Question

How do you forcibly cancel a zooming open pinch gesture on a UIScrollView, say when the user has zoomed “sufficiently” far to trigger a new action?

Answer 1

To prevent user-controller zooming and panning but still allow programmatic zooming and panning of a scrollview, the best approach is to override the UIScrollView's –addGestureRecognizer: method in a subclass.

-(void)addGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer
{
    //Prevent any of the default panning and zooming controls from working
    gestureRecognizer.enabled = NO;
    [super addGestureRecognizer:gestureRecognizer];
    return;
}

Each gesture recognizer is simply disabled, for finer control (for ex. allowing the pan control but only allow zooming via a double tap for instance) you’d simply check the incoming gesture recognizer via -isKindOfClass: and disabling as appropriate.

-(void)addGestureRecognizer:(UIGestureRecognizer *)gestureRecognizer 
{
    //Prevent zooming but not panning
    if ([gestureRecognizer isKindOfClass:[UIPinchGestureRecognizer class]]) 
    {
        gestureRecognizer.enabled = NO;
    }
    [super addGestureRecognizer:gestureRecognizer];
    return;
}

Hope this helps.