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Observer pattern in JS

Implementation

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. -wikipedia

Basically, if you've ever worked with PubSub , it's the same thing. It's one of the 23 pattherns defined in Gang of four pattern book. And it's used when, You need to maintain a one to many relation between objects.

Benefits:

  • It supports the principle of loose coupling between objects that interact with each other
  • It allows sending data to other objects effectively without any change in the Subject or Observer classes
  • Observers can be added/removed at any point in time

- Learning Python Design Patterns - O'Reilly

Let us create the Subscriber first. From the definition we realize the Publisher should have a list of subscribers, So lets just do that.

function Publisher() {
  this.subscribers = [];
}

Now, there should be an easy way to append a new subscriber to the subscribers array. Let's add that too.

function Publisher() {
  this.subscribers = [];

  this.subscribe = (subscriber) => {
    this.subscribers.push(subscriber)
  }
}

The same way, subscribers should be able to unsubscribe easily.

this.unsubscribe = (unsubscriber) => {
  this.subscribers = this.subscribers.filter(subscriber => {
    if(subscriber !== unsubscriber) {
      return subscriber;
    }
 })
}

And, finally, the publisher should be able to publish.

this.publish = () => {
  this.subscribers.forEach(sub => sub.call())
}

So, the final code should look like

function Publisher() {
  this.subscribers = [];

  this.subscribe = (subscriber) => {
    this.subscribers.push(subscriber)
  }

  this.unsubscribe = (unsubscriber) => {
    this.subscribers = this.subscribers.filter(subscriber => {
      if(subscriber !== unsubscriber) {
        return subscriber;
      }
    })
  }

  this.publish = () => {
    this.subscribers.forEach(sub => sub.call())
  }
}

In short, each of the publisher object will have it's own list of subscribers, on which it can append or remove subscribers easily, and when the publish method is invoked, each of the subscribers should be notified.

Now, let's create our subscribers. For brevity, we'll be using console.log instead of actual api calls 😃

const subscriberOne = () => {
  console.log("calling subscriber one")
}

const subscriberTwo = () => {
  console.log("calling subscriber two")
}

And Voila!. we're done. Now let's put it on test

p = new Publisher();

// subscrbe
p.subscribe(subscriberOne);
p.subscribe(subscriberTwo);

// unsubscribe
p.unsubscribe(subscriberTwo);

p.publish()
=> "calling subscriber one"

Live code

You can find the implementation here, modify it as you want and play.

https://repl.it/@SSalekin/ObserverPatternJs

Useful resources

Wikipedia (as always)

Medium article 'The Observer Pattern' by Steven Levia

This excellent youtube video


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