iOS -根据网络状态加载图片

#import <UIImageView+WebCache.h>
#import "SDWebImage.h"
#import <AFNetworking.h>

/** 小图 */
@property (nonatomic, copy) NSString *image0;
/** 中图 */
@property (nonatomic, copy) NSString *image2;
/** 大图 */
@property (nonatomic, copy) NSString *image1;

// 占位图片
    UIImage *placeholder = nil;
    // 根据网络状态来加载图片
    AFNetworkReachabilityManager *mgr = [AFNetworkReachabilityManager sharedManager];
    // 获得原图（SDWebImage的图片缓存是用图片的url字符串作为key）
    UIImage *originImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:topic.image1];
    if (originImage) { // 原图已经被下载过
        self.imageView.image = originImage;
    } else { // 原图并未下载过
        if (mgr.isReachableViaWiFi) {
            [self.imageView sd_setImageWithURL:[NSURL URLWithString:topic.image1] placeholderImage:placeholder];
        } else if (mgr.isReachableViaWWAN) {
#warning downloadOriginImageWhen3GOr4G配置项的值需要从沙盒里面获取
            // 3G4G网络下时候要下载原图
            BOOL downloadOriginImageWhen3GOr4G = YES;
            if (downloadOriginImageWhen3GOr4G) {
                [self.imageView sd_setImageWithURL:[NSURL URLWithString:topic.image1] placeholderImage:placeholder];
            } else {
                [self.imageView sd_setImageWithURL:[NSURL URLWithString:topic.image0] placeholderImage:placeholder];
            }
        } else { // 没有可用网络
            UIImage *thumbnailImage = [[SDImageCache sharedImageCache] imageFromDiskCacheForKey:topic.image0];
            if (thumbnailImage) { // 缩略图已经被下载过
                self.imageView.image = thumbnailImage;
            } else { // 没有下载过任何图片
                // 占位图片;
                self.imageView.image = placeholder;
            }
        }
    }