UIScrollView頻道滑動（授人以魚，不如授人以漁）

        內容型APP（比如新聞，視頻），通常採用多個頁面（頻道）滑動；另外，自動滾動的焦點圖用到比較多的。本文簡單介紹使用UIScrollView實現上述功能的原理，並附部分代碼。無論是頻道的滑動，還是焦點圖自動滑動，都是基於UIScrollView的特性，viewController 實現UIScrollView的代理，並做一些控制即可。
核心思想：
    ★使用UIScrollerview作爲承載view。各個頻道添加到UIScrollerview上。注意：UIScrollerview需要提前知道頻道數量，進而確定UIScrollerview的frame.width
    ★當滑動屏幕一半兒的時候，調用UIScrollView 的- (void)scrollRectToVisible:(CGRect) animated:(BOOL)方法，實現整頁滾動。
    ★當滑動屏幕不到一半兒的時候，由於啓用了UIScrollView的pagingEnabled屬性，所以，UIScrollView會自動彈回原位置。

關鍵代碼：

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.scrollView = [[UIScrollView alloc] initWithFrame:self.bounds];   /*創建scrollerview*/
    self.scrollView.pagingEnabled = YES; /*scroller 一頁頁滑動,而不是一直滑下去*/
    self.scrollView.scrollsToTop = NO;       /*點擊狀態欄，是否滾到頂部。不*/
    self.scrollView.bounces = YES;           /*到達邊界回彈*/
    self.scrollView.alwaysBounceHorizontal = YES;   /*如果水平方向，內容寬度小於bound，仍可以滑動回彈*/
    self.scrollView.alwaysBounceVertical = NO;         /*如果垂直方向，內容高度小於bound，不讓滑動回彈*/
    self.scrollView.showsHorizontalScrollIndicator = NO;/*橫向滾動條，不顯示*/
    self.scrollView.showsVerticalScrollIndicator = NO;    /*縱向滾動條，不顯示*/
    
    /*添加到viewController中的view之上*/
    [self.view addSubview:self.scrollView];
}
========================================================================
//內存警告的時候，卸載多餘的page。保留current前後兩個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 {
    //在滑動到page時候，處理邏輯。子類可重寫該方法
}


- (void)didScrollToPage:(NSInteger)page animated:(BOOL)animated {
    //在滑動到page時候，處理邏輯。子類可重寫該方法
    [self scrollToPage:index animated:YES];
}

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

//跳轉任意頻道。
- (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參數，一定要NO，不然滑動時候產生抖動 */
    [self.scrollView scrollRectToVisible:bounds animated:NO];
}

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

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

    //加載後一個，前一個page
    [self addViewControllerByPage:self.currentPage - 1];
    [self addViewControllerByPage:self.currentPage + 1];
}

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

    //加載後一個，前一個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];
        }
    }
}