 
     func roundAvatarWithAvatarURLString(avatarURLString: String, withRadius radius: CGFloat, completion: (UIImage) -> ()) {
 
-        completion(defaultRoundAvatarOfRadius(radius))
-
         if avatarURLString.isEmpty {
+            completion(defaultRoundAvatarOfRadius(radius))
+
             return
         }
 

                            }
                            
                            self.completeWithImage(image, avatarURLString: avatarURLString)
                        }
                    }
                }
            }
        }
    }
}

                avatarCompletions.append(avatarCompletion)

                if avatarCompletions.filter({ $0.avatarURLString == avatarURLString }).count > 1 {
                    // 不用做事，等着具有同样 avatarURLString 的 avatarCompletion 帮忙完成

                } else {
                    // 再看看是否已下载
