Programming iOS 6, 3rd Edition

Chapter 7. Nib Management

A nib file, or simply nib, is a file containing a drawing of a piece of your interface. The term nib is not really an English word (it has nothing to do with fountain pens or bits of chocolate); it is based on the file extension .nib, which originated as an acronym for “NeXTStep Interface Builder”. Nowadays, you will usually develop your interface using a file format whose extension is .xib; when your app is built, your target’s .xib files are translated (“compiled”) into .nib format (Chapter 6). But a .xib file is still referred to as a nib file. I will speak of the same nib file as having either a .xib extension (if you’re editing it) or a .nib extension (if it’s in the built app).

You construct your program in two ways — writing code, and drawing the interface. But these are really two ways of accomplishing the same ends; drawing the interface is a way of writing code. When the app runs and your drawing of the interface in a nib file is loaded, it is translated into instructions for instantiating and initializing the objects in the nib file. You could equally have instantiated and initialized those same objects in code. (This point is crucial; see Nib-Based Instantiation.) Indeed, deciding whether to create an interface object in code or through a nib file is not always easy; each approach has its advantages. The important thing is to understand how interface objects drawn in a nib file are instantiated and connected to your code when the app runs. ...