R – Can you swap the NIB file for a UIViewController that’s already on-screen

There should be no problem using the same NIB file for multiple controllers when each one is initialized using initWithNibName:bundle:. You don't normally load the controller's own nib file from within the controller though.

If you load a nib file using loadNibNamed::: then you get a NSArray with the objects defined therein, so you can't use it as a view directly. One way to get a view is to search through the array using for() or something to find the object you want, but if you set owner:self then it should connect to outlets connected to File's Owner in self as File's Owner will be self. But you can discard the return value in this case; you don't need the returned array. That may be your main problem (clobbering the outlet with the array) if you have that outlet connected.

It is possible to use loadNibNamed to load a specific view object (assuming you pick it out of the returned array), then display it somehow, but it's usually easier to use initWithNibName on the controller (in which case File's Owner will be the controller).

Oh, and you can also set the nib file for a controller in Interface Builder. There shouldn't be a problem with using the same nib for multiple controllers since essentially you'd just be telling Interface Builder to set up the nib file to do something like initWithNibNamed. Click the controller object and check the inspector window.

Update

I'd probably do this in order to use only one view in multiple controllers:

    NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"FooView" 
                                                 owner:self options:nil];
    for (id obj in nib)
        if ([obj isKindOfClass:[FooView class]])
            myNewView = (FooView *)obj;

(Stolen from Chapter 8: Cells 2 in the sample code from Beginning iPhone 3 Development.)

You could do this with outlets in IB, in which case you could leave out the for loop, but you'll probably need a superclass for both controllers declaring the outlet, and something to tell Interface Builder that File's Owner is an instance of that superclass so it knows about the outlet. Probably not worth the trouble.

Normally, you should only use a specific nib in the init method, and not the initWithNibName:bundle:, for this reason.

@implementation MotherViewController
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    if((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil])) {
        //custom initialization
    }
    return self;
}
- (id)init {
    return [self initWithNibName:@"MotherViewController" bundle:nil];
}

Then, to use the default nib for MotherViewController, you just use [[MotherViewController alloc] init];.

As an alternate, you could define a different initializer in MotherViewController for this reason.

@implementation MotherViewController
- (id)_initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    if((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil])) {
        //custom initialization
    }
    return self;
}
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    return [self _initWithNibName:@"MotherViewController" bundle:nibBundleOrNil];
}

Then, use a private category interface to tell SonViewController about this method.

//SonViewController.m
@interface MotherViewController (PrivateInit)
- (id)_initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil;
@end
@implementation SonViewController
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil {
    if((self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil])) {
        //custom initialization
    }
    return self;
}