                             }
                             
                             self.completeWithImage(image, avatarURLString: avatarURLString)
+
+                        } else {
+                            self.completeWithImage(self.defaultRoundAvatarOfRadius(radius), avatarURLString: avatarURLString)
                         }
                     }
                 }
             }
+
+        } else {
+            completion(defaultRoundAvatarOfRadius(radius))
         }
     }
 }

                            }

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


    func roundAvatarOfUser(user: User, withRadius radius: CGFloat, completion: (UIImage) -> ()) {

        completion(defaultRoundAvatarOfRadius(radius))

        // 为下面切换线程准备，Realm 不能跨线程访问
        let avatarURLString = user.avatarURLString
        let userID = user.userID

        if avatarURLString.isEmpty {
            return
        }


                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
