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:@"%02ld",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