The Core iOS Developer’s Cookbook, Fifth Edition

Recipe: Adding Pan Gesture Recognizers

With gesture recognizers, you can achieve the same kind of interaction shown in Recipe 1-1 without working quite so directly with touch handlers. Pan gesture recognizers detect dragging gestures. They allow you to assign a callback that triggers whenever iOS senses panning.

Recipe 1-2 mimics Recipe 1-1’s behavior by adding a recognizer to the view when it is first initialized. As iOS detects the user dragging on a DragView instance, the handlePan: callback updates the view’s center to match the distance dragged.

This code uses what might seem like an odd way of calculating distance. It stores the original view location in an instance variable (previousLocation) and then calculates the offset from that point ...