Programming iOS 11

Chapter 5. Touches

[Winifred the Woebegone illustrates hit-testing:] Hey nonny nonny, is it you? — Hey nonny nonny nonny no! — Hey nonny nonny, is it you? — Hey nonny nonny nonny no!

Marshall Barer, Once Upon a Mattress

A touch is an instance of the user putting a finger on the screen. The system and the hardware, working together, know when a finger contacts the screen and where it is. A finger is fat, but its location is cleverly reduced to a single point.

A UIResponder is a potential recipient of touches. A UIView is a UIResponder, and is thus the visible recipient of touches. There are other UIResponder subclasses, but none of them is visible on the screen. The user sees a view by virtue of its underlying layer; the user touches a view by virtue of the fact that it is a UIResponder.

A touch is represented as an object (a UITouch instance) which is bundled up in an envelope (a UIEvent) which the system delivers to your app. It is then up to your app to deliver the envelope to the appropriate UIView. In the vast majority of cases, this will happen automatically the way you expect, and you will respond to a touch by way of the view in which the touch occurred.

In fact, usually you won’t concern yourself with UIEvents and UITouches at all. Most built-in interface views deal with these low-level touch reports themselves, and notify your code at a higher level — you hear about functionality and intention rather than raw touches. When a UIButton emits an action message to report ...