新版的SDWebImage   已经计算好文件大小 只需要调用清理就够了

//获取缓存文件大小

        float tmpSize = [[SDImageCache sharedImageCache] getSize];

        NSString *clearCacheName =  [NSString stringWithFormat:@"确定清理缓存(%.2fM)？",tmpSize/1024/1024]; 

 UIAlertView *alertView = [[UIAlertView alloc]initWithTitle:nil message:clearCacheName delegate:self cancelButtonTitle:@"取消" otherButtonTitles:@"确定", nil];

        [alertView setTag:1001];

        [alertView show];

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {

    if (alertView.tag == 1001) {  

        if (buttonIndex == 1)   {

            [[SDImageCache sharedImageCache] clearDisk];

          }

    }

}

/*

注意  是clearDisk  不是cleanDisk

*/ 

利用SDWebImage 清理缓存