用3个label实现scrollView的循环滚动效果

今日公司给定一个实现广告位循环滚动效果的任务，翻查多篇博客文章，总结经验，写出下面的demo，感觉效果还可以，所以记录下来，以便日后翻查。

#import "ViewController.h"@interface ViewController ()<UIScrollViewDelegate>@property (nonatomic,strong)UIScrollView * scrollView;@property (nonatomic,strong)UIPageControl * pageControl;@property (nonatomic,strong)UILabel * firstLabel;@property (nonatomic,strong)UILabel * secondLabel;@property (nonatomic,strong)UILabel * thirdLabel;@property (nonatomic,strong)NSMutableArray * allArr;@property (nonatomic,strong)NSTimer * timer;@property (nonatomic,assign)NSInteger currentPage;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    self.view.backgroundColor = [UIColor whiteColor];    self.automaticallyAdjustsScrollViewInsets = NO;    _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 64, appWidth, appHeight - 164)];    _scrollView.pagingEnabled = YES;    _scrollView.showsHorizontalScrollIndicator = YES;    _scrollView.delegate = self;    [self.view addSubview:_scrollView];    _pageControl = [[UIPageControl alloc] initWithFrame:CGRectMake(0, appHeight - 124, appWidth, 20)];    _pageControl.pageIndicatorTintColor = [UIColor lightGrayColor];    _pageControl.currentPageIndicatorTintColor = [UIColor greenColor];    [self.view addSubview:_pageControl];    _firstLabel = [self createLabelWith:CGRectMake(0, 0, appWidth, appHeight - 164) andTitle:@"0"];    _secondLabel = [self createLabelWith:CGRectMake(appWidth, 0, appWidth, appHeight - 164) andTitle:@"1"];    _thirdLabel = [self createLabelWith:CGRectMake(appWidth * 2, 0, appWidth, appHeight - 164) andTitle:@"2"];    [_scrollView addSubview:_firstLabel];    [_scrollView addSubview:_secondLabel];    [_scrollView addSubview:_thirdLabel];    _allArr = [NSMutableArray array];    [_allArr addObject:@"0"];    [_allArr addObject:@"1"];    [_allArr addObject:@"2"];    _currentPage = 0;    _pageControl.currentPage = _currentPage;    _pageControl.numberOfPages = _allArr.count;    _scrollView.contentSize = CGSizeMake(_allArr.count * appWidth, 0);    [self reloadData];    _timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(autoShow) userInfo:nil repeats:YES];}- (void)reloadData{  // 确定了每一次要显示的页数之后，根据页数确定要显示的具体页面内容，然后将此具体页面内容展示到中间page上，根据逻辑顺序，确定左右页面要显示的内容    // 确保每次滚动，都只是内容发生变化，位置不变    if (_currentPage == 0) {// 2 0 1        _firstLabel.text = [_allArr lastObject];        _secondLabel.text = [_allArr objectAtIndex:_currentPage];        _thirdLabel.text = [_allArr objectAtIndex:_currentPage + 1];    }else if (_currentPage == [_allArr count] - 1){ // 1 2 0        _firstLabel.text = [_allArr objectAtIndex:_currentPage - 1];        _secondLabel.text = [_allArr objectAtIndex:_currentPage];        _thirdLabel.text = [_allArr firstObject];    }else{ // 0 1 2        _firstLabel.text = [_allArr objectAtIndex:_currentPage - 1];        _secondLabel.text = [_allArr objectAtIndex:_currentPage];        _thirdLabel.text = [_allArr objectAtIndex:_currentPage + 1];    }    _firstLabel.frame = CGRectMake(0, 0, appWidth, appHeight - 164);    _secondLabel.frame = CGRectMake(appWidth, 0, appWidth, appHeight - 164);    _thirdLabel.frame = CGRectMake(appWidth * 2, 0, appWidth, appHeight - 164);    _pageControl.currentPage = _currentPage;    _scrollView.contentOffset = CGPointMake(appWidth, 0);}- (UILabel *)createLabelWith:(CGRect)frame andTitle:(NSString *)title{    UILabel * label = [[UILabel alloc] initWithFrame:frame];    label.text = title;    label.textAlignment = NSTextAlignmentCenter;    label.font = [UIFont systemFontOfSize:25];    label.textColor = [UIColor blackColor];    [self.view addSubview:label];    return label;}#pragma mark ---- UIScrollViewDelegate- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{    if (_timer.isValid) {        [_timer invalidate];        _timer = nil;    }}- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{    // 拖拽结束，启动定时器    if (!_timer) {       _timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(autoShow) userInfo:nil repeats:YES];    }    float x = scrollView.contentOffset.x;       // 因为每次scrollView的起始的contentOffset.x 都是保持在appWidth位置上，所以，如果x <= 0就可以确定是向前翻的;x >= 2 * appWidth是向后翻的    if (x <= 0) { // 向前翻(向左方向滚动)        if (_currentPage <= 0) {            _currentPage = [_allArr count] - 1;        }else{            _currentPage --;        }    }    // 因为，起始的contentOffset.x 就是appWidth，所以一旦向右移动，就会达到>= 2 * appWidth的目的，使得_currentPage ++; 当第三次滑动达到_currentPage == [_allArr count] - 1的时候，_currentPage = 0;    if (x >= 2 * appWidth) { // 向后翻(向右方向滚动)        if (_currentPage == [_allArr count] - 1) {            _currentPage = 0;        }else{            _currentPage ++;        }    }    NSLog(@"---%f ---%ld",x,_currentPage);    // 1.调换label的位置 2.设置pageControl的currentPage    [self reloadData];}- (void)autoShow{// 自动滚动，都是向右滚动    if (_currentPage == _allArr.count - 1) {        _currentPage = 0;    }else{        _currentPage ++;    }    [self reloadData];}