.getWidth() * scaleFactor - p.x) / 2;
        } else if (newX < -((view.getWidth() * scaleFactor - p.x) / 2)){
            newX = -((view.getWidth() * scaleFactor - p.x) / 2);
        }

        if (newY > (view.getHeight() * scaleFactor - p.y) / 2){
            newY = (view.getHeight() * scaleFactor - p.y) / 2;
        } else if (newY < -((view.getHeight() * scaleFactor - p.y) / 2)){
            newY = -((view.getHeight() * scaleFactor - p.y) / 2);
        }

        view.setX(newX);
        view.setY(newY);

        return true;
    }

    @Override
    public void onShowPress(MotionEvent event) {
    }

    @Override
    public boolean onSingleTapUp(MotionEvent event) {
        return true;
    }

    @Override
    public boolean onDoubleTap(MotionEvent event) {
        view.setVisibility(View.GONE);
        return true;
    }

    @Override
    public boolean onDoubleTapEvent(MotionEvent event) {
        return true;
    }

    @Override
    public boolean onSingleTapConfirmed(MotionEvent event) {
        return true;
    }

    @Override
    public boolean onScale(ScaleGestureDetector detector) {

        scaleFactor *= detector.getScaleFactor();
        scaleFactor = scaleFactor < 1 ? 1 : scaleFactor; // prevent our image from becoming too small
        scaleFactor = (float) (int) (scaleFactor * 100) / 100; // Change precision to help with jitter when user just rests their fingers //
        view.setScaleX(scaleFactor);
        view.setScaleY(scaleFactor);
        onScroll(null, null, 0, 0); // call scroll to make sure our bounds are still ok //
        return true;
    }

    @Override
    public boolean onScaleBegin(ScaleGestureDetector detector) {
        inScale = true;
        return true;
    }

    @Override
    public void onScaleEnd(ScaleGestureDetector detector) {
        inScale = false;
        onScroll(null, null, 0, 0); // call scroll to make sure our bounds are still ok //
    }
}