# Events API
breaking

# Overview

$on, $off and $once instance methods are removed. Application instances no longer implement the event emitter interface.

# 2.x Syntax

In 2.x, Vue instance could be used to trigger handlers attached imperatively via the event emitter API ($on, $off and $once). This was used to create event hubs to create global event listeners used across the whole application:

// eventHub.js

const eventHub = new Vue()

export default eventHub
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// ChildComponent.vue
import eventHub from './eventHub'

export default {
  mounted() {
    // adding eventHub listener
    eventHub.$on('custom-event', () => {
      console.log('Custom event triggered!')
    })
  },
  beforeDestroy() {
    // removing eventHub listener
    eventHub.$off('custom-event')
  }
}
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// ParentComponent.vue
import eventHub from './eventHub'

export default {
  methods: {
    callGlobalCustomEvent() {
      eventHub.$emit('custom-event') // if ChildComponent is mounted, we will have a message in the console
    }
  }
}
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# 3.x Update

We removed $on, $off and $once methods from the instance completely. $emit is still a part of the existing API as it's used to trigger event handlers declaratively attached by a parent component

# Migration Strategy

In Vue 3, it is no longer possible to use these APIs to listen to a component's own emitted events from within a component, there is no migration path for that use case.

But the eventHub pattern can be replaced by using an external library implementing the event emitter interface, for example mitt (opens new window) or tiny-emitter (opens new window).

Example:

//eventHub.js
import emitter from 'tiny-emitter/instance'

export default {
  $on: (...args) => emitter.on(...args),
  $once: (...args) => emitter.once(...args),
  $off: (...args) => emitter.off(...args),
  $emit: (...args) => emitter.emit(...args),
}
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This provides the same event emitter API as in Vue 2.

These methods may also be supported in a future compatibility build of Vue 3.

Migration build flag: INSTANCE_EVENT_EMITTER

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Last updated: 2021-06-07, 16:47:59 UTC