Sunday, February 25, 2018

Observer Pattern in Node.js

The Observer is a design pattern where an object (known as a subject) maintains a list of objects depending on it (observers), automatically notifying them of any changes to state.

The Observer pattern requires that the observer (or object) wishing to receive topic notifications must subscribe this interest to the object firing the event (the subject).

One or more observers are interested in the state of a subject and register their interest with the subject by attaching themselves. When something changes in our subject that the observer may be interested in, a notify message is sent which calls the update method in each observer. When the observer is no longer interested in the subject's state, they can simply detach themselves

We will try to implement Observer pattern.

Observable.js

    var EventEmitter = require('events').EventEmitter;

    class Observable extends EventEmitter {
        constructor() {
        super()
        }

        RaiseEvent(event_name, event_args) {
        console.log("Raising Event:" + event_name);
        this.emit(event_name, event_args);
        }

        Subscribe(event_name, listener) {
        console.log("Adding Listener for Event :" + event_name);
        this.on(event_name, listener);
        }

        Unsubscribe(event_name, listener) {
        console.log("Removing Listener for Event:" + event_name);
        this.removeListener(event_name, listener);
        }

        ListenerCount(event_name)
        {
        this.listenerCount(this, event_name)
        }
    }

    module.exports = Observable;


Testing the 'Observable' class.

index.js

    var Observable = require('./Observable');

    var observable = new Observable();

    var greet1 = function (name) {
        console.log('Hello ,' + name);
    }

    var greet2 = function (name) {
        console.log('How are you,' + name);
    }

    var bye1 = function (name) {
      console.log('Bye Bye ,' + name);
    }


    //two listeners get attached to 'greet' event
    observable.Subscribe('greet', greet1);
    observable.Subscribe('greet', greet2);

    observable.Subscribe('bye', bye1);

    console.log("Total listeners for 'greet' event = " + observable.listenerCount('greet') );
    console.log("Total listeners for 'bye' event = " + observable.listenerCount('bye') );

    //raising event for greet
    observable.RaiseEvent('greet', 'sangram')

    //raising event for swara
    observable.RaiseEvent('greet', 'swara')

    //raising event for sangram
    observable.RaiseEvent('bye', 'sangram')


    //remove greet1 listener
    observable.Unsubscribe('greet', greet1)
    console.log("Total listeners for 'greet' event = " + observable.listenerCount('greet'));


    //raising event greet for sangram
    observable.RaiseEvent('greet', 'sangram')

Run node index.js


output:
    Adding Listener for Event :greet
    Adding Listener for Event :greet
    Adding Listener for Event :bye
    Total listeners for 'greet' event = 2
    Total listeners for 'bye' event = 1
    Raising Event:greet
    Hello ,sangram
    How are you,sangram
    Raising Event:greet
    Hello ,swara
    How are you,swara
    Raising Event:bye
    Bye Bye ,sangram
    Removing Listener for Event:greet
    Total listeners for 'greet' event = 1
    Raising Event:greet
    How are you,sangram   

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