How to 'cancel' view appearance transitions for custom container controller transitions

Question

I have created a custom container controller that works similarly to a UIPageViewController so that I could implement some custom transitions & data source logi

Answer 1

So as is often the way with SO, you spend hours fruitlessly working on a problem, and then right after you post the question, you find the answer...

I looked at the appearance callbacks of UINavigationController's built-in interactivePopGestureRecognizer to see what happens when the cancelling the transition, and it appears that my assumption about what appearance methods should be called was wrong.

For a view controller that is successfully transitioned to, the following callbacks are received (as you'd expect):

                 transition
                  finished
viewWillAppear:  ---------->  viewDidAppear:

But for a view controller transition that is unsuccessful (i.e. the transition was cancelled), the view controller receives the following callbacks:

                 transition                       immediately
                 cancelled                        followed by
viewWillAppear:  ---------->  viewWillDisappear:  ---------->  viewDidDisappear:

So we get a bit more of those dodgy view appearance semantics; I'm not sure how a view can disappear if it hasn't yet appeared! Oh well... I suppose it's slightly better than a view saying it will appear, and then nothing happening.

So, returning to my CustomTransitionContext's transitionCompletionHandler code, the solution looks like this:

...

// Register a completion handler to run when the context's completeTransition: method is called.
__weak CustomContainerController *weakSelf = self;
__weak CustomTransitionContext *weakContext = context;
context.transitionCompletionHandler = ^(BOOL complete) {
    if (complete) {
        // End the appearance transitions we began earlier.
        [oldVC endAppearanceTransition];
        [newVC endAppearanceTransition];

        [oldVC removeFromParentViewController];
        [newVC didMoveToParentViewController:weakSelf];
    } else {
        // Before ending each appearance transition, begin an
        // appearance transition in the opposite direction.
        [newVC beginAppearanceTransition:NO animated:[weakContext isAnimated]];
        [newVC endAppearanceTransition];
        [oldVC beginAppearanceTransition:YES animated:[weakContext isAnimated]];
        [oldVC endAppearanceTransition];

        [newVC removeFromParentViewController];
        [oldVC didMoveToParentViewController];
    }
}

....

So now the appearance callbacks look like this:

oldVC viewWillDisappear:
newVC viewWillAppear:
// ... some time later transition is cancelled ...
newVC viewWillDisappear:
newVC viewDidDisappear:
oldVC viewWillAppear:
oldVC viewDidAppear:

Also, subsequent transitions now trigger the callbacks as expected (since we're closing off all transitions with calls to endAppearanceTransition).