UIScrollView频道滑动（授人以鱼，不如授人以渔）

        内容型APP（比如新闻，视频），通常采用多个页面（频道）滑动；另外，自动滚动的焦点图用到比较多的。本文简单介绍使用UIScrollView实现上述功能的原理，并附部分代码。无论是频道的滑动，还是焦点图自动滑动，都是基于UIScrollView的特性，viewController 实现UIScrollView的代理，并做一些控制即可。
核心思想：
    ★使用UIScrollerview作为承载view。各个频道添加到UIScrollerview上。注意：UIScrollerview需要提前知道频道数量，进而确定UIScrollerview的frame.width
    ★当滑动屏幕一半儿的时候，调用UIScrollView 的- (void)scrollRectToVisible:(CGRect) animated:(BOOL)方法，实现整页滚动。
    ★当滑动屏幕不到一半儿的时候，由于启用了UIScrollView的pagingEnabled属性，所以，UIScrollView会自动弹回原位置。
关键代码：
- (void)viewDidLoad {
[super viewDidLoad];

self.scrollView = [[UIScrollView alloc] initWithFrame:self.bounds]; /*创建scrollerview*/
self.scrollView.pagingEnabled = YES; /*scroller 一页页滑动,而不是一直滑下去*/
self.scrollView.scrollsToTop = NO; /*点击状态栏，是否滚到顶部。不*/
self.scrollView.bounces = YES; /*到达边界回弹*/
self.scrollView.alwaysBounceHorizontal = YES; /*如果水平方向，内容宽度小于bound，仍可以滑动回弹*/
self.scrollView.alwaysBounceVertical = NO; /*如果垂直方向，内容高度小于bound，不让滑动回弹*/
self.scrollView.showsHorizontalScrollIndicator = NO;/*横向滚动条，不显示*/
self.scrollView.showsVerticalScrollIndicator = NO; /*纵向滚动条，不显示*/

/*添加到viewController中的view之上*/
[self.view addSubview:self.scrollView];
}
========================================================================
//内存警告的时候，卸载多余的page。保留current前后两个page
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];

for (NSInteger i = 0; i < [self.viewControllers count]; i++) {
if (i < self.currentPage - 1 || i > self.currentPage + 1) {
[self unloadViewControllerByPage:i];
}
}
}
========================================================================
- (void)willScrollToPage:(NSInteger)page animated:(BOOL)animated {
//在滑动到page时候，处理逻辑。子类可重写该方法
}

- (void)didScrollToPage:(NSInteger)page animated:(BOOL)animated {
//在滑动到page时候，处理逻辑。子类可重写该方法
[self scrollToPage:index animated:YES];
}

========================================================================

//跳转任意频道。
- (void)scrollToPage:(NSInteger)page animated:(BOOL)animated {
NSInteger lastPage = self.currentPage;
[self setCurrentPage:page animated:animated];
if (self.currentPage != lastPage) {
[self removeViewControllerByPage:lastPage];
}

CGRect bounds = UIEdgeInsetsInsetRect(self.scrollView.bounds, self.scrollView.contentInset);
bounds.origin.x = CGRectGetWidth(bounds) * page;
bounds.origin.y = 0;
/* animated参数，一定要NO，不然滑动时候产生抖动 */
[self.scrollView scrollRectToVisible:bounds animated:NO];
}

========================================================================
#pragma mark - UIScrollViewDelegate

/*手指开始拖拽*/
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
if (scrollView != self.scrollView) {
return;
}

//加载后一个，前一个page
[self addViewControllerByPage:self.currentPage - 1];
[self addViewControllerByPage:self.currentPage + 1];
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView {
if (scrollView != self.scrollView) {
return;
}

//加载后一个，前一个page
[self addViewControllerByPage:self.currentPage - 1];
[self addViewControllerByPage:self.currentPage + 1];
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
if (scrollView != self.scrollView) {
return;
}

if (decelerate) {
return;
}

CGFloat width = CGRectGetWidth(scrollView.bounds);
CGFloat position = self.scrollView.contentOffset.x;
[self setCurrentPage:round(position / width) animated:YES];

if (!decelerate) {
for (NSInteger i = 0; i < self.numberOfPages; i++) {
if (i != self.currentPage) {
[self removeViewControllerByPage:i];
}
}
}
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
if (scrollView != self.scrollView) {
return;
}

CGFloat width = CGRectGetWidth(scrollView.bounds);
CGFloat position = self.scrollView.contentOffset.x;
[self setCurrentPage:round(position / width) animated:YES];

for (NSInteger i = 0; i < self.numberOfPages; i++) {
if (i != self.currentPage) {
//非移除currentPage
[self removeViewControllerByPage:i];
}
}
}