UIControl Subclass - Events called twice

Question

I'm currently working on a custom UIControl Subclass. To track the touches I use the following Method:

- (BOOL)beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event {
 NSLog(@"Start");
 CGPoint location = [touch locationInView:self];
 if ([self touchIsInside:location] == YES) {
    //Touch Down
    [self sendActionsForControlEvents:UIControlEventTouchDown];
    return YES;
 }
 else {
    return NO;
 }  
}

This works as expected and @"Start" is loged exactely once. The next step is that I add a Target and a Selector with UIControlEventTouchDown.

[markItem addTarget:self action:@selector(action:) forControlEvents:UIControlEventTouchUpInside];

This works also and the action: method is called. But that's my problem. The action is called twice. What am I doing wrong? I just use [self sendActionsForControlEvents:UIControlEventTouchDown]; once and the target action is called twice. What's wrong with my code?

Sandro Meier

Answer 1

The first call to the action method happens automatically by the event dispatcher once you've called:

[markItem addTarget:self action:@selector(action:) forControlEvents:UIControlEventTouchUpInside];

to register the handler.

So when you then call:

//Touch Down
[self sendActionsForControlEvents:UIControlEventTouchDown];

you are generating the second call to your handler (and any others that are hooked up).

So it seems like you don't need both the action handler and the beginTracking - use one or the other.

Update:

Given your comment and further thought: since you are a subclass of UIControl, I think you probably don't want to be registering for event handlers for yourself.

Instead you should exclusively use:

- (BOOL)beginTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (BOOL)continueTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (void)endTrackingWithTouch:(UITouch *)touch withEvent:(UIEvent *)event;
- (void)cancelTrackingWithEvent:(UIEvent *)event;   // event may be nil if cancelled for non-event reasons, e.g. removed from window

Also the tracking instance variable.

So I think you should not be posting events or listening to events. Further, is it actually possible to get a beginTrackingWithTouch event if it's not in your view? Doesn't seem like it would be. So I don't think you need the testing to see if it's in your view.

So I think it might be worth stepping back and thinking about what you are trying to do and re-reading UIControl documentation. Specifically:

Subclassing Notes You may want to extend a UIControl subclass for either of two reasons:

To observe or modify the dispatch of action messages to targets for particular events To do this, override sendAction:to:forEvent:, evaluate the passed-in selector, target object, or UIControlEvents bit mask, and proceed as required.

To provide custom tracking behavior (for example, to change the highlight appearance) To do this, override one or all of the following methods: beginTrackingWithTouch:withEvent:, continueTrackingWithTouch:withEvent:, endTrackingWithTouch:withEvent:.

The first part is for having your UIControl subclass do non-standard handling of target action processing for clients or users of your control (that doesn't sound like what you are trying to do, though you didn't really give a high-level description).

The second part sounds more like what you are wanting to do - custom tracking within your UIControl subclass.