ios - PageViewController 之前和之后调用两次并且索引不计为真

Question

- (void)viewDidLoad {
self.pageController = [[UIPageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil];
self.pageController.dataSource = self;
[[self.pageController view] setFrame:CGRectMake(0, 0, self.viewParent.frame.size.width, self.viewParent.frame.size.height)];
initialViewController = [self viewControllerAtIndex:[[NSUserDefaults standardUserDefaults] integerForKey:@"indexAllQuestion"]];
viewControllers = [NSArray arrayWithObject:initialViewController];

[self.pageController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];
[self  addChildViewController:self.pageController];
[[self viewParent] addSubview:[self.pageController view]];
[self.pageController didMoveToParentViewController:self];
self.pageController.delegate = self;
self.pageController.dataSource = self;
}


- (AllComponentQuestionViewController *)viewControllerAtIndex:(NSUInteger)index {
current_question = self.all_question_keys[index];
AllComponentQuestionViewController *childViewController = [[AllComponentQuestionViewController alloc] initCurrent_question:current_question initCurrent_worksheet:current_worksheet iniTotalQuestion:maxDataCount];

[childSwipeVC replaceObjectAtIndex:index withObject:childViewController];
childViewController.indexChild = index;
return childViewController;}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
NSUInteger index = [(AllComponentQuestionViewController *)viewController indexChild];
if ((index == NSNotFound) || index <= 0) {
    index = 0;
    return nil;
}
else{
    index--;
    return [self viewControllerAtIndex:index];
}
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
NSUInteger index = [(AllComponentQuestionViewController *)viewController indexChild];
if (index >= maxDataCount) {
    index = maxDataCount;
    return nil;
}
else{
    index++;
    return [self viewControllerAtIndex:index];
}}

http://pasted.co/de8d5246这个链接和那个一样

案例是:

1. 从索引 0 向右滑动到索引 1
2. 索引增加到 2，所以我的索引 +2
3. 如果我再次滑动它会+1
4. 告诉我现在是否在索引 8 中，我向左滑动
5. 指数会下降到-6，所以我的指数是-2
6. 如果我再次向左滑动，它将索引-- 或 index = index - 2

提前需要帮助，一周前卡在这里 谢谢你

Answer 1

viewControllerAfterViewController:和viewControllerBeforeViewController:是UIPageViewControllerDataSource方法，由UIPageViewController内部 Controller 调用

这些方法在一次滑动时被调用两次的原因是，例如，如果您向右滑动(或向前滑动)，它会提前为您准备下一个 View Controller 和另一个 View Controller ，因此，如果您需要拥有用户当前所在的确切索引，您可能需要使用委托(delegate)方法在滑动时获取当前索引，例如:

objective-C :

- (void)pageViewController:(UIPageViewController *)pageViewController 
willTransitionToViewControllers:(NSArray<UIViewController *> *)pendingViewControllers;

swift 4:

func pageViewController(_ pageViewController: UIPageViewController,
                        willTransitionTo pendingViewControllers: [UIViewController])

和

objective-C :

- (void)pageViewController:(UIPageViewController *)pageViewController 
        didFinishAnimating:(BOOL)finished 
   previousViewControllers:(NSArray<UIViewController *> *)previousViewControllers 
       transitionCompleted:(BOOL)completed;

swift 4:

func pageViewController(_ pageViewController: UIPageViewController,
                        didFinishAnimating finished: Bool,
                        previousViewControllers: [UIViewController],
                        transitionCompleted completed: Bool)

比如用户可以向右滑动，但是transition不会落空，pageViewController滑动会自动返回到之前的index，所以需要使用completed参数查看index是否改变等。 .

在这里阅读更多:https://developer.apple.com/reference/uikit/uipageviewcontrollerdelegate/1614090-pageviewcontroller?language=objc