// BOOL cache = [[SDWebImageManager sharedManager] cachedImageExistsForURL:[NSURL URLWithString:headImageString]];
//
// if (cache) {
// [[SDImageCache sharedImageCache] removeImageForKey:headImageString fromDisk:YES];
//
// }
不断地刷新 下载的URL
[_headProtrail sd_setImageWithURL:<#(NSURL *)#> placeholderImage:<#(UIImage *)#> options:SDWebImageRefreshCached];