Blank Page after switching to UIPageViewControllerTransitionStyleScroll - ios

Question

I'm trying to switch my UIPageViewController's transition style from UIPageViewControllerTransitionStylePageCurl to UIPageViewControllerTransitionStyleScroll.
When using UIPageViewControllerTransitionStylePageCurl, Pages transition as desired. However, with UIPageViewControllerTransitionStyleScroll, blank pages mysteriously appear between page turns. The pattern with which blank pages show isn't consistent between builds.
Any idea what might be happening here? I'm stumped.
//AppDelegate.m
PageVC *pageVC = [[PageVC alloc]
initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll
navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal
options:NULL];
//PageVC.h
#interface PageVC : UIPageViewController <UIPageViewControllerDataSource>
//PageVC.m
- (void)viewDidLoad {
[super viewDidLoad];
self.graphVC = [[GraphVC alloc]init];
self.aboutVC = [[AboutVC alloc] init];
self.exerciseVC = [[ExerciseVC alloc] init];
self.viewControllerArray = [[NSArray alloc] initWithObjects:self.exerciseVC, self.graphVC, self.aboutVC, nil];
self.currentPage = 1;
if (self.graphVC != nil) {
self.dataSource = self;
[self setViewControllers:#[self.graphVC]
direction:UIPageViewControllerNavigationDirectionForward
animated:NO
completion:NULL];
}}
- (UIViewController *) pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
if (self.currentPage >= 2) {
return nil;
} else {
self.currentPage += 1;
return [self.viewControllerArray objectAtIndex:self.currentPage];
}
}
- (UIViewController *) pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
if (self.currentPage <= 0) {
return nil;
} else {
self.currentPage -= 1;
return [self.viewControllerArray objectAtIndex:self.currentPage];
}
}

Answer 1

As #pbasdf worote, tracking currentPage in viewControllerAfter/Before was a bad idea. When using TransitionStyleScroll, these two methods are called multiple times per page turn. As a result, they sometimes incremented incorrectly and returned nil. (TransitionStylePageCurl only calls each method once, after a page turn.)
A better solution is to check for the viewController passed to each method, and return before and after accordingly:
// PageVC.m
- (void)viewDidLoad {
[super viewDidLoad];
// initialize array of ViewControllers
viewControllerArray = [[NSArray alloc] initWithObjects:exerciseVC, graphVC, aboutVC, nil];
}
- (UIViewController *) pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
NSUInteger index = [viewControllerArray indexOfObject:viewController];
if (index >= [viewControllerArray count]-1) {
return nil;
} else {
return [viewControllerArray objectAtIndex:index+1];
}
}
- (UIViewController *) pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
NSUInteger index = [viewControllerArray indexOfObject:viewController];
if (index <= 0) {
return nil;
} else {
return [viewControllerArray objectAtIndex:index-1];
}