OBSERVER PATTERN

Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically

UML Diagram

Participants

• Subject: The Subject interface declares a set of methods for managing subscribers.
• ConcreteSubject: The ConcreteSubject class is responsible for storing the state of interest to ConcreteObserver objects and sending notifications to observers in the observer pattern.
• Observer: The Observer interface declares the update method, which is called by the subject.
• ConcreteObserver: The ConcreteObserver class implements the Observer interface and stores a reference to the ConcreteSubject object. ConcreteObserver's update method is called by the subject when the state of the subject changes.

When to use the Observer Pattern

• When a change to one object requires changing others, and you don't know how many objects need to be changed.
• When an object should be able to notify other objects without making assumptions about who these objects are.
• When an object should be able to notify other objects without knowing the details of the objects.

Pros

• Open/Closed Principle: You can introduce new subscriber classes without having to change the publisher's code (and vice versa if there's a publisher interface).
• Single Responsibility Principle: You can segregate the publisher and the subscriber into two classes.
• Liskov Substitution Principle: You can introduce new subscriber classes without breaking the existing publisher's code.
• Interface Segregation Principle: You can introduce new subscriber classes without changing the publisher's interface.
• Dependency Inversion Principle: You can refer to the abstract publisher interface instead of specific publisher classes.

Cons

• Subscribers are notified in random order*.
• The publisher may continue to send updates to subscribers that are no longer interested in receiving them.
• The publisher may send updates to subscribers that are in an inconsistent state.
• The publisher may send updates to subscribers that are no longer in memory.

Example

Check the src directory for the implementation of the observer pattern in TypeScript.

To run the main file (src/index.ts) and see the output, run ts-node src/index.ts in your terminal.

N/B: You must have ts-node installed globally to run the command above. If you don't have it installed, you can install it by running npm install -g ts-node.