iPhone: Click view behind transparent UIScrollView

Question

I have a UIScrollView that is set to have a clear background. Part of the scrollview does have content, but part does not (so it shows other views behind it). I would like to be able to click through the UIScrollView and to the MKMapView behind, but only for the transparent portion of the UIScrollView.

I have found some code which I am having a real hard time understanding how to get working:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    if (![self yourMethodThatDeterminesInterestingTouches:touches withEvent:event])
        [self.nextResponder touchesBegan:touches withEvent:event]; 
}

Could someone help me wrap my mind around how to forward a touch event to a view that is behind another view? Can I call - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event from a UIViewController?

Answer 1

Basically, it's up to you to determine what touch events you care to forward to another responder. If you simply want to forward all touch events, just remove that if statement in the code you posted so the next responder will receive all the touch events.

Answer 2

What we did was to subclass UIScrollView and implement logic that passes responsibility down to views under it, if the touch happens inside of the transparent area.

In our case the transparent area is defined by contentOffset of 120 on Y axis, meaning our content starts 120 points below the start of the UIScrollView, and the code looks like this:

- (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event
{
    if (self.contentOffset.y < 0 && point.y < 0.0) {
        return NO;
    } else {
        return YES;
    }
}

Obviously this response is well past its prime but hopefully this is helpful to anyone searching for a solution.