Why ContentVC's viewWill/DidAppear is called before its parent(PageVC)? - ios

Question

In my case, I use ViewController as parent of PageViewController. And it also be the dataSource of PageViewController by providing ContentViewController.
It's strange that ContentViewController's -viewWill/DidAppear: is called before it's parent, here is what I got from console:
[ContentViewController viewWillAppear:]
[ContentViewController viewDidAppear:]
[ViewController viewWillAppear:]
[PageViewController viewWillAppear:]
[ViewController viewDidAppear:]
[PageViewController viewDidAppear:]
Here are some code from ViewController.m:
- (void)viewDidLoad {
[super viewDidLoad];
PageViewController *pageVC = [[PageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil];;
pageVC.dataSource = self;
[pageVC setViewControllers:#[[self contentViewControllerWithIndex:0]]
direction:UIPageViewControllerNavigationDirectionForward
animated:NO completion:nil];
pageVC.view.backgroundColor = [UIColor whiteColor];
pageVC.view.frame = self.view.bounds;
[self addChildViewController:pageVC];
[self.view addSubview:pageVC.view];
[pageVC didMoveToParentViewController:self];
}
-(ContentViewController *)contentViewControllerWithIndex:(NSInteger )index {
ContentViewController *vc = [[ContentViewController alloc] init];
vc.index = index;
return vc;
}
Thanks.

Answer 1

If you go through line by line of your code
This line just alloc your PageViewController. It has actually not added it on top of your ViewController. So ViewWillAppear for PageViewController will not get called here.
Code:
PageViewController *pageVC = [[PageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil];
Below line is calling [self contentViewControllerWithIndex:0] which will be allocating the ContentViewController also adding it on PageVC so ViewWillAppear for ContentViewController is called
Code:
[pageVC setViewControllers:#[[self contentViewControllerWithIndex:0]]
direction:UIPageViewControllerNavigationDirectionForward
animated:NO completion:nil];
Below line is adding PageVC as childViewControllerof ViewController. So once the ViewWillAppear of ViewController will be called then only it can call ViewWillAppear of PageVC and add it as its childView.
Code:
[self addChildViewController:pageVC];
[self.view addSubview:pageVC.view];
[pageVC didMoveToParentViewController:self];