Programming iOS 4

Chapter 18. 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. (Fingers are fat, but the system and the hardware cleverly reduce the finger’s location to a single appropriate point.)

A UIView, by virtue of being a UIResponder, is the visible locus of touches. There are other UIResponder subclasses, but none of them is visible on the screen. What the user sees are views; what the user is touching are views. (The user may also see layers, but a layer is not a UIResponder and is not involved with touches. I’ll talk later about how to make it seem as if the user can touch a layer.)

It would make sense, therefore, if every touch were reported directly to the view in which it occurred. However, what the system “sees” is not particular views but an app as a whole. So 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 an 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 ...