UITabBarController子类从不调用setViewControllers：或setViewControllers：animated：

Question

I have a simple question: My custom UITabBarController subclass overrides both setViewControllers: and setViewControllers:animated: . When using my subclass in a storyboard, neither method is called. Why? How is the viewControllers property being set? Can I somehow hook into the moment when they are being set?

My code:

MyTabBarController.h

#import <UIKit/UIKit.h>

@interface MyTabBarController : UITabBarController <UITabBarControllerDelegate>
@end

MyTabBarController.m

#import "MyTabBarController.m"

@implementation MyTabBarController
- (void)viewDidLoad
{
    NSLog(@"I do get called, and at this point I have viewControllers");
}

- (void)setViewControllers:(NSArray *)viewControllers
{
    NSLog(@"in setViewControllers:");
    [super setViewControllers:viewControllers];
}

- (void)setViewControllers:(NSArray *)viewControllers animated:(BOOL)animated
{
    NSLog(@"in setViewControllers:animated:");
    [super setViewControllers:viewControllers animated:animated];
}
@end

Note: I know that my subclass is being used because viewDidLoad gets called.

Answer 1

I agree with @Duncan, that the controllers are set by initWithCoder (or rather loadView ).

Based on that assumption it's kind of obvious why the accessors aren't used: a subclass could override them (and maybe forget the call to super or assume the instance is fully initialized).

Although Apple missed to give advice you can find the recommendation of avoiding accessors during init in Google's Obj-C Styleguide (and I believe I read that Stanford courses recommend that, too). And I heavily doubt that Apple would have written code to emit KVO-events in the initialisation phase.

So unless you need to manipulate the ViewControllers before they are added, why not go with the straight-forward solution?

- (id)initWithCoder:(NSCoder*)aDecoder 
{
    if(self = [super initWithCoder:aDecoder]) 
    {
        [whateverYouWantToDoWithTheControllers:self.viewControllers];
    }
    return self;
}

- (void)setViewControllers:(NSArray *)viewControllers
{
    [super setViewControllers:viewControllers];
    [whateverYouWantToDoWithTheControllers:viewControllers];
}

- (void)setViewControllers:(NSArray *)viewControllers animated:(BOOL)animated
{
    [super setViewControllers:viewControllers animated:animated];
    [whateverYouWantToDoWithTheControllers:viewControllers];
}

Answer 2

From the docs it looks to me like setViewControllers:animated is a method intended for 3rd party developers to switch the list of view controllers installed in aa tab bar controller.

My guess is that when a view controller is loaded from an XIB or a storyboard, it sets itself up using initWithCoder rather than using the public interfaces. It is likely manipulating the instance variable that holds the array of view controllers manually.