#1 add observer

DesignPattern/Observer/README.md

@@ -0,0 +1,13 @@
+# Observer
+
+![observer](https://refactoring.guru/images/patterns/content/observer/observer.png)
+
+Thuộc trong nhóm các mô hình chức năng, cung cấp một **cơ chế đăng kí** nhằm thông báo cho
+nhiều đối tượng đang về bất cứ sự kiện xảy ra đối với đối tượng đang bị theo dõi. 
+
+## Mô hình chức năng 
+
+Bao gồm các mô hình liên quan đến các thuật toán và phép gán trách nghiệm giữa các dối tượng. 
+
+
+## 

DesignPattern/Observer/conceptual.py

@@ -1,17 +1,134 @@
-# coding: utf-8
-"""
+from __future__ import annotations
+from abc import ABC, abstractmethod
+from random import randrange
+from typing import List
+
+
+class Subject(ABC):
+    """
+    The Subject interface declares a set of methods for managing subscribers.
+    """
+
+    @abstractmethod
+    def attach(self, observer: Observer) -> None:
+        """
+        Attach an observer to the subject.
+        """
+        pass
+
+    @abstractmethod
+    def detach(self, observer: Observer) -> None:
+        """
+        Detach an observer from the subject.
+        """
+        pass
+
+    @abstractmethod
+    def notify(self) -> None:
+        """
+        Notify all observers about an event.
+        """
+        pass
+
+
+class ConcreteSubject(Subject):
+    """
+    The Subject owns some important state and notifies observers when the state
+    changes.
+    """
+
+    _state: int = None
+    """
+    For the sake of simplicity, the Subject's state, essential to all
+    subscribers, is stored in this variable.
+    """
+
+    _observers: List[Observer] = []
+    """
+    List of subscribers. In real life, the list of subscribers can be stored
+    more comprehensively (categorized by event type, etc.).
+    """
+
+    def attach(self, observer: Observer) -> None:
+        print("Subject: Attached an observer.")
+        self._observers.append(observer)
+
+    def detach(self, observer: Observer) -> None:
+        self._observers.remove(observer)
+
+    """
+    The subscription management methods.
+    """
+
+    def notify(self) -> None:
+        """
+        Trigger an update in each subscriber.
+        """
+
+        print("Subject: Notifying observers...")
+        for observer in self._observers:
+            observer.update(self)
+
+    def some_business_logic(self) -> None:
+        """
+        Usually, the subscription logic is only a fraction of what a Subject can
+        really do. Subjects commonly hold some important business logic, that
+        triggers a notification method whenever something important is about to
+        happen (or after it).
+        """
+
+        print("\nSubject: I'm doing something important.")
+        self._state = randrange(0, 10)
+
+        print(f"Subject: My state has just changed to: {self._state}")
+        self.notify()
+
+
+class Observer(ABC):
+    """
+    The Observer interface declares the update method, used by subjects.
+    """
+
+    @abstractmethod
+    def update(self, subject: Subject) -> None:
+        """
+        Receive update from subject.
+        """
+        pass
+
 
-Abstract::
-    - 
-History::
-    - Ver.      Date            Author        History
-    - 
-Copyright (C) 2020 HACHIX Corporation. All Rights Reserved.
 """
-# 標準ライブラリ
+Concrete Observers react to the updates issued by the Subject they had been
+attached to.
+"""
+
+
+class ConcreteObserverA(Observer):
+    def update(self, subject: Subject) -> None:
+        if subject._state < 3:
+            print("ConcreteObserverA: Reacted to the event")
+
+
+class ConcreteObserverB(Observer):
+    def update(self, subject: Subject) -> None:
+        if subject._state == 0 or subject._state >= 2:
+            print("ConcreteObserverB: Reacted to the event")
+
+
+if __name__ == "__main__":
+    # The client code.
+
+    subject = ConcreteSubject()
+
+    observer_a = ConcreteObserverA()
+    subject.attach(observer_a)
 
+    observer_b = ConcreteObserverB()
+    subject.attach(observer_b)
 
-# 関連外部ライブラリ
+    subject.some_business_logic()
+    subject.some_business_logic()
 
+    subject.detach(observer_a)
 
-# 内部ライブラリ
+    subject.some_business_logic()