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Observer pattern - Design Patterns

oop programming design-patterns

The observer pattern is a behavioural pattern. A one-to-many relationship between objects is created so that when the state of one object changes all its children objects are notified and updated automatically.

Pattern type

This is a behavioural pattern.

Intent

A one-to-many relationship between objects is created so that when the state of one object changes all its children objects are notified and updated automatically.

The pattern intent is to make it easier to maintain consistency between related objects without making them tightly coupled.

Problem

You have many clients that need to be updated when the state of of object changes.

Pattern

Observer
+update()
ConcreteObserver
+update()
Subject
+register_observer()
+remove_observer()
+notify_observer()
ConcreteSubject
+register_observer()
+remove_observer()
+notify_observer()
+get_state()
+set_state()
  • the Subject class is the object that is being observed
  • the Observer class is the object that is observing the Subject
  • the ConcreteSubject implements the Subject interface
  • the ConcreteObserver can be any class that implements the Observer interface

Code example

Consider this psuedo code example:

class Subject:
    def __init__(self):
        pass
 
    def register_observer(self, observer):
        pass
 
    def remove_observer(self, observer):
        pass
 
    def notify_observers(self):
        pass
 
 
class Observer:
    def __init__(self):
        pass
 
    def update(self):
        pass
 
 
class ConcreteSubject:
    def __init__(self):
        pass
 
    def register_observer(self, observer):
        # add observer to list of observers
        pass
 
    def remove_observer(self, observer):
        # remove observer from list of observers
        pass
 
    def notify_observers(self):
        # notify all observers
        pass
 
    def get_state(self):
        # get state of subject
        pass
 
    def set_state(self, state):
        # set state of subject
        pass
 
class ConcreteObserver:
    def __init__(self):
        pass
 
    def update(self):
        # update observer
        pass
 
    # any other methods

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