IOS图片无限轮播器的实现原理

//
// PBCollectionCell.h
// 无限滚动
//
// Created by 裴波波 on 16/3/30.
// Copyright © 2016年 裴波波. All rights reserved.
//
#import <UIKit/UIKit.h>
@interface PBCollectionCell : UICollectionViewCell
@property(nonatomic,strong)UIImage * img;
@end
•cell.m文件
//
// PBCollectionCell.m
// 无限滚动
//
// Created by 裴波波 on 16/3/30.
// Copyright © 2016年 裴波波. All rights reserved.
//
#import "PBCollectionCell.h"
@interface PBCollectionCell ()
//不要忘记给collectionView的cell上设置图片框,并拖线到cell分类中
@property (strong, nonatomic) IBOutlet UIImageView *imgView;
@end
@implementation PBCollectionCell
//重写img的set方法来个cell进行赋值.(当调用cell.img = img的时候回调用set ..img的方法)
-(void)setImg:(UIImage *)img{
_img = img;
self.imgView.image = img;
}
@end

//
// ViewController.m
// 无限滚动
//
// Created by 裴波波 on 16/3/30.
// Copyright © 2016年 裴波波. All rights reserved.
//
#import "ViewController.h"
#import "PBCollectionCell.h"
//定义宏图片的个数
#define kPictureCount 3
@interface ViewController ()
@property (strong, nonatomic) IBOutlet UICollectionViewFlowLayout *flowLayout;
/**
* 图片的索引
*/
@property(nonatomic,assign) NSInteger index;
@end
•声明全局变量index为了拼接滚动过程中切换的图片的索引
@implementation ViewController
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section{
return kPictureCount;
}
-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
static NSString * ID = @"cell";
PBCollectionCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:ID forIndexPath:indexPath];
//图片索引只有下一站或者上一张,即 1,或者-1,为了切换图片
//中间的cell的脚标是1,滚动后脚标是2或者0,凑成next值为 1或者-1(让图片索引 1或者-1来完成切换图片),则
NSInteger next = indexPath.item - 1;
//为了不让next越界,进行取余运算 --------- next为了切换图片
next = (self.index   kPictureCount   next) % kPictureCount;
NSString * imgName = [NSString stringWithFormat:@"ld",next   1];
UIImage * img = [UIImage imageNamed:imgName];
cell.img = img;
return cell;
}

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
//计算偏移的整数倍,偏移了0或者是2, -1是让self.index图片索引 1 或者 -1以切换图片;
NSInteger offset = scrollView.contentOffset.x / scrollView.bounds.size.width - 1;
self.index = (offset   self.index   kPictureCount) % kPictureCount;
//本质就是改变cell的索引,再根据self.index来切换图片,使用取余让图片索引不至于越界
//异步主队列执行,为了不让连续滚动停止后,图片有闪动.
dispatch_async(dispatch_get_main_queue(), ^{
[self scrollToSecondCell];
});
}

-(void)scrollToSecondCell{
NSIndexPath * idxPath = [NSIndexPath indexPathForItem:1 inSection:0];
[self.collectionView scrollToItemAtIndexPath:idxPath atScrollPosition:UICollectionViewScrollPositionLeft animated:NO];
}
- (void)viewDidLoad {
[super viewDidLoad];
self.flowLayout.itemSize = self.collectionView.bounds.size;
self.flowLayout.scrollDirection = UICollectionViewScrollDirectionHorizontal;
self.flowLayout.minimumLineSpacing = 0;
self.collectionView.backgroundColor = [UIColor whiteColor];
self.collectionView.pagingEnabled = YES;
self.collectionView.bounces = NO;
self.collectionView.showsHorizontalScrollIndicator = NO;
[self scrollToSecondCell];
}
@end