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Custom Event Publisher-Subscriber Model on Top of Reactive Programming for Non-DOM Elements

by The Mighty ProgrammerFebruary 28th, 2021

Too Long; Didn't Read

RXJS offers Subject API which can be used to build custom in memory event pub sub construct.

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featured image - Custom Event Publisher-Subscriber Model on Top of Reactive Programming for Non-DOM Elements

Event-Based Programming is natural to any GUI based interface. HTML DOM¹ offers an inbuilt Event Notification model, but it is only useful when dealing with DOM Elements. There is no support for non-DOM elements.

Web applications have grown in complexity with time; also, JavaScript started participating in backend applications, which in turn made the world see JavaScript as more than HTML DOM manipulation gig.

This article showcases building up a custom event publisher-subscriber model on top of Reactive Programming for non-DOM elements.

Setting Up Stage

Event-Driven Programming

Event-Driven programming paradigm² introduces an intuitive flow of control based upon events.

Button is clicked => show dialog.

Vocabulary

One that emits events can be called Event Publisher or Event Emitter or Event Dispatcher.
One that listens to events is called Event Subscriber or Event Listener or Event Handler.

In general, Publisher-Subscriber or PubSub term is used collectively for event publishers and even subscribers.

Reactive Programming

Reactive Programming paradigm² introduces the processing of streams of data asynchronously. It is a trending paradigm² these days.

RxJS is a reactive programming extension for JavaScript. You are going to build an event notification model on top of it by modelling events as a stream of data.

In Action

Problem Statement

Consider modelling Thermostat, when room temperature crosses a certain threshold (

30°C

), AC would turn up, and windows would close.

Design

Consider class

Thermostat

which encapsulates the logic of monitoring and publishes events:

above: when temperature crossed above 30°C (threshold) from lower value.
below: when temperature crossed below or equal to 30°C (threshold) from higher value.

Data Structure for holding event structure

RxJS offers

Subject

which can be used to emit events to multiple subscribers.

class Thermostat {
  constructor(sensor) {
   
    // data structure for holding subscriptions 
    this.subscriptions = {
      above: new Subject(),
      below: new Subject()
    };
  }
  
}

Notice, there are two

Subject

objects. You can also create a single

Subject

object named

ThresholdCrossed

with direction

above

below

. Separating the event or combining the event is Design Concern, which may depend upon many factors.

Subscription API for Event

on

API for registering action based upon event.

Subject

offers

subscribe

API which would be called whenever there is new data is available.

on(event, action) {
  // sanatize event .. left code
  this.subscriptions[event].subscribe(v => action(v));
}

Publishing Events

Consider

monitor

method which detects the rise and falls in temperature and publishes event accordingly using

next

API.

monitor(currentTemperature) {
  // when temperature cross above THRESHOLD
  if (
    currentTemperature > THRESHOLD 
    && this.lastRecordedTemperature <= THRESHOLD
  ) {

    // publish event
    this.subscriptions["above"].next(currentTemperature);

  } else if (
    currentTemperature <= THRESHOLD 
    && this.lastRecordedTemperature >= THRESHOLD
  ) {

    // publish event
    this.subscriptions["below"].next(currentTemperature);
  }

  this.lastRecordedTemperature = currentTemperature;
 }

Whenever

next

is called the

on

API would be triggered; by design, the callback would be called.

Subscribing for event

When the temperature is

below

30°C then ac should go off, and the window should open; the opposite should happen for

above

event 30°C.

thermostat.on("below", t => {
  this.ac = false;
  this.window = true;
});

thermostat.on("above", t => {
  this.ac = true;
  this.window = false;
});

Wiring all pieces together.

Fiddling

Any variation you make with the above code itself a pattern:

Keeping event subscription data in the mediator class of Event-Emitter and Event-Listener is an in-memory Event Bus pattern.
If the event holds enough information that Event-Listener doesn’t look back to Event-Emitter; it is Event Carried State Transfer pattern
If you store all events in datastore, it is Event Sourcing pattern (possible for backend side).
If there is a network partition between Event Publisher and Event Subscriber, then collectively, it is a Reactive System. It is build using queuing technologies (possible for backend side).
An event may contain, reference or link back to its source; it is Source Aware Event pattern. In HTML DOM, each event has a field named target to point back to its source.