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Note About ViewController Containment

📓

After learning from WWDC session about the view controller containment topic, I'm still curious that what exactly the methods like addChildViewController and removeFromParentViewController do. Therefore I did some experiment to figure it. First, setup some log message in my view controller to see what does it happen.

- (void)viewWillAppear:(BOOL)animated
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)viewDidAppear:(BOOL)animated
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)viewWillDisappear:(BOOL)animated
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)viewDidDisappear:(BOOL)animated
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
}

Then I did add a subview of childviewcontroller into root viewcontroller without calling addChildViewController method.

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    self.childVC = [VCChildViewController alloc] init];
    [self.view addSubview:self.childVC.view];
}

The log console showed that

2016-03-11 00:04:40.113 VCContainment[69157:4688892] -[VCParentViewController viewWillAppear:]
2016-03-11 00:04:40.114 VCContainment[69157:4688892] -[VCChildViewController viewWillAppear:]
2016-03-11 00:04:40.116 VCContainment[69157:4688892] -[VCChildViewController viewDidAppear:]
2016-03-11 00:04:40.116 VCContainment[69157:4688892] -[VCParentViewController viewDidAppear:]

To compare the above, I did the same thing and call addChildViewController method.

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    self.childVC = [VCChildViewController alloc] init];
    [self addChildViewController:self.childVC];
    [self.childVC didMoveToParentViewController:self];
    [self.view addSubview:self.childVC.view];
}

The log console showed that

2016-03-11 00:03:11.751 VCContainment[68100:4685301] -[VCParentViewController viewWillAppear:]
2016-03-11 00:03:11.752 VCContainment[68100:4685301] -[VCChildViewController viewWillAppear:]
// This time, VCParentViewController's `viewDidAppear` called earlier than VCChildViewController's
2016-03-11 00:03:11.755 VCContainment[68100:4685301] -[VCParentViewController viewDidAppear:]
2016-03-11 00:03:11.755 VCContainment[68100:4685301] -[VCChildViewController viewDidAppear:]

Thus, we could find that appearance callback would be called on the right moment with calling method addChildViewController. (Of course, it also need to be called with method didMoveToParentViewController)

NOTE: With parent and child relationship between VC, we could access VC.navigationController, VC.parentViewController and VC.childViewControllers properly.

Reference