Programming iOS 6, 3rd Edition

Nib Loading and File’s Owner

A nib file is useless until your app runs and the nib file is loaded. If a nib is designated by the Info.plist key “Main nib file base name” (NSMainNibFile, see Chapter 6), it is loaded automatically as the app launches; but this is an exceptional case, and has now fallen out of favor — there are no automatically loaded main nib files in the current project templates. In general, nibs are loaded explicitly as needed while the app runs. In our Empty Window application, you can actually see where this happens, in AppDelegate.m:

self.viewController =
    [[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];

That line of code does several things, one of which is that (for reasons to be explained more fully in Chapter 19) it causes the nib named @"ViewController" (i.e., the nib file compiled from ViewController.xib, the nib file we’ve been editing) to be loaded, and the resulting views to be put into our app’s interface — which is how we were able to obtain the outcome shown in Figure 7-4.

So a nib is not loaded until the app runs and our code decides, at some point in the life of the app, that that nib is needed. This architecture is a source of great efficiency. For example, imagine our app has two complete sets of interface, and the user might never ask to see the second one. It makes obvious sense not to load a nib containing the second set of interface until the user does ask to see it. By this strategy, a nib is loaded when its instances are ...