UIScrollView zoom by tap

tapToZoom.m

# define DEFAULT_ZOOM_SCALE 5.0
# define SCALE_RATE 2.0

- (IBAction)onDoubleTapOnMap:(UITapGestureRecognizer *)sender {
    
    float zoom = self.scrollView.zoom;
    
    CGPoint tapPoint = [sender locationInView:self.scrollView];

    // Zoom=self.scrollView.zoom → Zoom=1.0 
    tapPoint.x /= zoom;
    tapPoint.y /= zoom;

    if (zoom >= self.scrollView.maximumZoomScale) {
        // already max zoom : set zoom to default
        [self.scrollView setZoomScale:DEFAULT_ZOOM_SCALE animated:YES];
    } else {
        // zoom up

        float newZoom = zoom * SCALE_RATE;
        if (newZoom >= self.scrollView.maximumZoomScale) {
            newZoom = self.scrollView.maximumZoomScale;
        }

        CGRect newRect = [self zoomRectForScrollView:self.scrollView withScale:newZoom withCenter:tapPoint];
        [self.scrollView zoomToRect:newRect animated:YES];
    }
}

// http://developer.apple.com/library/ios/#documentation/WindowsViews/Conceptual/UIScrollView_pg/ZoomZoom/ZoomZoom.html#//apple_ref/doc/uid/TP40008179-CH102-SW7
- (CGRect)zoomRectForScrollView:(UIView *)scrollView withScale:(float)scale withCenter:(CGPoint)center {
    
    CGRect zoomRect;
    
    // The zoom rect is in the content view's coordinates.
    // At a zoom scale of 1.0, it would be the size of the
    // imageScrollView's bounds.
    // As the zoom scale decreases, so more content is visible,
    // the size of the rect grows.
    zoomRect.size.height = scrollView.frame.size.height / scale;
    zoomRect.size.width  = scrollView.frame.size.width  / scale;
    
    // choose an origin so as to get the right center.
    zoomRect.origin.x = center.x - (zoomRect.size.width  / 2.0);
    zoomRect.origin.y = center.y - (zoomRect.size.height / 2.0);
    
    return zoomRect;
}