iphone - iOS: Can I override pinch in/out behavior of UIScrollView?

Question

I'm drawing a graph on a UIView, which is contained by a UIScrollView so that the user can scroll horizontally to look around the entire graph.

Now I want to zoom the graph when a user pinches in with two fingers, but instead of zooming in a view with the same rate for X and Y direction, I want to zoom only in the X direction by changing the X scale, without changing the Y scale.

I think I have to catch the pinch in/out gesture and redraw the graph, overriding the default zooming behavior.

But is there a way to do this?

I've been having a very difficult time to catch the pinch gesture on the UIScrollView, as it cancels the touches when it starts to scroll. I want the zooming to work even after the UIScrollView cancels the touches. :(

Thanks, Kura

Answer 1

Although you cannot delete the existing pinch gesture recognizer, you can disable it and then add your own:

// Disable existing recognizer
for (UIGestureRecognizer* recognizer in [_scrollView gestureRecognizers]) {
    if ([recognizer isKindOfClass:[UIPinchGestureRecognizer class]]) {
        [recognizer setEnabled:NO];
    }
}

// Add our own
UIPinchGestureRecognizer* pinchRecognizer = 
  [[UIPinchGestureRecognizer alloc] initWithTarget:self 
                                            action:@selector(pinch:)];
[_scrollView addGestureRecognizer:pinchRecognizer];
[pinchRecognizer release];

Then in

- (void) pinch:(UIPinchGestureRecognizer*)recognizer { .. }

use

[recognizer locationOfTouch:0 inView:..]
[recognizer locationOfTouch:1 inView:..]

to figure out if the user is pinching horizontally or vertically.