直接从SDWebImage的缓存中取图片

#import "SDWebImageManager.h"

#import "SDImageCache.h"

NSString* strUrl = @"http://xxx.com/x.jpg";

SDWebImageManager *manager = [SDWebImageManager sharedManager];

NSString* key = [manager cacheKeyForURL:[NSURL URLWithString:strUrl]];

SDImageCache* cache = [SDImageCache sharedImageCache];

//此方法会先从memory中取。

image = [cache imageFromDiskCacheForKey:key];