Preface

It’s been a long time since I used the less common APIs of UIViewController, and I’ve gradually lost my memory of them. In iOS apps, pages with multiple childViewControllers are a very common interaction design — the early NetEase News, Toutiao, and so on. This post reviews the old APIs for manually controlling the life cycle of view controllers.

UIViewController

When using addChildViewController:, we run into a question: how to manually control the life cycle of the added controller.

Here’s the code:

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self.vc1 = [[VC1ViewController alloc] init]; //子控制器
self.vc2 = [[VC2ViewController alloc] init]; //子控制器
    
[self addChildViewController:self.vc1]; //添加到父控制器中
[self.view addSubview:self.vc1.view];   //把子控制器的 view 添加到父控制器的 view 上面
self.vc1.view.frame = CGRectMake(0, 0, 100, 100); //设置 frame
[self.vc1 didMoveToParentViewController:self];//子控制器被通知有了一个父控制器


    
[self addChildViewController:self.vc2];
[self.view addSubview:self.vc2.view];
self.vc2.view.frame = CGRectMake(0, 0, 100, 100);
[self.vc2 didMoveToParentViewController:self];//子控制器被通知有了一个父控制器

To remove one, use the following code:

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// Remove a childViewController
[self.vc1 willMoveToParentViewController:nil];//子控制器被通知即将解除父子关系
[self.vc1.view removeFromSuperview];//把子控制器的 view 从到父控制器的 view 上面移除
[self.vc1 removeFromParentViewController];//真正的解除关系,会自己调用 [self.vc1 didMoveToParentViewController:nil]

When we add a child to the parent controller, its

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

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

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

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

these methods are called automatically by the system.

Methods for manually managing the child ViewController’s life cycle

You need to override the following method in the parent ViewController and return NO:

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- (BOOL)shouldAutomaticallyForwardAppearanceMethods{
    // Manually manage the child VC's life cycle
    return NO;
}

However, note that you must not call methods like viewWillAppear, viewDidAppear, etc. manually. Instead, you should call:

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- (void)beginAppearanceTransition:(BOOL)isAppearing animated:(BOOL)animated;
- (void)endAppearanceTransition;

Note: use these two methods to indirectly trigger the child controller’s life cycle, and they must be used in pairs

Setting isAppearing to YES triggers viewWillAppear:.

Setting isAppearing to NO triggers viewWillDisappear:.

The endAppearanceTransition method will call viewDidAppear: and viewDidDisappear: based on the isAppearing value we passed in.

To test this, I wrote a piece of code:

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- (IBAction)click:(UIButton *)sender {
    sender.selected = !sender.selected;
    if (sender.selected) {
        [self.vc1 beginAppearanceTransition:NO animated:YES];  //调用vc1的 viewWillDisappear:
        [self.vc2 beginAppearanceTransition:YES animated:YES];  //调用vc2的 viewWillAppear:
        [self.vc1 endAppearanceTransition]; //调用vc1的viewDidDisappear: 
        [self.vc2 endAppearanceTransition]; //调用vc2的viewDidAppear:
    } else {
        [self.vc1 beginAppearanceTransition:YES animated:YES];
        [self.vc2 beginAppearanceTransition:NO animated:YES];
        [self.vc1 endAppearanceTransition];
        [self.vc2 endAppearanceTransition];
    }
}

Demo

Common transition animation

transitionFromViewController:toViewController:duration:options:animations:completion: is the “officially recommended” way for a custom container controller to switch between two child controllers. It automatically forwards the appearance life cycle (viewWill/DidAppear/Disappear) for you, so you don’t need to manually call beginAppearanceTransition/endAppearanceTransition.

A minimal working pattern (fromVC is the currently displayed child controller, toVC is the child controller you’re switching to):

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- (void)switchFrom:(UIViewController *)fromVC to:(UIViewController *)toVC 
{
    if (fromVC == toVC) return;

    // 1) Prepare the parent-child relationship
    [fromVC willMoveToParentViewController:nil]; // 即将移除旧的
    [self addChildViewController:toVC];          // 先把新的加为子控制器

    // 2) Prepare the view's size/position (important)
    toVC.view.frame = self.containerView.bounds; // 或者设置自动布局约束
    // If using Auto Layout, usually set the frame first, then add constraints afterwards.

    // 3) Switch (the system handles the appearance life cycle automatically)
    [self transitionFromViewController:fromVC
                      toViewController:toVC
                              duration:0.25
                               options:UIViewAnimationOptionTransitionCrossDissolve
                            animations:^{
                                // Optional: extra animations, such as layout changes, alpha, transform, etc.
                                // [self.containerView layoutIfNeeded];
                            }
                            completion:^(BOOL finished) {
                                // 4) Finalize the parent-child relationship changes
                                [toVC didMoveToParentViewController:self];
                                [fromVC removeFromParentViewController];

                                self.currentVC = toVC;
                            }];
}

Key Points

Use Cases

This is the utility method for switching between two child controllers in a “custom container controller” — for example, custom Tab switching, paged content switching, and so on.

Life Cycle

This method automatically calls the appropriate appearance callbacks for fromVC’s disappearance and toVC’s appearance (viewWill/DidDisappear/Appear). Don’t manually write beginAppearanceTransition/endAppearanceTransition, or they’ll be triggered twice.

Order of Parent-Child Relationship Calls

Before switching: call willMoveToParentViewController:nil on the old controller, and addChildViewController: on the new controller first.

In the completion block: call didMoveToParentViewController:self on the new controller, and removeFromParentViewController on the old controller.

View Hierarchy and Size

Set the final size (frame) of toVC.view, or prepare its constraints, before calling. transitionFrom inserts toVC.view into the superview of fromVC.view and removes fromVC.view when finished (you don’t need to manually call addSubview/removeFromSuperview).

If you’re using Auto Layout, a common approach:

Simple solution: switch using frames first, then add/update toVC.view’s constraints after completion. Or use the UIViewAnimationOptionShowHideTransitionViews option: add both child views to the same container with constraints in advance; during the switch the system only toggles show/hide. But this requires the views to be installed before the transition.

Common values for options

• UIViewAnimationOptionTransitionCrossDissolve: cross-fade
• UIViewAnimationOptionTransitionFlipFromLeft/Right: flip
• UIViewAnimationOptionCurveEaseInOut etc.: animation curve
• UIViewAnimationOptionShowHideTransitionViews: only toggles the hidden property (both views must be siblings and already in the parent view)

The animations block can contain extra layout or visual changes (alpha, transform, layoutIfNeeded after constraint changes, etc.). If it’s a simple transition, you can also leave it empty.

Pitfalls fromVC must already be a child controller of self; toVC must be added with addChild before calling transitionFrom, otherwise it will crash or have no effect.

Don’t manually trigger the life cycle

The end.