Vue 3 Event Bus with Composition API

I have setup mitt and trying to dispatch event to another component but I am having hard time because in the setup() method it doesn’t have this for accessing app instance.

Here is what I tried:

import App from './App.vue'
const el = document.getElementById('app')

import mitt from 'mitt';
const emitter = mitt();

const app = createApp(App)
app.config.globalProperties.emitter = emitter;
app.mount(el);

And in the component, I want to dispatch an event

export default {
   setup() {
      function toggleSidebar() {
          this.emitter.emit('toggle-sidebar');

          console.log(this); // binds to setup(), not the vue instance.
      }
   }
}

As this doesn’t exist, I can’t access the .emitter. What am I missing? How to use officially suggested mitt in Vue 3 composition api?

---

By the way if I use the v2 syntax, I can access this.emitter. But I am curious about Composition API way

export default {
  mounted() {
    console.log(this.emitter); // works
  }
} 

Answer

To use an event bus in Vue 3 Composition API, use Vue 3’s new provide api in main.js, and then inject in any component:

1. Install mitt:

npm install mitt

2. Provide:

main.js

import { createApp } from 'vue';
import App from './App.vue';

import mitt from 'mitt';                  // Import mitt
const emitter = mitt();                   // Initialize mitt

const app = createApp(App);
app.provide('emitter', emitter);          // ✅ Provide as `emitter`
app.mount('#app');

3. Inject

3a. Any Component – Emit an event

import { inject } from 'vue'

export default {
  setup() {
    const emitter = inject('emitter'); // Inject `emitter`
    const mymethod = () => {
      emitter.emit('myevent', 100);
    };
    return {
      mymethod
    }
  }
}

^{Call mymethod from a button click or something.}

3b. Any Component – Listen for the event

import { inject } from 'vue'

export default {
  setup() {
    const emitter = inject('emitter');   // Inject `emitter`

    emitter.on('myevent', (value) => {   // *Listen* for event
      console.log('myevent received!', `value: ${value}`);
    });
  },
}

Console

myevent received! value: 100