Question

I have transparent UIView (we’ll call it receiver) layered over a UIScrollView (scrollView) which has a UIScrollViewDelegate (scrollViewDelegate).

In some situations, receiver needs to consume the touch events which land on it. In others, I need to glean some positional information and then either pass the touch events intact through to scrollView or alternatively spoof the events which would comprise a drag movement and result in the appropriate deceleration etc. How can I achieve this?

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EDIT – Research thus far:

I’ve put a breakpoint inside scrollViewDelegate‘s scrollViewWillBeginDragging, which reveals the call sequence to be:

(...various calls filtering down from UIApplicationMain...)
[UIWindow sendEvent:]
[UIWindow _sendGesturesForEvent:]
...
[UIGestureRecognizer _delayedUpdateGesture:]
[UIGestureRecognizer _updateGestureWithEvent]
_UIGestureRecognizerSendActions
[UIScrollView handlePan:]
[UIScrollView _updatePanGesture]
....
[UIScrollViewDelegate scrollViewWillBeginDragging:]

Ideally, I’d like to be able to call into UIScrollView handlePan: with the appropriate information, but I can’t find anything which details how to correctly call handlePan (I assume it must be possible, since _UIGestureRecognizerSendActions does it?).

Alternatively I suppose I could subclass UIGestureRecognizer, but this seems a pretty heavy handed way of doing it (and again I have no idea how to communicate the sequence of touch events).

Answer 1

The easiest way to do this is to place your UIScrollView inside of your receiver.

You can then do one of two things:

• Set userInteractionsEnabled on your scroll view to NO if you know ahead-of-time that the events shouldn’t be delivered to it. The events will be delivered to your receiver view instead.

• Override hitTest:withEvent: to note the incoming events. To squelch them, just return your receiver (indicating that it should be the target of those events); to pass them along, return the value from super’s implementation (which will correctly identify the scroll view as the target when appropriate).

If you really need to synthesize events, you can construct them and pass them to UIApplication‘s - (void)sendEvent: method. (Calling handlePan: directly probably won’t work, since it takes a UIGestureRecognizer and so will be hard to fake.)