View Controllers - iOS Programming: The Big Nerd Ranch Guide - page 242

A view controller will destroy its view if the system is running low on memory and if its
view isn't currently on the screen. Run HypnoTime in the simulator. Select the Time tab
so that both views are loaded.
Now, in the simulator, select Simulate Memory Warning from the Hardware menu. This
simulates what happens when the operating system is running low on memory and tells
your application to clean up stuff it isn't using.
Switch back to the Hypnosis tab; notice that the console reports that Hyp-
nosisViewController loaded its view again. Now switch to the Time tab and no-
tice that TimeViewController did not reload its view.
TimeViewController 's view was on the screen when the memory warning oc-
curred, so its view was not destroyed. (Destroying a view that is on the screen would res-
ult in a miserable user experience.) HypnosisViewController 's view , on the other
hand, was not on the screen during the memory warning, so it was destroyed. When you
returned to that view, it was recreated and reloaded.
When the Hypnosis tab bar item was tapped, the UITabBarController asked the
HypnosisViewController for its view so it could add it as a subview of its own
view . HypnosisViewController , like all view controllers, automatically calls
loadView if it is sent the message view and does not yet have its view . The imple-
mentation of UIViewController 's view method looks something like this:
- (UIView *)view
{
if ([self isViewLoaded] == NO)
{
// If I don't currently have a view, then create it
[self loadView];
[self viewDidLoad];
}
// The view is definitely going to exist here, so return it
return view;
}
UIViewController s that load their view from a XIB file follow the same behavior. In
fact, the default implementation of loadView loads the XIB file specified by
initWithNibName:bundle: . So when view controllers that load their views pro-
grammatically override loadView , they intentionally don't call the superclass's imple-
mentation. If they did, it would kick off a search for a XIB file.