UIPageViewController behavior different for Scroll & PageCurl - ios

Question

I am seeing different behavior between a UIPageViewController using Scroll transitions versus PageCurl. PageCurl works the way I expect but when using Scroll-ing I sometimes see extra (nonsensical) calls to viewControllerBeforeViewController: and viewControllerAfterViewController:
Does anyone recognize this pattern of extra Data Source calls?
I am displaying a series of images and I can start anywhere in the sequence.
When using the Scroll transition, but not PageCurl, after the initial page is displayed if I move to the "right", I get 3 DataSource calls instead of the single one I expect. For the example run below I began at my index value of 3. When I swipe to move to the "next" image I expect to get a single viewControllerAfterViewController: call on index 3 to get to index 4. If I run a test I get two extra calls: one for Before index 3 (i.e. 2) and one for After index 4 (i.e. 5). The output is:
2013-04-19 12:37:33.964 Clouds[496:907] Page 3 - viewControllerAfterViewController called on this index
2013-04-19 12:37:33.988 Clouds[496:907] Page 3 - viewControllerBeforeViewController called on this index
2013-04-19 12:37:34.010 Clouds[496:907] willTransitionToViewControllers to indices
2013-04-19 12:37:34.014 Clouds[496:907] Page 4
2013-04-19 12:37:34.461 Clouds[496:907] Page 4 - viewControllerAfterViewController called on this index
The view that is displayed after all this is indeed the view for index 4. After this first transition the UIPageViewController seems to behave the way I expect. When I use the PageCurl transition it always behaves the way I expect and I don't get any of these extraneous calls.
The code that actually produced the output is:
- (void)viewDidLoad
{
[super viewDidLoad];
self.pageViewController = [[UIPageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil];
self.pageViewController.delegate = self;
self.pageViewController.dataSource = self;
CloudImageVC *startingViewController = [self viewControllerAtIndex:self.currentPage storyboard:self.storyboard];
NSArray *viewControllers = #[startingViewController];
[self.pageViewController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:NULL];
[self addChildViewController:self.pageViewController];
[self.view addSubview:self.pageViewController.view];
CGRect pageViewRect = self.view.bounds;
self.pageViewController.view.frame = pageViewRect;
[self.pageViewController didMoveToParentViewController:self];
// Add the page view controller's gesture recognizers to the book view controller's view so that the gestures are started more easily.
self.view.gestureRecognizers = self.pageViewController.gestureRecognizers;
}
- (CloudImageVC *)viewControllerAtIndex:(NSUInteger)index storyboard:(UIStoryboard *)storyboard {
// Return the data view controller for the given index.
if (index >= [self.imageNames count]) {
return nil;
}
self.dataViewController = [self makeDataViewControllerAtIndex:index storyboard:storyboard];
return self.dataViewController;
}
- (CloudImageVC *) makeDataViewControllerAtIndex:(NSUInteger)index storyboard:(UIStoryboard *)storyboard {
// Make and return the data view controller for the given index.
if (index >= [self.imageNames count]) {
return nil;
}
// Create a new view controller and pass suitable data.
self.dataViewController = [storyboard instantiateViewControllerWithIdentifier:#"CloudImageVC"];
self.dataViewController.imageIndex = index;
//self.dataViewController.cloudImage = nil;
return self.dataViewController;
}
- (NSUInteger)indexOfViewController:(CloudImageVC *)viewController
{
// Return the index of the given data view controller.
// For simplicity, we store the index value in the view controller.
return viewController.imageIndex;
}
#pragma mark - Page View Controller Data Source
- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
NSUInteger index = [self indexOfViewController:(CloudImageVC *)viewController];
NSLog(#"Page %d - viewControllerBeforeViewController called on this index", index);
if ((index == 0) || (index == NSNotFound)) {
return nil;
}
index--;
return [self viewControllerAtIndex:index storyboard:viewController.storyboard];
}
- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
NSUInteger index = [self indexOfViewController:(CloudImageVC *)viewController];
NSLog(#"Page %d - viewControllerAfterViewController called on this index", index);
if (index == NSNotFound) {
return nil;
}
index++;
if (index == self.imageNames.count) {
return nil;
}
return [self viewControllerAtIndex:index storyboard:viewController.storyboard];
}
#pragma mark - Protocol UI PageViewController Delegate
- (void) pageViewController:(UIPageViewController *)pageViewController willTransitionToViewControllers:(NSArray *)pendingViewControllers {
NSLog(#"willTransitionToViewControllers to indices");
for (CloudImageVC *vc in pendingViewControllers) {
NSLog(#"Page %d",vc.imageIndex);
}
}

Answer 1

The answer is in the comment by rdelmar.