Type Based Global Events in Vue.js

In one of the latest freelance projects of mine, my client prefers Vue Framework which is recently super popular on the frontend side.

So, I dived into Vue. I can say that it is very practical and effective on the very first sight.

Besides, when we compare it with other predominant competitors like Angular and Aurelia, we can easily notice Vue has a very slight learning curve.

However, it didn’t take so long for me to stumble on a bad feeling that my code is getting unmanageable and becoming unable to be followed.

Undoubtfully, it wasn’t a big surprise to me because this is mostly what dynamic-typed languages make us resent the harsh trade-off along with their super cool advantages.

Today, I am going to show a practical and effective way of using global events in Vue Framework.

A Simple Event Bus in Vue

The typical way of implementing a global event bus in Vue is just using the Vue object itself:

// create a Vue instance somewhere you can access globally
let eventBus = new Vue()

// register an event handler
eventBus.$on("onAppStarted", () => console.log("App Started!"))

// publish an event
eventBus.$emit("onAppStarted")

// Super easy, right?

The Problem Coming From Strings

As long as our application has more than a couple of lines, sooner or later, we start stressing to follow which components publish and which others listen to them.

Therefore, we can imagine how hard to identify a simple typo like onApStarted instead of onAppStarted as an event name in a large project:

eventBus.$on("onApStarted", () => {
  // some business logic here  
})

Implicit Event Parameters

Moreover, because we don’t define any corresponding type or interface for our event parameters only God knows what and how many parameters might be for the onAppStarted event.

In order to identify, we resent doing this kind of tests everytime we confuse:

eventBus.$on("onAppStarted", (...args) => {
  args.forEach(e => console.log(e))    
})

A Proper Solution Comes From ES6+

As being a fan of static-typed Java world, whatever language I do, I prefer using types clearly unless it’s super unconventional for the specific language.

Thus, I will show a solution to get rid of these string-based event names by using the capabilities ECMAScript 6 and later offers.

Defining Event Types

Let’s create a separate file called app-events.js to define our event types:

/**
* Event type to publish when app loads
* ProducedBy: components/preload.js
* ConsumedBy: App.vue, views/MainPanel.vue
**/
export class AppStartEvent {

  constructor(){
    // An event type with no arguments
  }

}

/**
* Event type to publish when code changes
* ProducedBy: views/CodePanel.vue
* ConsumedBy: views/MainPanel.vue
* @param {object} editor editor instance
* @param {string} code changed code value inside the editor
**/
export class CodeChangeEvent {

  constructor(editor, code){
    this.editor = editor
    this.code = code
  }

}

As we can notice, defining event type classes and parameters in constructor explicitly offers us great readability.

Although it is optional, we recommend keeping comments updated. This provides a way to follow the components which deal with a certain event type.

Importing Event Types

When we want to use our events, we should import them into our components:

import {AppStartEvent, CodeChangeEvent} from "@/app-events"

As we specify explicitly every event type we need, it brings us another important benefit that we can easily identify what events are involved in a component.

Registering an Event

In order to register our event, simply we use our event types and their static name properties:

import {AppStartEvent} from "@/app-events"

eventBus.$on(AppStartEvent.name, () => console.log("App Started!"))

Plus, we can expect the event type instance itself as a single argument instead of more than one arguments:

import {AppStartEvent, CodeChangeEvent} from "@/app-events"

// we can access the event type instance as a single argument
eventBus.$on(AppStartEvent.name, event => console.log(event))

// also can access to event parameters
eventBus.$on(CodeChangeEvent.name, event => {
  console.log(event.editor)  
  console.log(event.code)  
})

Publishing an Event

Now we can publish our events by creating a new instance of that event type:

// no parameters
eventBus.$emit(AppStartEvent.name, new AppStartEvent())

// with parameters
eventBus.$emit(CodeChangeEvent.name, new CodeChangeEvent(editor, "some code here..."))

Implementing a Wrapper Class

Certainly, we may proceed to define a class as EventBus and wrap the basic methods of Vue instance.

class EventBus {

  $eventbus = new Vue()

  listen (eventClass, handler) {
    this.$eventBus.$on(eventClass.name, handler)
  }

  publish (event) {
    this.$eventBus.$emit(event.constructor.name, event)
  }

}

Therefore, we can use it in a more practical way:

// register an event handler
EventBus.listen(AppStartEvent, () => console.log("App Started!"))

// publish an event
EventBus.publish(new AppStartEvent())

Using as a Plugin

We may prefer to use our EventBus as a Vue Plugin :

export default {

  $eventBus: null,

  install (Vue, options) {
    this.$eventBus = new Vue()
  },

  listen (eventClass, handler) {
    this.$eventBus.$on(eventClass.name, handler)
  },

  listenOnce (eventClass, handler) {
    this.$eventBus.$once(eventClass.name, handler)
  },

  remove (eventClass, handler) {
    if (handler) {
      this.$eventBus.$off(eventClass.name, handler)
    } else {
      this.$eventBus.$off(eventClass.name)
    }
  },

  removeAll () {
    this.$eventBus.$off()
  },

  publish (event) {
    this.$eventBus.$emit(event.constructor.name, event)
  }

}

In order to use the plugin, we should import and register to our Vue instance:

import EventBus from '@/plugin/vue-event-bus';

Vue.use(EventBus)

Consequently, we can simply import and use in any other Vue component as well:

import EventBus from '@/plugin/vue-event-bus';
import {AppStartEvent} from "@/app-events"

// register an event handler
EventBus.listen(AppStartEvent, () => console.log("App Started!"))

// publish an event
EventBus.publish(new AppStartEvent())

Finally

In this short tutorial, I explained how to implement type-based global events and use them in Vue.

You can find the plugin over on GitHub.

So, what do you think about this approach or anything to extend? I would like to see your comments below!