Question

我是Obj-C的新手，也是学习它的舞台。刚刚看到那些代码并试图得到它。但是，在第一个以 - （UIViewController *）开头的代码中，pageViewController ....对我来说太复杂了。

任何可以一步一步解释的人？

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController
{
    NSUInteger index = ((PageContentViewController*) viewController).pageIndex;

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

    index--;
    return [self viewControllerAtIndex:index];
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController
{
    NSUInteger index = ((PageContentViewController*) viewController).pageIndex;

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

    index++;
    if (index == [self.pageTitles count]) {
        return nil;
    }
    return [self viewControllerAtIndex:index];
}

Answer 1

基本上你有一个UIPageViewController。它目前在所有N页中显示索引K处的页面。 UIPageViewController需要知道在显示的当前页面之前（在索引K处）和当前页面之后的页面之前出现了什么页面。

以下是它的外观：

//Called on the data source. Returns the UIViewController before the 
//current one (viewController) within the UIPageViewController (pageViewController)
- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController
{
    //gets the index of the page currently showing
    NSUInteger index = ((PageContentViewController*) viewController).pageIndex;

    //Checks that it's not the first page and that it's
    //actually a part of the UIPageViewController
    if ((index == 0) || (index == NSNotFound)) {
        //If it is, then there is no page before it.
        return nil;
    }

    //otherwise, lets go back one index
    index--;
    //and then this is the UIViewController before it
    return [self viewControllerAtIndex:index];
}

对于第二种方法，它正在做同样的事情，但是试图获取后面的页面。同样，iOS需要了解这些内容，因此，在高级别，它可以在用户更改页面时快速显示该页面。

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController
{
    //gets the index of the current page being shown
    NSUInteger index = ((PageContentViewController*) viewController).pageIndex;

    //if it's not found, then there isn't a page after it
    if (index == NSNotFound) {
        return nil;
    }

    //go to the next index
    index++;
    //if we're at the end of the pages
    if (index == [self.pageTitles count]) {
        //there's no page after it
        return nil;
    }
    //otherwise, return the next UIViewController
    return [self viewControllerAtIndex:index];
}

页面视图控制器数据源

1 个答案: