Determine whether a component is linked to an event listener

Question

Determine whether a component is linked to an event listener

If a <Form> component is called with a @cancel event listener attached to it, the cancel button that triggers this event should be visible. If no @cancel event is present, the cancel button should not be displayed.

Is there a method to verify if a component has an event listener attached to it?

Currently, I use:

<template>
  <form>
    <button v-if="cancelEventPassed" @click="$emit('cancel')">Cancel</button>
  </form>
</template>

To call it:

<Form :cancelEventPassed="true" @cancel="handle_cancel" />

or

<Form/>

Is it feasible to accomplish this without utilizing an additional property like cancelEventPassed?

javascript vue.js vuejs2 vue-component

Answer 1

Answer №1

Whenever a component has listeners attached to it, those listeners can be accessed through the $listeners property of the component.

You can utilize this property to check if a specific listener exists. Here's an example of a computed property that verifies the presence of a cancel listener.

computed:{
  hasCancelListener(){
    return this.$listeners && this.$listeners.cancel
  }
}

Below is how you can implement this in a component:

console.clear()

Vue.component("CustomForm", {
  template:`
    <div>
      <h1>Custom Form</h1>
      <button v-if="hasCancelListener" @click="$emit('cancel')">I have a listener!</button>
    </div>
  `,
  computed:{
    hasCancelListener(){
      return this.$listeners && this.$listeners.cancel
    }
  },
})

new Vue({
  el: "#app",
  methods:{
    onCancel(){
      alert('canceled')
    }
  }
})

<script src="https://unpkg.com/<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="5422213114667a607a66">[email protected]</a>"></script>
<div id="app">
  <custom-form @cancel="onCancel"></custom-form>
  <hr>
  <custom-form></custom-form>
</div>

Answer 2

Whenever a component has listeners attached to it, those listeners can be accessed through the $listeners property of the component.

You can utilize this property to check if a specific listener exists. Here's an example of a computed property that verifies the presence of a cancel listener.

computed:{
  hasCancelListener(){
    return this.$listeners && this.$listeners.cancel
  }
}

Below is how you can implement this in a component:

console.clear()

Vue.component("CustomForm", {
  template:`
    <div>
      <h1>Custom Form</h1>
      <button v-if="hasCancelListener" @click="$emit('cancel')">I have a listener!</button>
    </div>
  `,
  computed:{
    hasCancelListener(){
      return this.$listeners && this.$listeners.cancel
    }
  },
})

new Vue({
  el: "#app",
  methods:{
    onCancel(){
      alert('canceled')
    }
  }
})

<script src="https://unpkg.com/<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="5422213114667a607a66">[email protected]</a>"></script>
<div id="app">
  <custom-form @cancel="onCancel"></custom-form>
  <hr>
  <custom-form></custom-form>
</div>

Answer 3

Answer №2

With the release of Vue 3, the use of the $listeners object has been phased out, as detailed in this resource. Now, listeners are integrated into the $attrs object and they have to be prefixed with on.

If you need to verify the existence of a specific listener in a child component, you can employ the following approach:

computed: {
  hasCancelListener() : boolean {
    return (this.$attrs && this.$attrs.onCancel) as boolean
  }
}

The child component's implementation should look like this:

<custom-form @cancel="onCancel"></custom-form>

Answer 4

With the release of Vue 3, the use of the $listeners object has been phased out, as detailed in this resource. Now, listeners are integrated into the $attrs object and they have to be prefixed with on.

If you need to verify the existence of a specific listener in a child component, you can employ the following approach:

computed: {
  hasCancelListener() : boolean {
    return (this.$attrs && this.$attrs.onCancel) as boolean
  }
}

The child component's implementation should look like this:

<custom-form @cancel="onCancel"></custom-form>

Answer 5

Answer №3

After facing an issue with @prerak-sola's solution not working while defining emits in this manner (as pointed out by Adam Reis):

const emit = defineEmits<{
  (e: 'click', v: MouseEvent): void;
  (e: 'update:modelValue', v: MouseEvent): void;
}>();

I realized that since Vue converts all props to an object and adds a prefix on before each event name, you can simply check if the event listener is defined in the vnode:

const hasClickEventListener = computed(() => !!getCurrentInstance()?.vnode.props?.onClick);
const hasModelValueUpdateListener = computed(() => !!getCurrentInstance()?.vnode.props?.['onUpdate:modelValue']);

However, I couldn't find any official documentation on this approach (and it's more complex than using useAttrs). Proceed with caution.

Answer 6

After facing an issue with @prerak-sola's solution not working while defining emits in this manner (as pointed out by Adam Reis):

const emit = defineEmits<{
  (e: 'click', v: MouseEvent): void;
  (e: 'update:modelValue', v: MouseEvent): void;
}>();

I realized that since Vue converts all props to an object and adds a prefix on before each event name, you can simply check if the event listener is defined in the vnode:

const hasClickEventListener = computed(() => !!getCurrentInstance()?.vnode.props?.onClick);
const hasModelValueUpdateListener = computed(() => !!getCurrentInstance()?.vnode.props?.['onUpdate:modelValue']);

However, I couldn't find any official documentation on this approach (and it's more complex than using useAttrs). Proceed with caution.

Answer 7

Answer №4

Vue 3 Simplified Example

this.$attrs.onCancel

Explanation

In Vue 3, the $listeners object has been removed and listeners are now separated from the $attrs object with the prefix on...

Reference

Check out more information here

Answer 8

Vue 3 Simplified Example

this.$attrs.onCancel

Explanation

In Vue 3, the $listeners object has been removed and listeners are now separated from the $attrs object with the prefix on...

Reference

Check out more information here

Answer 9

Answer №5

Understanding Vue 3

const thisInstance = getCurrentInstance();

const hasTaskClickedListener = computed(() => {
  if (!thisInstance) return null;

  return thisInstance.attrs?.onClicked;
});

Insights:

this is similar to using getCurrentInstance() in Vue 3
The listeners object is modified by the attrs property with the prefix "on" such as onClick, onDelete, and others
Always include getCurrentInstance() in the setup script to avoid receiving null values (which occurs when Vue cannot detect its instance)

Answer 10

Understanding Vue 3

const thisInstance = getCurrentInstance();

const hasTaskClickedListener = computed(() => {
  if (!thisInstance) return null;

  return thisInstance.attrs?.onClicked;
});

Insights:

this is similar to using getCurrentInstance() in Vue 3
The listeners object is modified by the attrs property with the prefix "on" such as onClick, onDelete, and others
Always include getCurrentInstance() in the setup script to avoid receiving null values (which occurs when Vue cannot detect its instance)

Answer 11

Answer №6

To verify the existence of a listener, use the following code snippet: this._events['listener-name']

Answer 12

To verify the existence of a listener, use the following code snippet: this._events['listener-name']

Answer 13

Answer №7

If you're searching for a solution using Vue 3's script setup or setup function, consider utilizing the attrs key within the getCurrentInstance function.

<template>
  <form>
    <button @click="$emit('cancel')">Cancel</button>
  </form>
</template>

<custom-form @cancel="onCancel"></custom-form>

onMounted(() => {
  const instance = getCurrentInstance() // only accessible inside lifecycle hooks
  console.log(instance?.attrs?.onCancel)
})

Answer 14

If you're searching for a solution using Vue 3's script setup or setup function, consider utilizing the attrs key within the getCurrentInstance function.

<template>
  <form>
    <button @click="$emit('cancel')">Cancel</button>
  </form>
</template>

<custom-form @cancel="onCancel"></custom-form>

onMounted(() => {
  const instance = getCurrentInstance() // only accessible inside lifecycle hooks
  console.log(instance?.attrs?.onCancel)
})

Answer 15

Answer №8

Example Vue 2.7 script setup with useListeners and computed

import {useListeners, computed} from 'vue';

const eventListeners = useListeners();
const hasCancelEventListener= computed(() => {
    return typeof eventListeners['cancel'] !== 'undefined';
});

Answer 16

Example Vue 2.7 script setup with useListeners and computed

import {useListeners, computed} from 'vue';

const eventListeners = useListeners();
const hasCancelEventListener= computed(() => {
    return typeof eventListeners['cancel'] !== 'undefined';
});

Answer 17

Answer №9

Gratitude to abdul-aziz-al-basyir for the insightful solution.
When working with Vue 3, utilize attrs instead of relying on emit and listeners

<script setup>
const attrs = useAttrs()
const cancelEventPassed = computed(() => {
  return !!attrs.onCancel
})
function toCancel() {
  attrs.onCancel()
}
</script>
<template>
  <form>
    <button v-if="cancelEventPassed" @click="toCancel">Cancel</button>
  </form>
</template>

<Form @cancel="handle_cancel" />

Answer 18

Gratitude to abdul-aziz-al-basyir for the insightful solution.
When working with Vue 3, utilize attrs instead of relying on emit and listeners

<script setup>
const attrs = useAttrs()
const cancelEventPassed = computed(() => {
  return !!attrs.onCancel
})
function toCancel() {
  attrs.onCancel()
}
</script>
<template>
  <form>
    <button v-if="cancelEventPassed" @click="toCancel">Cancel</button>
  </form>
</template>

<Form @cancel="handle_cancel" />

Determine whether a component is linked to an event listener

Answer №1

Answer №2

Answer №3

Answer №4

Vue 3 Simplified Example

Explanation

Reference

Answer №5

Understanding Vue 3

Answer №6

Answer №7

Answer №8

Example Vue 2.7 script setup with useListeners and computed

Answer №9

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