What is UIControlEventTouchCancel?

Question

When the user taps and holds down a UIButton in my app, after a while a UIControlEventTouchCancel event is fired. The Apple documentation for UIControl doesn't have any detailed explanation about this event. Why is it triggered? What is it used for?

[button addTarget:key action:@selector(keyUp:) forControlEvents:UIControlEventTouchUpInside];
[button addTarget:key action:@selector(keyUp:) forControlEvents:UIControlEventTouchUpOutside];
[button addTarget:key action:@selector(keyCancel:) forControlEvents:UIControlEventTouchCancel];
[button addTarget:key action:@selector(keyDown:) forControlEvents:UIControlEventTouchDown];

Competence

Answer 1

According to the documentation, a UIControlEventTouchCancel event is due to:

A system event canceling the current touches for the control.

Therefore, it's not due to user input that the touch has ended, it's due to a system event (such as a low memory warning, or the app moving into the background). I have no idea why you are able to trigger it from simply pressing and holding on the button.

I'm pretty sure it's a similar (if not caused by the) event to the UIResponder's touchesCancelled:withEvent: event. The documentation on this is slightly more detailed:

This method is invoked when the Cocoa Touch framework receives a system interruption requiring cancellation of the touch event; for this, it generates a UITouch object with a phase of UITouchPhaseCancel. The interruption is something that might cause the application to be no longer active or the view to be removed from the window

Propst

Memory is fine and the app wasn't moved to the background. I was seeing some anomalies with the button keyDown action and when I added the touch cancel event, it explained the anomaly. However, I can't figure out what is causing the cancel event to trigger. – Competence

hmm... weird. Are you doing anything in the background that could maybe affect the button, such as animations? – Propst

I figured it out! My code was removing the button from the view in another method that was refreshing the display. – Competence

ah, makes sense, glad you found it! – Propst

Answer 2

One scenario is the screen rotation. If you rotate your screen while holding down the button you will get a touch cancel event.

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