iOS 实现UIScrollView的无限轮播 （转）

原文地址：https://www.jianshu.com/p/7c4b79e5b123

在摆放图片时需要注意，我们需要在第一张图片的位置摆放最后一张图片（可能有点懵哈，不过不要着急慢慢往下看），然后我们依次摆放图片（从第一张到最后一张），最后在所有图片的尾部我们再放上第一张图片。这样我们就多放了两张图片（分别在首尾多放了一张图）。我把对应的方法写一下：

/// 将图片放置在UIScrollView上
-(void)setupImage {
    /// 在UIScrollView的最前面添加一张图片
    UIImageView *firstImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, kScreenWidth, self.scrollView.frame.size.height)];
    /// 图片名是最后一张图片
    firstImageView.image = [UIImage imageNamed:self.imageNameList.lastObject];
    [self.scrollView addSubview:firstImageView];
    
    /// 添加图片
    for (NSInteger index = 0; index < self.imageNameList.count; index ++) {
        /// UIScrollView上的每一张图片
        UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake((index + 1) * kScreenWidth, 0, kScreenWidth, self.scrollView.frame.size.height)];
        imageView.image = [UIImage imageNamed:self.imageNameList[index]];
        
        [self.scrollView addSubview:imageView];
        self.scrollView.contentSize = CGSizeMake((index + 2) * self.scrollView.bounds.size.width, 0);
    }
    
    /// 在UIScrollView的最后面添加一张图片
    UIImageView *lastImageView = [[UIImageView alloc] initWithFrame:CGRectMake((self.imageNameList.count + 1) * kScreenWidth, 0, kScreenWidth, self.scrollView.frame.size.height)];
    /// 图片名是第一张图片
    lastImageView.image = [UIImage imageNamed:self.imageNameList.firstObject];
    [self.scrollView addSubview:lastImageView];
    
    /// 设置UIScrollView的偏移量
    self.scrollView.contentSize = CGSizeMake((self.imageNameList.count + 2) * self.scrollView.bounds.size.width, 0);
    
    /// 设置UIScrollView的起始偏移距离（将第一张图片跳过）
    self.scrollView.contentOffset = CGPointMake(kScreenWidth, 0);
    
    /// 图片总数
    self.pageControl.numberOfPages = self.imageNameList.count;
    self.pageControl.currentPage = 0;
}

其实，如果大家看到这里，应该就会大致明白无线轮播的实现原理了。接下来就是最后一步，在UIScrollView的代理方法里面写逻辑：判断UIScrollView的偏移量，当其滑动到首位时（显示的是最后一张图片），滑动停止，就把偏移量修改最后面图片的位置上（倒数第二张）。同理，当UIScrollView滑动到最后时（显示的是第一张图片），滑动停止，就把偏移量修改到第一张图片的位置上（正数第二张）。

if (scrollView.contentOffset.x == 0) {
        scrollView.contentOffset = CGPointMake(self.imageNameList.count * kScreenWidth, 0);
        self.pageControl.currentPage = self.imageNameList.count;
    /// 当UIScrollView滑动到最后一位停止时，将UIScrollView的偏移位置改变
    } else if (scrollView.contentOffset.x == (self.imageNameList.count + 1)* kScreenWidth) {
        scrollView.contentOffset = CGPointMake(kScreenWidth, 0);
        self.pageControl.currentPage = 0;
    } else {
        self.pageControl.currentPage = scrollView.contentOffset.x / kScreenWidth - 1;
    }

 ok，原理其实就是这样。在首尾多加两张图片当做占位符，然后当UIScrollView滑动到占位符的位置时，改变UIScrollView的偏移量，简单且方便。下面就是全部代码：

#import "ViewController.h"

#define kScreenWidth [UIScreen mainScreen].bounds.size.width

@interface ViewController () <UIScrollViewDelegate>

/// 滑动控制器
@property (nonatomic, strong) UIScrollView *scrollView;
/// 图片数组
@property (nonatomic, strong) NSArray<NSString *> *imageNameList;
/// 页码控制器
@property (nonatomic, strong) UIPageControl *pageControl;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    // 设置图片名的数组
    self.imageNameList = @[@"image0", @"image1", @"image2", @"image3"];
    
    // 添加图片
    [self setupImage];
}

/// 将图片放置在UIScrollView上
-(void)setupImage {
    /// 在UIScrollView的最前面添加一张图片
    UIImageView *firstImageView = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, kScreenWidth, self.scrollView.frame.size.height)];
    /// 图片名是最后一张图片
    firstImageView.image = [UIImage imageNamed:self.imageNameList.lastObject];
    [self.scrollView addSubview:firstImageView];
    
    /// 添加图片
    for (NSInteger index = 0; index < self.imageNameList.count; index ++) {
        /// UIScrollView上的每一张图片
        UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake((index + 1) * kScreenWidth, 0, kScreenWidth, self.scrollView.frame.size.height)];
        imageView.image = [UIImage imageNamed:self.imageNameList[index]];
        
        [self.scrollView addSubview:imageView];
        self.scrollView.contentSize = CGSizeMake((index + 2) * self.scrollView.bounds.size.width, 0);
    }
    
    /// 在UIScrollView的最后面添加一张图片
    UIImageView *lastImageView = [[UIImageView alloc] initWithFrame:CGRectMake((self.imageNameList.count + 1) * kScreenWidth, 0, kScreenWidth, self.scrollView.frame.size.height)];
    /// 图片名是第一张图片
    lastImageView.image = [UIImage imageNamed:self.imageNameList.firstObject];
    [self.scrollView addSubview:lastImageView];
    
    /// 设置UIScrollView的偏移量
    self.scrollView.contentSize = CGSizeMake((self.imageNameList.count + 2) * self.scrollView.bounds.size.width, 0);
    
    /// 设置UIScrollView的起始偏移距离（将第一张图片跳过）
    self.scrollView.contentOffset = CGPointMake(kScreenWidth, 0);
    
    /// 图片总数
    self.pageControl.numberOfPages = self.imageNameList.count;
    self.pageControl.currentPage = 0;
}

#pragma mark - UIScrollViewDelegate
-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    /// 当UIScrollView滑动到第一位停止时，将UIScrollView的偏移位置改变
    if (scrollView.contentOffset.x == 0) {
        scrollView.contentOffset = CGPointMake(self.imageNameList.count * kScreenWidth, 0);
        self.pageControl.currentPage = self.imageNameList.count;
    /// 当UIScrollView滑动到最后一位停止时，将UIScrollView的偏移位置改变
    } else if (scrollView.contentOffset.x == (self.imageNameList.count + 1)* kScreenWidth) {
        scrollView.contentOffset = CGPointMake(kScreenWidth, 0);
        self.pageControl.currentPage = 0;
    } else {
        self.pageControl.currentPage = scrollView.contentOffset.x / kScreenWidth - 1;
    }
}

#pragma mark - Get方法
-(UIScrollView *)scrollView {
    if (!_scrollView) {
        _scrollView = [[UIScrollView alloc] initWithFrame:CGRectMake(0, 0, kScreenWidth, 200)];
        _scrollView.pagingEnabled = YES;
        _scrollView.clipsToBounds = NO;
        _scrollView.scrollEnabled = YES;
        _scrollView.delegate = self;
        _scrollView.bounces = NO;
        _scrollView.showsHorizontalScrollIndicator = NO;
        _scrollView.showsVerticalScrollIndicator = NO;
        
        [self.view addSubview:_scrollView];
    }
    
    return _scrollView;
}

-(UIPageControl *)pageControl {
    if (!_pageControl) {
        _pageControl = [[UIPageControl alloc] initWithFrame:CGRectMake(0, 150, kScreenWidth, 50)];
        
        _pageControl.userInteractionEnabled = NO;
        _pageControl.pageIndicatorTintColor = [UIColor blackColor];
        _pageControl.currentPageIndicatorTintColor = [UIColor grayColor];
        
        [self.view addSubview:_pageControl];
    }
    
    return _pageControl;
}

@end