UIScrollView实现图片轮播器的无限滚动

简介

在现在的一些App中常常见到图片轮播器，一般用于展示广告、新闻等数据，在iOS内并没有现成的控件直接实现这种功能，但是通过UIScrollView的允许分页设置，可以实现滚动轮播的功能。

轮播原理

UIScrollView对象有pagingEnable成员，如果设置为YES，那么每一个scrollView尺寸这么大的区域就会被当作一页，在滚动时会根据滚动的比例自动计算应该切换到哪一页。

无限滚动原理

要实现无限滚动，需要额外的两张图片，假设我们的图片有五张，存在images数组中，那么我们在将图片插入到scrollView中时，在第一张图片前面插入一个最后一张图片作为辅助图片，在最后一张后面插入一个第一张图片作为辅助图片。这样，当滚动到第一张前面一张时，在页面切换结束后无动画的切换scrollView的偏移量为最后一张图片（不包含最后一张后面的第一张那个辅助图片），这样就实现了由辅助图片到真实图片的过渡，之所以设置辅助图片是为了在滚动中看到那个真实图片。同理，当滚动到最后一张的后面一张时，我们吧scrollView的偏移量设置为第一张图片即可。

具体的代码实现

这个代码是在开发一个项目中所写的，已经封装称一个View，只需要调用initWithFrame指定轮播器尺寸，然后通过设置images成员的值即可实现无限滚动的轮播。

// .h
//
//  ESPicPageView.h
//  享技
//
//  Created by 11 on 11/30/15.
//  Copyright © 2015 soulghost. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface ESPicPageView : UIView

@property (nonatomic, strong) NSArray *images;

@end
// --------------------------------------------
// .m
//
//  ESPicPageView.m
//  享技
//
//  Created by 11 on 11/30/15.
//  Copyright © 2015 soulghost. All rights reserved.
//

#import "ESPicPageView.h"
#import "UIImageView+WebCache.h"

@interface ESPicPageView () <UIScrollViewDelegate>

@property (nonatomic, weak) UIScrollView *scrollView;
@property (nonatomic, weak) UIPageControl *pageControl;
@property (nonatomic, assign) CGFloat imgW;
@property (nonatomic, assign) CGFloat imgH;
@property (nonatomic, strong) NSTimer *timer;
@property (nonatomic, strong) NSArray *imageViews;
@property (nonatomic, assign) NSInteger imageCount;

@end

@implementation ESPicPageView

- (instancetype)initWithFrame:(CGRect)frame{

    if (self = [super initWithFrame:frame]) {

        self.backgroundColor = [UIColor blueColor];
        UIScrollView *scrollView = [[UIScrollView alloc] init];
        self.scrollView = scrollView;
        self.scrollView.delegate = self;
        self.scrollView.pagingEnabled = YES;
        self.scrollView.showsHorizontalScrollIndicator = NO;
        self.scrollView.backgroundColor = [UIColor whiteColor];
        [self addSubview:scrollView];
        self.imgW = frame.size.width;
        self.imgH = frame.size.height;
        [self setNeedsLayout];

        UIPageControl *pageControl = [[UIPageControl alloc] initWithFrame:CGRectMake(frame.size.width - 50, frame.size.height - 10, 0, 0)];
        self.pageControl = pageControl;
        self.pageControl.numberOfPages = 1;
        [self addSubview:pageControl];

        self.timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(nextImage) userInfo:nil repeats:YES];
        [[NSRunLoop currentRunLoop] addTimer:self.timer forMode:NSRunLoopCommonModes];

    }

    return self;

}

- (void)nextImage{
    NSInteger page = self.pageControl.currentPage;
    page = self.pageControl.currentPage + 1;
    CGPoint offset = CGPointMake((1 + page) * self.imgW, 0);
    [self.scrollView setContentOffset:offset animated:YES];
}

- (void)setupImageViews{

    for (int i = 0; i < self.images.count + 2; i++) {
        UIImageView *imageView = [[UIImageView alloc] init];
        CGFloat imageX = i * self.imgW;
        CGFloat imageY = 0;
        CGFloat imageW = self.imgW;
        CGFloat imageH = self.imgH;
        imageView.frame = CGRectMake(imageX, imageY, imageW,imageH);
        [self.scrollView insertSubview:imageView atIndex:0];
    }

}

- (NSArray *)imageViews{

    if (_imageViews == nil) {
        NSMutableArray *arr = [NSMutableArray array];
        for (int i = 0; i < self.images.count + 2; i++) {
            UIImageView *imageView = [[UIImageView alloc] init];
            CGFloat imageX = i * self.imgW;
            CGFloat imageY = 0;
            CGFloat imageW = self.imgW;
            CGFloat imageH = self.imgH;
            imageView.frame = CGRectMake(imageX, imageY, imageW,imageH);
            [self.scrollView insertSubview:imageView atIndex:0];
            [arr addObject:imageView];
        }
        _imageViews = arr;
    }

    return _imageViews;

}

- (void)setImages:(NSArray *)images{

    _images = images;
    self.imageCount = images.count;
    self.pageControl.numberOfPages = self.imageCount;
    [self addPics];

}

- (void)addPics{

    for (int i = 0; i < self.images.count + 2; i++) {
        UIImageView *imageView = self.imageViews[i];
        if (i == 0) {
            imageView.image = [self.images lastObject];
        }else if(i == self.images.count + 1){
            imageView.image = [self.images firstObject];
        }else{
            imageView.image = self.images[i - 1];
        }
    }

}

- (void)layoutSubviews{

    [super layoutSubviews];
    self.scrollView.frame = self.bounds;
    self.scrollView.contentSize = CGSizeMake((self.imageCount + 2) * self.imgW, 0);
    [self.scrollView setContentOffset:CGPointMake(self.imgW, 0) animated:NO];

}

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView{

    [self.timer invalidate];
    self.timer = nil;
}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate{

    self.timer = [NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(nextImage) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop ] addTimer:self.timer forMode:NSRunLoopCommonModes];

}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView{

    if (scrollView.contentOffset.x == self.imgW * (self.imageCount + 1)) {
        [self.scrollView setContentOffset:CGPointMake(self.imgW, 0) animated:NO];
    }else if(scrollView.contentOffset.x == 0){
        [self.scrollView setContentOffset:CGPointMake(self.imgW * (self.imageCount), 0) animated:NO];
    }

    self.pageControl.currentPage = (self.scrollView.contentOffset.x + self.imgW * 0.5f) / self.imgW - 1;

}

@end