Defining Custom Gestures

You've now seen how to detect the most commonly used iPhone gestures. The real fun begins when you start defining your own custom gestures! You've already learned how to use a few of UIGestureRecognizer's subclasses, so now it's time to learn how to create your own gestures, which can be easily attached to any view you like.

Defining a custom gesture is tricky. You've already mastered the basic mechanism, and that wasn't too difficult. The tricky part is being flexible when defining what constitutes a gesture.

Most people are not precise when they use gestures. Remember the variance we used when we implemented the swipe, so that even a swipe that wasn't perfectly horizontal or vertical still counted? That's a perfect example of the subtlety you need to add to your own gesture definitions. If you define your gesture too strictly, it will be useless. If you define it too generically, you'll get too many false positives, which will frustrate the user. In a sense, defining a custom gesture can be hard because you must be precise about a gesture's imprecision. If you try to capture a complex gesture like, say, a figure eight, the math behind detecting the gesture is also going to get quite complex.