iOS 9 Programming Fundamentals with Swift

Chapter 11. Cocoa Events

All of your app’s executable code lies in its functions. The impetus for a function being called must come from somewhere. One of your functions may call another, but who will call the first function in the first place? How, ultimately, will any of your code ever run? As I said in Chapter 6, after your app has launched, “UIApplicationMain is just sitting there, watching for the user to do something, maintaining the event loop, which will respond to user actions as they occur.”

The event loop is the key. The runtime is watching and waiting for certain things to happen, such as the user making a gesture on the screen, or some specific stage arriving in the lifetime of your app. When such things do happen, the runtime calls your code. But the runtime can call your code only if your code is there to be called. Your code is like a panel of buttons, waiting for Cocoa to press one. If something happens that Cocoa feels your code needs to know about and respond to, it presses the right button — if the right button is there.

The art of Cocoa programming lies in knowing what Cocoa wants to do. You organize your code, right from the start, with Cocoa’s behavior in mind. Cocoa makes certain promises about how and when it will dispatch messages to your code. These are Cocoa’s events. You know what these events are, and you arrange for your code to be ready when Cocoa delivers them.

The specific events that you can receive are listed in the documentation. The overall architecture ...