View controller container sporadically misses viewWillAppear in child controllers

Question

I have a root view controller with subview to be wrappers for views of child view controllers. The basic idea is my root controller has a left and right view controller, both present on screen (similar to a splitviewcontroller). On load a modal view pops up over the root view controller and asks for details. The modal view then contacts a server, and is dismissed after getting a response. The root controller then adds the child view controllers with the following code:

[self addViewController:self.leftViewController];
[self addViewController:self.rightViewController];

[self addView:self.rightViewController.view ToWrapper:self.rightViewWrapper];
[self addView:self.leftViewController.view ToWrapper:self.leftViewWrapper];

Where add view controller is:

[self addChildViewController:controller];
[controller didMoveToParentViewController:self];

and addViewToWrapper just adds the view controller's view to the relevant subview of the rootViewController as follows:

[[viewWrapper.contentView subviews] makeObjectsPerformSelector:@selector(removeFromSuperview)];
newSubview.frame = viewWrapper.contentView.bounds;
newSubview.autoresizingMask = UIViewAutoresizingFlexibleHeight |    UIViewAutoresizingFlexibleWidth;
[viewWrapper.contentView addSubview:newSubview];

99% of the time this works fine. Both views appear instantly and viewWillAppear fires in both child controllers. However, occasionally the screen stays white for a brief period and viewWillAppear doesn't fire in the right view controller (EDIT: and the left). All of the other view lifecycle methods fire, just not viewWillAppear.

Unfortunately, I can't give code for the whole class as it is complex and proprietary. But are there any clues in this description for this intermittent behaviour?

I should also add that this is happening way after appDelegate has started the app. Behind the modal view are 2 blank placeholders. When the modal view is dismissed these controllers are removed, and the new child controllers added. — Nick Jones, Apr 23 '14 at 16:35
A pretty major clue is that removing the modal screen, removes the issue. Something during the dismissal of the modal screen blocks the view will appear of the child view controllers. — Nick Jones, Apr 23 '14 at 17:35

score 2 · Accepted Answer · answered Apr 23 '14 at 18:16

Some thoughts on this:

ONE

It looks like your containment methods are not being called properly. They should look like this for each viewController:

    [self addChildViewController:controller];
    [self.view addSubview:controller.view]; 
    [controller didMoveToParentViewController:self];

The view should be added to the superview in between the addChild and didMove calls.

TWO

To my knowledge, there cannot be more than one presentation or dismissal occurring at a time. I.e. If you are trying to present (add) your child viewControllers at the same time as the modal is being dismissed, you will see an error in the console log and your "add" operation will not occur.

My recommendation would be to create a delegate protocol on the modal viewcontroller. And in the completion block of dismissViewControllerAnimated:completion:, call your delegate method:

[self dismissViewControllerAnimated:YES completion:^{
   if ([weakSelf.delegate respondsToSelector:@selector(settingsViewControllerDidDismiss:)])
   {
       [weakSelf.delegate settingsViewControllerDidDismiss:self];
   }
}];

And in your rootViewController, you would begin adding its children inside of settingsViewControllerDidDismiss or whatever you decide to call that method. The point is that the "add" operation begins AFTER the dismissal operation.

Hope this helps.

Ah, I didn't realise the order of adding the view was important. It does seem to be the modal view that is causing the issue. Annoyingly, the add operation is triggered by events entirely unrelated to the dismissal operation so I'll have to somehow trigger it from both. — Nick Jones, Apr 24 '14 at 08:53
Sounds good, if you end up thinking that this is the correct answer, please dont forget to mark it as so. I'm fairly certain that the issue you're experiencing is due to concurrent presentation/dismissal. — Alfie Hanssen, Apr 24 '14 at 12:30
It is almost definitely the answer. I added a notification to tell the modal to dismiss once the view controllers behind it are fully loaded (perhaps not the right way around, but the cleanest from a UI perspective). I haven't seen the issue since. — Nick Jones, Apr 24 '14 at 16:21
That's great news! Sounds like that flow works best for your UI case, as opposed to the other way around via the delegate stuff outlines above. Either way, nice! — Alfie Hanssen, Apr 24 '14 at 20:24

View controller container sporadically misses viewWillAppear in child controllers

1 Answers1