这里用到AFN框架和SDWebImage框架

首先需要AFN监控网络状态
在AppDelegate.m中

[[AFNetworkReachabilityManager sharedManager] startMonitoring];

创建一个UIImageView的Category
封装各种判断网络的状态的代码,以及缓存问题
可以防止tableViewCell的循环利用导致出现的各种问题
贴代码:

#import <UIKit/UIKit.h>
#import <UIImageView+WebCache.h>
#import <AFNetworking.h>
@interface UIImageView (YYDownload)
/**
 *   设置imageView显示的图片
 *
 *  @param originalImageURL 原图URL
 *  @param thumbnailImageURL   缩略图URL
 *  @param placeholderImage 占位图片
 *  @param completedBlock    获取图片完毕之后会调用的block
 */
- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL placeholderImage:(UIImage *)placeholderImage completed:(SDWebImageCompletionBlock)completedBlock;

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL completed:(SDWebImageCompletionBlock)completedBlock;

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL ;

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL placeholderImage:(UIImage *)placeholderImage;
@end
#import "UIImageView+YYDownload.h"

@implementation UIImageView (YYDownload)

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL completed:(SDWebImageCompletionBlock)completedBlock
{
    [self downloadOriginalImageURL:OriginalImageURL thumbnailImageURL:thumbnailImageURL placeholderImage:nil completed:completedBlock];
}

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL
{
    [self downloadOriginalImageURL:OriginalImageURL thumbnailImageURL:thumbnailImageURL placeholderImage:nil completed:nil];
    
}
- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL placeholderImage:(UIImage *)placeholderImage
{
    [self downloadOriginalImageURL:OriginalImageURL thumbnailImageURL:thumbnailImageURL placeholderImage:placeholderImage completed:nil];
}

/*
 sd_setImageWithURL:placeholderImage:方法的执行步骤
 1.取消当前imageView之前关联的请求
 2.设置占位图片到当前imageView上面
 3.如果缓存中有对应的图片，那么就显示到当前imageView上面
 4.如果缓存中没有对应的图片，发送请求给服务器下载图片
 */

- (void)downloadOriginalImageURL:(NSString *)OriginalImageURL thumbnailImageURL:(NSString *)thumbnailImageURL placeholderImage:(UIImage *)placeholderImage completed:(SDWebImageCompletionBlock)completedBlock
{
      UIImage *originalImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:OriginalImageURL];
      if (originalImage) {//如果有,sd_setImageWithURL:内部会取消上次操作的请求,
      [self sd_setImageWithURL:[NSURL URLWithString:OriginalImageURL] completed:completedBlock];
      }else {//没有原图
       AFNetworkReachabilityManager *manager = [AFNetworkReachabilityManager sharedManager];
    if (manager.isReachableViaWiFi) {//wifi情况下,下载原图
        [self sd_setImageWithURL:[NSURL URLWithString:OriginalImageURL] placeholderImage:placeholderImage completed:completedBlock];
       }else if (manager.isReachableViaWWAN){
        
        // 在使用手机自带网络
        //     用户的配置项假设利用NSUserDefaults存储到了沙盒中
        //    [[NSUserDefaults standardUserDefaults] setBool:NO forKey:@"alwaysDownloadOriginalImage"];
        //    [[NSUserDefaults standardUserDefaults] synchronize];
        //考虑用户的设置,如果仍然下载原图
#warning 从沙盒中读取用户的配置项：在3G\4G环境是否仍然下载原图
        //手机网络下,下载缩略图
        BOOL alwaysDownloadOriginalImage = [NSUserDefaults standardUserDefaults];
        if (alwaysDownloadOriginalImage) {
            //用户设置的YES下载原图
            [self sd_setImageWithURL:[NSURL URLWithString:OriginalImageURL]placeholderImage:placeholderImage completed:completedBlock];
        }else {
            //下载小图
            [self sd_setImageWithURL:[NSURL URLWithString:thumbnailImageURL] placeholderImage:placeholderImage completed:completedBlock];
        }
        
        
    }else{//没有网络,(没有原图)
        //从缓存/沙盒中看有没有小图
        UIImage *thumbnailImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:thumbnailImageURL];
        if (thumbnailImage){
            //如果缓存/沙盒中有小图
            [self sd_setImageWithURL:[NSURL URLWithString:thumbnailImageURL] completed:completedBlock];
        }else{//没有设置为空
            [self sd_setImageWithURL:nil placeholderImage:placeholderImage completed:completedBlock];
        }
        
    }
}
}

@end