Implementing pinch-zoom and pan/drag in an Android view on the canvas

by vivin

As far as what I’ve covered so far, here is the code its entirety; I hope you find it useful:

public class ZoomView extends View {

    //These two constants specify the minimum and maximum zoom
    private static float MIN_ZOOM = 1f;
    private static float MAX_ZOOM = 5f;

    private float scaleFactor = 1.f;
    private ScaleGestureDetector detector;

    //These constants specify the mode that we're in
    private static int NONE = 0;
    private static int DRAG = 1;
    private static int ZOOM = 2;

    private int mode;

    //These two variables keep track of the X and Y coordinate of the finger when it first
    //touches the screen
    private float startX = 0f;
    private float startY = 0f;

    //These two variables keep track of the amount we need to translate the canvas along the X
    //and the Y coordinate
    private float translateX = 0f;
    private float translateY = 0f;

    //These two variables keep track of the amount we translated the X and Y coordinates, the last time we
    //panned.
    private float previousTranslateX = 0f;
    private float previousTranslateY = 0f;    

    public ZoomView(Context context) {
        super(context);
        detector = new ScaleGestureDetector(getContext(), new ScaleListener());
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

        switch (event.getAction() & MotionEvent.ACTION_MASK) {

            case MotionEvent.ACTION_DOWN:
                mode = DRAG;

                //We assign the current X and Y coordinate of the finger to startX and startY minus the previously translated
                //amount for each coordinates This works even when we are translating the first time because the initial
                //values for these two variables is zero.               
                startX = event.getX() - previousTranslateX;
                startY = event.getY() - previousTranslateY;
                break;

            case MotionEvent.ACTION_MOVE:               
                translateX = event.getX() - startX;
                translateY = event.getY() - startY;
 
                //We cannot use startX and startY directly because we have adjusted their values using the previous translation values. 
                //This is why we need to add those values to startX and startY so that we can get the actual coordinates of the finger.
                double distance = Math.sqrt(Math.pow(event.getX() - (startX + previousTranslateX), 2) + 
                                            Math.pow(event.getY() - (startY + previousTranslateY), 2)
                                           );
 
                if(distance > 0) {
                   dragged = true;
                }               

                break;

            case MotionEvent.ACTION_POINTER_DOWN:
                mode = ZOOM;
                break;
   
            case MotionEvent.ACTION_UP:
                mode = NONE;
                dragged = false;

                //All fingers went up, so let's save the value of translateX and translateY into previousTranslateX and 
                //previousTranslate           
                previousTranslateX = translateX;
                previousTranslateY = translateY;
                break;

            case MotionEvent.ACTION_POINTER_UP:
                mode = DRAG;

                //This is not strictly necessary; we save the value of translateX and translateY into previousTranslateX
                //and previousTranslateY when the second finger goes up
                previousTranslateX = translateX;
                previousTranslateY = translateY;
                break;       
        }

        detector.onTouchEvent(event);

        //We redraw the canvas only in the following cases:
        //
        // o The mode is ZOOM 
        //        OR
        // o The mode is DRAG and the scale factor is not equal to 1 (meaning we have zoomed) and dragged is
        //   set to true (meaning the finger has actually moved)
        if ((mode == DRAG && scaleFactor != 1f && dragged) || mode == ZOOM) {
            invalidate();
        }

        return true;
    }

    @Override
    public void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        canvas.save();

        //We're going to scale the X and Y coordinates by the same amount
        canvas.scale(scaleFactor, scaleFactor);

        //If translateX times -1 is lesser than zero, let's set it to zero. This takes care of the left bound
        if((translateX * -1) < 0) {
           translateX = 0;
        }

        //This is where we take care of the right bound. We compare translateX times -1 to (scaleFactor - 1) * displayWidth.
        //If translateX is greater than that value, then we know that we've gone over the bound. So we set the value of 
        //translateX to (1 - scaleFactor) times the display width. Notice that the terms are interchanged; it's the same
        //as doing -1 * (scaleFactor - 1) * displayWidth
        else if((translateX * -1) > (scaleFactor - 1) * displayWidth) {
           translateX = (1 - scaleFactor) * displayWidth;
        }

        if(translateY * -1 < 0) {
           translateY = 0;
        }

        //We do the exact same thing for the bottom bound, except in this case we use the height of the display
        else if((translateY * -1) > (scaleFactor - 1) * displayHeight) {
           translateY = (1 - scaleFactor) * displayHeight;
        }

        //We need to divide by the scale factor here, otherwise we end up with excessive panning based on our zoom level
        //because the translation amount also gets scaled according to how much we've zoomed into the canvas.
        canvas.translate(translateX / scaleFactor, translateY / scaleFactor);

        /* The rest of your canvas-drawing code */
        canvas.restore();
    }

    private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
        @Override
        public boolean onScale(ScaleGestureDetector detector) {
            scaleFactor *= detector.getScaleFactor();
            scaleFactor = Math.max(MIN_ZOOM, Math.min(scaleFactor, MAX_ZOOM));
            return true;
        }
    }
}

UPDATE

Chris Rogers figured out how to make sure that the display stays centered on the screen. In the canvas.scale(…) call, do the following instead:

canvas.scale(this.scaleFactor, this.scaleFactor, this.detector.getFocusX(), this.detector.getFocusY());