UIPageViewController gesture is calling viewControllerAfter: but doesn't animate

Question

I have a really interesting issue with UIPageViewController.

My project is set up very similarly to the example Page Based Application template. Every now and then (

Answer 1

Since I was still receiving mysterious crashes with the implementation in my first answer, I kept searching for a "good enough" solution which depends less on personal assumptions about the page view controller's (PVC) underlying behavior. Here is what I managed to come up with.

My former approach was kind of intrusive and was more of a workaround than an acceptable solution. Instead of fighting the PVC to force it to do what I thought it was supposed to do, it seems that it's better accept the facts that:

• the pageViewController:viewControllerBeforeViewController: and pageViewController:viewControllerAfterViewController: methods can be called an arbitrary number of times by UIKit, and
• there is absolutely no guarantee that either of these correspond to a paging animation, nor that they will be followed by a call to pageViewController:didFinishAnimating:previousViewControllers:transitionCompleted:

That means we cannot use the before/after methods as "animation-begin" (note, however, that didFinishAnimating still serves as "animation-end" event). So how do we know an animation has indeed started?

Depending on our needs, we may be interested in the following events:

1. the user begins fiddling with the page: A good indicator for this is the before/after callbacks, or more precisely the first of them.

2. first visual feedback of the page turning gesture: We can use KVO on the state property of the tap and pan gesture recognizers of the PVC. When a UIGestureRecognizerStateBegan value is observed for panning, we can be pretty sure that visual feedback will follow.

3. the user finishes dragging the page around by releasing the touch: Again, KVO. When the UIGestureRecognizerStateRecognized value is reported either for panning or tapping, it is when the PVC is actually going to turn the page, so this may be used as "animation-begin".

4. UIKit starts the paging animation: I have no idea how to get a direct feedback for this.

5. UIKit concludes the paging animation: Piece of cake, just listen to pageViewController:didFinishAnimating:previousViewControllers:transitionCompleted:.

For KVO, just grab the gesture recognizers of the PVC as below:

@interface MyClass () 
{
    UIPanGestureRecognizer* pvcPanGestureRecognizer;
    UITapGestureRecognizer* pvcTapGestureRecognizer;
}
...
for ( UIGestureRecognizer* recognizer in pageViewController.gestureRecognizers )
{
    if ( [recognizer isKindOfClass:[UIPanGestureRecognizer class]] )
    {
        pvcPanGestureRecognizer = (UIPanGestureRecognizer*)recognizer;
    }
    else if ( [recognizer isKindOfClass:[UITapGestureRecognizer class]] )
    {
        pvcTapGestureRecognizer = (UITapGestureRecognizer*)recognizer;
    }
}

Then register your class as observer for the state property:

[pvcPanGestureRecognizer addObserver:self
                          forKeyPath:@"state"
                             options:NSKeyValueObservingOptionNew
                             context:NULL];
[pvcTapGestureRecognizer addObserver:self
                          forKeyPath:@"state"
                             options:NSKeyValueObservingOptionNew
                             context:NULL];

And implement the usual callback:

- (void)observeValueForKeyPath:(NSString *)keyPath
        ofObject:(id)object
        change:(NSDictionary *)change
        context:(void *)context
{
    if ( [keyPath isEqualToString:@"state"] && (object == pvcPanGestureRecognizer || object == pvcTapGestureRecognizer) )
    {
        UIGestureRecognizerState state = [[change objectForKey:NSKeyValueChangeNewKey] intValue];
        switch (state)
        {
            case UIGestureRecognizerStateBegan:
            // trigger for visual feedback
            break;

            case UIGestureRecognizerStateRecognized:
            // trigger for animation-begin
            break;

            // ...
        }
    }
}

When you are done, don't forget to unsubscribe from those notifications, otherwise you may get leaks and strange crashes in your app:

[pvcPanGestureRecognizer removeObserver:self
                             forKeyPath:@"state"];
[pvcTapGestureRecognizer removeObserver:self
                             forKeyPath:@"state"];

That's all folks!