Question

I’m fairly new to UI programming on the Mac and iPhone, and I’ve run across something that somewhat puzzles me.

A UIViewController has 3 methods that involve the initialization of it and its view:

1. init (and init-like methods)
2. loadView
3. viewDidLoad (delegate method)

I’d expect these to occur in the order above. First UIViewController is alloc’ed by some other object, then init is immediately called (or some other init method, like initWithStyle).

Only once the object is initialized would I expect it to call its own loadView function, after which the view, once loaded, calls the viewDidLoad delegate method.

This doesn’t happen, for instance:

@implementation UIViewControllerSubclass

- (id)init {
        NSLog(@"0");
    if (self = [super init]) {
        NSLog(@"1");
    }
    return self;
}

- (void)loadView {
    [super loadView];
    NSLog(@"2");
}

- (void)viewDidLoad {
    [super viewDidLoad];
    NSLog(@"3");
}

@end

Produces the console output:

0
2
3
1

The loadView and viewDidLoad methods, therefore, cannot make delegate calls, as the delegate is usually set after the call to [super init], which (as shown above) is called after loadView and viewDidLoad have run:

UIViewControllerSubClass *someViewController = [[UIViewControllerSubclass alloc] init];
[viewController setDelegate:self];

If I want to run code that sets up the ViewController in some way, notifying the delegate as it goes, should the code reside in the init method? Isn’t the reason for loadView existing to allow such code to be run at the appropriate moment?

It looks to me like I’ll have to create a new initWithDelegate method which sets the delegate ivar before calling [super init], is this right, or am I going about this the wrong way?

Thanks in advance 🙂

Answer 1

The view loading system on the iPhone works like this:

When you initialize a view controller (either with -init or -initWithNibName:bundle:), it doesn’t actually create and initialize the view. When you call -view for the first time, it calls -loadView. By default, -loadView just loads the view from the xib file (nibName). If you override this, though, you’re responsible for creating the view and assigning it to the view controller’s view property. As an example:

- (void)loadView
{
   UIView *view = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] applicationFrame]];
   // add subviews 
   self.view = view;
   [view release];
}

Every time you create the view, which is different from the view becoming visible and showing onscreen, it calls -viewDidLoad. (-viewDidAppear/-viewDidDisappear is for the visibility of the view on-screen)

Since we’re already off-track, let’s consider memory management. When the view is offscreen, the system will automatically set the view property of a view controller to nil. The problem is that all the subviews of that view are leaking. How so? Well, the retain count for each subview is 2 (views retain subviews, and your view controller has an outlet/ivar to it). When the view is nil, the retain count of that view is 1. It doesn’t make sense for a view to stick around if a view isn’t showing, so you set it to nil in -viewDidUnload (which is a hook for whenever the view is set to nil).