UIScrollview 无缝循环滚动实现

头文件：
#import 

@class ArticleViewController;

@interface ArticleScrollViewController : UIViewController  {
        //不使用数组，看起来更明了，为了节省内存同时还要看起来无缝，3个view最好
	ArticleViewController	*article1;
	ArticleViewController	*article2;
	ArticleViewController	*article3;
}

@end
实现：
#import "ArticleScrollViewController.h"
#import "ArticleViewController.h"
#import "ViewSwitcher.h"
@implementation ArticleScrollViewController

// vieDidLoad函数不重要，只要初始化了三个view并放在uiscrollview里面，正确设定uiscrollview的content size就行了
- (void)viewDidLoad {
    [super viewDidLoad];

    UIScrollView *scrollView = (UIScrollView *)self.view;
    scrollView.contentSize = CGSizeMake(self.view.frame.size.width*3, self.view.frame.size.height);
    CGRect frame = self.view.frame;
    frame.origin.y = 0.0f;
    NSInteger article2Index = [ViewSwitcher getInstance].currentArticleIndex;
    NSInteger article1Index = article2Index - 1;
    article1Index = article1Index < 0 ? [ViewSwitcher getInstance].articleCount - 1 : article1Index;
    article1Index = article1Index >= [ViewSwitcher getInstance].articleCount ? 0 : article1Index;
    NSInteger article3Index = article2Index + 1;
    article3Index = article3Index < 0 ? [ViewSwitcher getInstance].articleCount - 1 : article3Index;
    article3Index = article3Index >= [ViewSwitcher getInstance].articleCount ? 0 : article3Index;

    article1 = [[ArticleViewController alloc] initWithArticleIndex:article1Index];
    [article1.view setFrame:frame];

    article2 = [[ArticleViewController alloc] initWithArticleIndex:article2Index];
    frame.origin.x += self.view.frame.size.width;
    [article2.view setFrame:frame];

    article3 = [[ArticleViewController alloc] initWithArticleIndex:article3Index];
    frame.origin.x += self.view.frame.size.width;
    [article3.view setFrame:frame];

    [scrollView addSubview:article1.view];
    [scrollView addSubview:article2.view];
    [scrollView addSubview:article3.view];

    CGPoint p = CGPointZero;
    p.x = scrollView.frame.size.width;
    [scrollView setContentOffset:p animated:NO];
    [article2 reloadData];
}
#pragma mark -
#pragma mark UIScrollViewDelegate

#define SET_FRAME(ARTICLEX) x = ARTICLEX.view.frame.origin.x + increase;\
                            if(x < 0) x = pageWidth * 2;\
                            if(x > pageWidth * 2) x = 0.0f;\
                            [ARTICLEX.view setFrame:CGRectMake(x, \
                                ARTICLEX.view.frame.origin.y,\
                                ARTICLEX.view.frame.size.width,\
                                ARTICLEX.view.frame.size.height)]
//将三个view都向右移动，并更新三个指针的指向，article2永远指向当前显示的view，article1是左边的，article3是右边的
- (void)allArticlesMoveRight:(CGFloat)pageWidth {
    //上一篇
    article3.articleIndex = article1.articleIndex - 1;
    if (article3.articleIndex < 0) {
        article3.articleIndex = [ViewSwitcher getInstance].articleCount - 1;
    }
    [article1 reloadData];
    ArticleViewController *tmpArticleViewController = article3;
    article3 = article2;
    article2 = article1;
    article1 = tmpArticleViewController;

    float increase = pageWidth;
    CGFloat x = 0.0f;
    SET_FRAME(article3);
    SET_FRAME(article1);
    SET_FRAME(article2);
}
- (void)allArticlesMoveLeft:(CGFloat)pageWidth {
    article1.articleIndex = article3.articleIndex + 1;
    if (article1.articleIndex >= [ViewSwitcher getInstance].articleCount) {
        article1.articleIndex = 0;
    }
    [article3 reloadData];//[article2 resetView];[article3 resetView];
    ArticleViewController *tmpArticleViewController = article1;
    article1 = article2;
    article2 = article3;
    article3 = tmpArticleViewController;

    float increase = -pageWidth;
    CGFloat x = 0.0f;
    SET_FRAME(article2);
    SET_FRAME(article3);
    SET_FRAME(article1);
}
/*
 循环滚动
 每次滚动后都将scrollview的offset设置为中间的一页
 若本次滚动是向前一页滚动，则把三页都向后放置，最后一页放到开头
 若本次滚动是向后一页滚动，则把三页都向前放置，第一页放到末尾
 */
- (void)scrollViewDidEndDecelerating:(UIScrollView *)theScrollView
{
    CGFloat pageWidth = theScrollView.frame.size.width;
    // 0 1 2
    int page = floor((theScrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
    if(page == 1) {
        //用户拖动了，但是滚动事件没有生效
        return;
    } else if (page == 0) {
        [self allArticlesMoveRight:pageWidth];
    } else {
        [self allArticlesMoveLeft:pageWidth];
    }
    CGPoint p = CGPointZero;
    p.x = pageWidth;
    [theScrollView setContentOffset:p animated:NO];
    [article1 resetView];
    [article3 resetView];
}
#pragma mark -
#pragma mark dealloc
- (void)didReceiveMemoryWarning {
    // Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];

    // Release any cached data, p_w_picpaths, etc. that aren't in use.
}

- (void)viewDidUnload {
    [super viewDidUnload];
    // Release any retained subviews of the main view.
    // e.g. self.myOutlet = nil;
    [article1    release];article1 = nil;
    [article2    release];article2 = nil;
    [article3    release];article3 = nil;
}

- (void)dealloc {
    [article1    release];
    [article2    release];
    [article3    release];
    [super dealloc];
}

@end
这里的雕虫小技主要在于：
以手指向右拖动为例，【屏幕】指的是scorllview的显示区域
  article1 article2 article3
           【屏幕】
  拖动以后成这样：
 article1 article2 article3
【屏幕】

将article3放到第一个去（设定article3的frame），这是屏幕还显示的是article1的内容
article3 article1 article2
【屏幕】
将屏幕移到中间：使用setContentOffset，禁用动画，这样骗过人眼
article3 article1 article2
          【屏幕】
最后更新指针顺序：
article1 article2 article3
          【屏幕】
无缝循环实现了。
by gqzhu