[iOS]通过UIScrollView和UIPageControl实现滑动切换的效果

UIPageControl是自带的控件，可以查看官方文档，下载官方示例学习。

如果对Xcode自带的文档不熟悉可以参见：苹果Xcode帮助文档阅读指南

接下来是我学习笔记，使用Storyboard实现滑动切换的效果。

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新建一个项目，拖上一个UIScrollView和UIPageControl，并且建立关联：

新建一个ContentViewController，作为页面切换的内容视图：

也就是说，当滑动屏幕切换的时候，其实就是多个ContentViewController在切换。

基本结构是：

在主页面，我们首先初始化一些设置：

- (void)viewDidLoad
{
    [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
    
    // 初始化page control的内容
    _contentList = [NSArray arrayWithObjects:@"1",@"2",@"3",@"4", nil];
    
    
    // 一共有多少页
    NSUInteger numberPages = self.contentList.count;
    
    // 存储所有的controller
    NSMutableArray *controllers = [[NSMutableArray alloc] init];
    for (NSUInteger i = 0; i < numberPages; i++)
    {
		[controllers addObject:[NSNull null]];
    }
    self.viewControllers = controllers;
    
    // 一个页面的宽度就是scrollview的宽度
    self.myScrollView.pagingEnabled = YES;  // 自动滚动到subview的边界
    self.myScrollView.contentSize =
    CGSizeMake(CGRectGetWidth(self.myScrollView.frame) * numberPages, CGRectGetHeight(self.myScrollView.frame));
    self.myScrollView.showsHorizontalScrollIndicator = NO;
    self.myScrollView.showsVerticalScrollIndicator = NO;
    self.myScrollView.scrollsToTop = NO;
    self.myScrollView.delegate = self;
    
    _numberOfPages = numberPages;
    _myPageControl.numberOfPages = numberPages;
    _currentPage = 0;
    
    
    [self loadScrollViewWithPage:0];
    [self loadScrollViewWithPage:1];
    
}

接下来，我们需要一个函数，来加载ContentView页面上的元素：

// 加载ScrollView中的不同SubViewController
- (void)loadScrollViewWithPage:(NSUInteger)page
{
    if (page >= self.contentList.count)
        return;
    
    // replace the placeholder if necessary
    LContentViewController *controller = [self.viewControllers objectAtIndex:page];
    if ((NSNull *)controller == [NSNull null])
    {
        controller = [[LContentViewController alloc] init];
        [self.viewControllers replaceObjectAtIndex:page withObject:controller];
    }
    
    // add the controller's view to the scroll view
    if (controller.view.superview == nil)
    {
        CGRect frame = self.myScrollView.frame;
        frame.origin.x = CGRectGetWidth(frame) * page;
        frame.origin.y = 0;
        controller.view.frame = frame;
        [controller setLabel:[_contentList objectAtIndex:page]];
        [self.myScrollView addSubview:controller.view];
    }
}

然后先来处理一下PageControl的切换事件：

- (void)gotoPage:(BOOL)animated
{
    NSInteger page = self.myPageControl.currentPage;
    
    // load the visible page and the page on either side of it (to avoid flashes when the user starts scrolling)
    [self loadScrollViewWithPage:page - 1];
    [self loadScrollViewWithPage:page];
    [self loadScrollViewWithPage:page + 1];
    
	// update the scroll view to the appropriate page
    CGRect bounds = self.myScrollView.bounds;
    bounds.origin.x = CGRectGetWidth(bounds) * page;
    bounds.origin.y = 0;
    [self.myScrollView scrollRectToVisible:bounds animated:animated];
}

// page control 选项修改监听
- (IBAction)changePage:(id)sender
{
    [self gotoPage:YES];    // YES = animate
}

滑动ScrollView的事件监听：

// 滑动结束的事件监听
- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    // switch the indicator when more than 50% of the previous/next page is visible
    CGFloat pageWidth = CGRectGetWidth(self.myScrollView.frame);
    NSUInteger page = floor((self.myScrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
    self.myPageControl.currentPage = page;
    NSLog(@"当前页面 = %d",page);
    // a possible optimization would be to unload the views+controllers which are no longer visible
    
    [self loadScrollViewWithPage:page - 1];
    [self loadScrollViewWithPage:page];
    [self loadScrollViewWithPage:page + 1];
}