gistfile1.m

// Assuming SomePageViewController has a property called idx that holds its page number.

// Some number
#define MAX_PAGES 5

// Factory method
- (UIViewController *)viewControllerAtIndex:(int)i {
    // Asking for a page that is out of bounds??
    if (i<0) {
        return nil;
    }
    if (i>=MAX_PAGES) {
        return nil;
    }
    
    // Assuming you have SomePageViewController.xib
    SomePageViewController *newController = [[SomePageViewController alloc] initWithNibName:@"SomePageViewController" bundle:nil];
    newController.idx = i;
    return newController;
}


- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController {
    SomePageViewController *p = (SomePageViewController *)viewController;
    return [self viewControllerAtIndex:(p.idx - 1)];
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController {
    SomePageViewController *p = (SomePageViewController *)viewController;
    return [self viewControllerAtIndex:(p.idx + 1)];
}