Implementing a Custom Gesture Recognizer

When the built-in UIKit gesture recognizers do not provide the behavior you want, you can define custom gesture recognizers. UIKit defines highly configurable gesture recognizers to handle touch sequences for taps, long presses, pans, swipes, rotations, and pinches. For other touch sequences, or to handle gestures that involve button presses, you can define a custom gesture recognizer.

You might also use a custom gesture recognizer to simplify the event-handling code in your app. For example, the Leveraging Touch Input for Drawing Apps sample uses a gesture recognizer to capture input and display it onscreen, as shown in Figure 1.

To define a custom gesture recognizer, subclass UIGestureRecognizer (or one of its subclasses). At the top of your source file, import the UIGestureRecognizerSubclass.h header file (for Objective-C) or the UIKit.UIGestureRecognizerSubclass module (for Swift), as shown in Listing 1. This header file defines the methods and properties that you must override to implement your custom gesture recognizer.

import UIKit
import UIKit.UIGestureRecognizerSubclass

In your custom subclass, implement whatever methods you need to process events. For example, if your gesture consists of touch events, implement the touchesBegan(_:with:), touchesMoved(_:with:), touchesEnded(_:with:), and touchesCancelled(_:with:) methods. Use incoming events to update the state property of your gesture recognizer. UIKit uses the gesture recognizer states to coordinate interactions with other objects in your interface.