pageViewControllerviewControllerBefore

The term pageViewControllerviewControllerBefore refers to a callback method used in the iOS UIPageViewController framework. It is part of the UIPageViewControllerDelegate protocol and is written in the form pageViewController(_:viewControllerBeforeViewController:). The method is called by the page view controller when it needs to retrieve the view controller that precedes the currently visible one, enabling navigation in the reverse direction. The delegate returns either the appropriate view controller or nil if there is no preceding controller.

When implementing this method, the developer receives the currently visible view controller as a parameter. Typically the method uses the index of that controller to determine the predecessor. For example, an array of view controllers can be maintained, and the method locates the index of the supplied view controller and then returns the previous item if the index is greater than zero. If no previous controller exists, the method returns nil to indicate that the user cannot swipe backward.

Typical usage looks like this in Swift:

func pageViewController(_ pageViewController: UIPageViewController,

viewControllerBefore viewController: UIViewController) -> UIViewController? {

guard let index = viewControllers.firstIndex(of: viewController), index > 0 else {

return nil

}

return viewControllers[index - 1]

}

And in Objective‑C:

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController

viewControllerBeforeViewController:(UIViewController *)viewController {

NSUInteger index = [self.viewControllers indexOfObject:viewController];

if (index == 0 || index == NSNotFound) { return nil; }

return self.viewControllers[index - 1];

}

This method, together with pageViewController(_:viewControllerAfterViewController:), forms the backbone of paging logic. When properly implemented, it gives the page view controller the information needed to present pages in correct order and to enable smooth swiping interactions.