UIPageViewController transition 'Unbalanced calls to begin/end appearance transitions for '

Question

When I navigate through UIPageViewController faster than its transition animation I am getting \'Unbalanced calls to begin/end appearance transitions for <

Answer 1

Good answer from Basem Saadawy but it has some defect.

Actually the delegate's gestureRecognizerShouldBegin: could be called with no further animation started. This is possible if you start your gesture by vertical finger's moving and its horizontal offset is not enough to start the animation (but is enough to launch gestureRecognizerShouldBegin:). Thus our variable pageAnimationFinished will be set to NO without an actual animation. Therefore the pageViewController: didFinishAnimating: will never be called and you get the current page frozen without a possibility to change it.

That's why a better place to assign NO to this variable is a gesture recognizer's action method with examination of its velocity and translation (we are interested in horizontal direction only).

So the final steps are:

1) Declare an instance variable (a flag):

BOOL pageAnimationFinished;

2) Set its initial value

- (void)viewDidLoad
{
    [super viewDidLoad];
    ...
    pageAnimationFinished = YES;
}

3) Assign a delegate and a custom action to the pan gesture recognizers

for (UIGestureRecognizer * gesRecog in self.pageViewController.gestureRecognizers)
{
    if ([gesRecog isKindOfClass:[UIPanGestureRecognizer class]])
    {
        gesRecog.delegate = self;
        [gr addTarget:self action:@selector(handlePan:)];
    }
}

3') Animation is really started when the gesture's translation is greater in horizontal direction and the finger is moving horizontally at a moment.
I guess the same logic is used in the internal recognizer's action assigned by UIPageViewController.

- (void) handlePan:(UIPanGestureRecognizer *)gestureRecognizer
{
    if (pageAnimationFinished && gestureRecognizer.state == UIGestureRecognizerStateChanged)
    {
        CGPoint vel = [gestureRecognizer velocityInView:self.view];
        CGPoint tr = [gestureRecognizer translationInView:self.view];
        if (ABS(vel.x) > ABS(vel.y) && ABS(tr.x) > ABS(tr.y))
            pageAnimationFinished = NO; // correct place
    }
}

4) Disallowing a gesture if an animation is not finished.

-(BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer
{
    if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]] && ([gestureRecognizer.view isEqual:self.view] || [gestureRecognizer.view isEqual:self.pageViewController.view]))
    {
        UIPanGestureRecognizer * panGes = (UIPanGestureRecognizer *)gestureRecognizer;
        if(!pageAnimationFinished || (currentPage < minimumPage && [panGes velocityInView:self.view].x < 0) || (currentPage > maximumPage && [panGes velocityInView:self.view].x > 0))
            return NO;
    }
    return YES;
}

5) Animation is finished

- (void)pageViewController:(UIPageViewController *)pageViewController didFinishAnimating:(BOOL)finished previousViewControllers:(NSArray *)previousViewControllers transitionCompleted:(BOOL)completed
{
    pageAnimationFinished = YES;
}

I played too much with it and seems this is a nice solution that works well.