UIKit-ViewAppear/Disappear

Sometimes we are confused about the firing of viewAppear/Disappear.

Basically, they are fired when super-view is on-screen/off-screen. (You can set a breakpoint to check the stack)

What’s more, we feel like some ViewControllers just don’t get the viewAppear/Disappear message, we can find out why.

Don’t AddSubview Before ParentViewController Appear ?

Occasionally we have logics in viewDidAppear like adding a observer & remove it in viewDidDisappear.

** Don’t do it in WillAppear/WillDisappear, because they are not pairing fired in some circumstance.**

e.g. During interactive transitioning

Let’s check this out with a simple example

UIViewController *vc1 = [[UIViewController alloc] init];
UIViewController *vc2 = [[UIViewController alloc] init];
UIViewController *vc3 = [[UIViewController alloc] init];

UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:vc1];
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [navigationController pushViewController:vc2 animated:YES];
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    [navigationController pushViewController:vc3 animated:YES];
});

The code above will log out like this.

2016-04-27 10:49:46.694 TestMe[67943:4951489] vc1 viewWillAppear
2016-04-27 10:49:51.692 TestMe[67943:4951489] vc2 viewWillAppear
2016-04-27 10:49:56.693 TestMe[67943:4951489] vc3 viewWillAppear

Sometimes we got codes like this, in vc3

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    self.vc4 = [[UIViewController alloc] init];
    [self.view addSubview:self.vc4.view];
}

But vc4 viewWillAppear will NOT be fired!

We can try again with a delay & see what happens

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        self.vc4 = [[UIViewController alloc] init];
        [self.view addSubview:self.vc4.view];
    });
}

Now we got the callback!

So if you try to addSubview before parentViewController viewWillAppear, then you won’t got the viewWillAppear callback.

Sometims we might solve it like this

- (void)viewDidLoad {
    [super viewDidLoad];

    self.vc4 = [[UIViewController alloc] init];
    [self.view addSubview:self.vc4.view];
}

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];

    [self.vc4 viewWillAppear:animated];
}

We won’t get the viewWillAppear from iOS, so sometimes we call it ourseleves, simple and problem solved.

But it’s crazy & stupid to do that if there is more than 3-level views!

The right way to achieve what we want

- (void)viewDidLoad {
    [super viewDidLoad];

    self.vc4 = [[UIViewController alloc] init];
    [self.view addSubview:self.vc4.view];
    
    // We have one more step here
    [self addChildViewController:self.vc4]
    [self.vc4 didMoveToParentViewController:self];

}

This time we also get the appear callback!

Now we know that iOS connect ViewControllers with addChildViewController & removeParentViewController, so the appear/disappear message can pass from parent to sons.

It’s very important for you when you assemble ViewControllers in a ParentViewController, if you don’t connect them, you will miss some viewAppear/Disappear message when using TabBarController or NavigationController like this.

- navigation
    1. push A-VC                     // receive view appear
             |
             -- AA-VC                // NOT receive view appear
    2. push B-VC                     // receive view appear
             |  
        (B-VC addChildViewController: BB-VC)
             -- BB-VC                // receive view appear
    3. push C-VC                     // receive view appear
             

If you have customized your own TabBarController or NavigationController, don’t forget to connect them.

As last, remember to call removeParentViewController or it might cause a leak.

For More informations from Apple

Adding a Child View Controller to Your Content

To incorporate a child view controller into your content programmatically, create a parent-child relationship between the relevant view controllers by doing the following:

1. Call the addChildViewController: method of your container view controller. This method tells UIKit that your container view controller is now managing the view of the child view controller.
2. Add the child’s root view to your container’s view hierarchy. Always remember to set the size and position of the child’s frame as part of this process.
3. Add any constraints for managing the size and position of the child’s root view.
4. Call the didMoveToParentViewController: method of the child view controller.

- (void) displayContentController: (UIViewController*) content {
   [self addChildViewController:content];
   content.view.frame = [self frameForContentController];
   [self.view addSubview:self.currentClientView];
   [content didMoveToParentViewController:self];
}

Removing a Child View Controller

To remove a child view controller from your content, remove the parent-child relationship between the view controllers by doing the following:

1. Call the child’s willMoveToParentViewController: method with the value nil.
2. Remove any constraints that you configured with the child’s root view.
3. Remove the child’s root view from your container’s view hierarchy.
4. Call the child’s removeFromParentViewController method to finalize the end of the parent-child relationship.

- (void) hideContentController: (UIViewController*) content {
   [content willMoveToParentViewController:nil];
   [content.view removeFromSuperview];
   [content removeFromParentViewController];
}

Apple - View Controller Programming Guide for iOS

Shared by

Ivan Chan (aintivanc@icloud.com)