iOS SDWebImage判断图片是否已缓存

#import "SDImageCache.h"
/// 判断是否有imageURL的缓存
BOOL isExist = [[SDImageCache sharedImageCache] diskImageDataExistsWithKey:imageURL];
/// 从缓存中读取imageURL的图像
[[SDImageCache sharedImageCache] imageFromDiskCacheForKey:imageURL];