Component Events #

This page assumes you've already read the Components Basics. Read that first if you are new to components.

Emitting and Listening to Events #

A component can emit custom events directly in template expressions (e.g. in a k-on handler) using the built-in $emit method:

<!-- MyComponent -->
<button @click="$emit('someEvent')">click me</button>

export default {
  methods: {
    submit() {
      this.$emit('submit')
    }
  }
}

The parent can then listen to it using k-on:

<MyComponent @some-event="callback" />

The .once modifier is also supported on component event listeners:

<MyComponent @some-event.once="callback" />

Like components and props, event names provide an automatic case transformation. Notice we emitted a camelCase event, but can listen for it using a kebab-cased listener in the parent. As with props casing, we recommend using kebab-cased event listeners in templates.

Event Arguments #

It's sometimes useful to emit a specific value with an event. For example, we may want the <BlogPost> component to be in charge of how much to enlarge the text by. In those cases, we can pass extra arguments to $emit to provide this value:

<button @click="$emit('increaseBy', 1)">
  Increase by 1
</button>

Then, when we listen to the event in the parent, we can use an inline arrow function as the listener, which allows us to access the event argument:

<MyButton @increase-by="(n) => count += n" />

Or, if the event handler is a method:

<MyButton @increase-by="increaseCount" />

Then the value will be passed as the first parameter of that method:

methods: {
  increaseCount(n) {
    this.count += n
  }
}

function increaseCount(n) {
  count.value += n
}

Declaring Emitted Events #

Emitted events can be explicitly declared on the component via the defineEmits() macroemits option:

<script setup>
defineEmits(['inFocus', 'submit'])
</script>

<script setup>
const emit = defineEmits(['inFocus', 'submit'])

function buttonClick() {
  emit('submit')
}
</script>

export default {
  emits: ['inFocus', 'submit'],
  setup(props, ctx) {
    ctx.emit('submit')
  }
}

export default {
  emits: ['inFocus', 'submit'],
  setup(props, { emit }) {
    emit('submit')
  }
}

export default {
  emits: ['inFocus', 'submit']
}

The emits option also supports an object syntax, which allows us to perform runtime validation of the payload of the emitted events:

<script setup>
const emit = defineEmits({
  submit(payload) {
    // return `true` or `false` to indicate
    // validation pass / fail
  }
})
</script>

<script setup lang="ts">
const emit = defineEmits<{
  (e: 'change', id: number): void
  (e: 'update', value: string): void
}>()
</script>

export default {
  emits: {
    submit(payload) {
      // return `true` or `false` to indicate
      // validation pass / fail
    }
  }
}

Although optional, it is recommended to define all emitted events in order to better document how a component should work. It also allows Kdu to exclude known listeners from fallthrough attributes.

Events Validation #

Similar to prop type validation, an emitted event can be validated if it is defined with the object syntax instead of the array syntax.

To add validation, the event is assigned a function that receives the arguments passed to the this.$emitemit call and returns a boolean to indicate whether the event is valid or not.

<script setup>
const emit = defineEmits({
  // No validation
  click: null,

  // Validate submit event
  submit: ({ email, password }) => {
    if (email && password) {
      return true
    } else {
      console.warn('Invalid submit event payload!')
      return false
    }
  }
})

function submitForm(email, password) {
  emit('submit', { email, password })
}
</script>

export default {
  emits: {
    // No validation
    click: null,

    // Validate submit event
    submit: ({ email, password }) => {
      if (email && password) {
        return true
      } else {
        console.warn('Invalid submit event payload!')
        return false
      }
    }
  },
  methods: {
    submitForm(email, password) {
      this.$emit('submit', { email, password })
    }
  }
}

Usage with k-model #

Custom events can also be used to create custom inputs that work with k-model. Remember that:

<input k-model="searchText" />

does the same thing as:

<input
  :value="searchText"
  @input="searchText = $event.target.value"
/>

When used on a component, k-model instead does this:

<CustomInput
  :modelValue="searchText"
  @update:modelValue="newValue => searchText = newValue"
/>

For this to actually work though, the <input> inside the component must:

• Bind the value attribute to the modelValue prop
• On input, emit an update:modelValue event with the new value

Here's that in action:

<!-- CustomInput.kdu -->
<script>
export default {
  props: ['modelValue'],
  emits: ['update:modelValue']
}
</script>

<template>
  <input
    :value="modelValue"
    @input="$emit('update:modelValue', $event.target.value)"
  />
</template>

<!-- CustomInput.kdu -->
<script setup>
defineProps(['modelValue'])
defineEmits(['update:modelValue'])
</script>

<template>
  <input
    :value="modelValue"
    @input="$emit('update:modelValue', $event.target.value)"
  />
</template>

Now k-model should work perfectly with this component:

<CustomInput k-model="searchText" />

Another way of implementing k-model within this component is to use a writable computed property with both a getter and a setter. The get method should return the modelValue property and the set method should emit the corresponding event:

<!-- CustomInput.kdu -->
<script>
export default {
  props: ['modelValue'],
  emits: ['update:modelValue'],
  computed: {
    value: {
      get() {
        return this.modelValue
      },
      set(value) {
        this.$emit('update:modelValue', value)
      }
    }
  }
}
</script>

<template>
  <input k-model="value" />
</template>

<!-- CustomInput.kdu -->
<script setup>
import { computed } from 'kdu'

const props = defineProps(['modelValue'])
const emit = defineEmits(['update:modelValue'])

const value = computed({
  get() {
    return props.modelValue
  },
  set(value) {
    emit('update:modelValue', value)
  }
})
</script>

<template>
  <input k-model="value" />
</template>

k-model arguments #

By default, k-model on a component uses modelValue as the prop and update:modelValue as the event. We can modify these names passing an argument to k-model:

<MyComponent k-model:title="bookTitle" />

In this case, the child component should expect a title prop and emit an update:title event to update the parent value:

<!-- MyComponent.kdu -->
<script setup>
defineProps(['title'])
defineEmits(['update:title'])
</script>

<template>
  <input
    type="text"
    :value="title"
    @input="$emit('update:title', $event.target.value)"
  />
</template>

<!-- MyComponent.kdu -->
<script>
export default {
  props: ['title'],
  emits: ['update:title']
}
</script>

<template>
  <input
    type="text"
    :value="title"
    @input="$emit('update:title', $event.target.value)"
  />
</template>

Multiple k-model bindings #

By leveraging the ability to target a particular prop and event as we learned before with k-model arguments, we can now create multiple k-model bindings on a single component instance.

Each k-model will sync to a different prop, without the need for extra options in the component:

<UserName
  k-model:first-name="first"
  k-model:last-name="last"
/>

<script setup>
defineProps({
  firstName: String,
  lastName: String
})

defineEmits(['update:firstName', 'update:lastName'])
</script>

<template>
  <input
    type="text"
    :value="firstName"
    @input="$emit('update:firstName', $event.target.value)"
  />
  <input
    type="text"
    :value="lastName"
    @input="$emit('update:lastName', $event.target.value)"
  />
</template>

<script>
export default {
  props: {
    firstName: String,
    lastName: String
  },
  emits: ['update:firstName', 'update:lastName']
}
</script>

<template>
  <input
    type="text"
    :value="firstName"
    @input="$emit('update:firstName', $event.target.value)"
  />
  <input
    type="text"
    :value="lastName"
    @input="$emit('update:lastName', $event.target.value)"
  />
</template>

Handling k-model modifiers #

When we were learning about form input bindings, we saw that k-model has built-in modifiers - .trim, .number and .lazy. In some cases, however, you might also want to add your own custom modifiers.

Let's create an example custom modifier, capitalize, that capitalizes the first letter of the string provided by the k-model binding:

<MyComponent k-model.capitalize="myText" />

Modifiers added to a component k-model will be provided to the component via the modelModifiers prop. In the below example, we have created a component that contains a modelModifiers prop that defaults to an empty object:

<script setup>
const props = defineProps({
  modelValue: String,
  modelModifiers: { default: () => ({}) }
})

defineEmits(['update:modelValue'])

console.log(props.modelModifiers) // { capitalize: true }
</script>

<template>
  <input
    type="text"
    :value="modelValue"
    @input="$emit('update:modelValue', $event.target.value)"
  />
</template>

<script>
export default {
  props: {
    modelValue: String,
    modelModifiers: {
      default: () => ({})
    }
  },
  emits: ['update:modelValue'],
  created() {
    console.log(this.modelModifiers) // { capitalize: true }
  }
}
</script>

<template>
  <input
    type="text"
    :value="modelValue"
    @input="$emit('update:modelValue', $event.target.value)"
  />
</template>

Notice the component's modelModifiers prop contains capitalize and its value is true - due to it being set on the k-model binding k-model.capitalize="myText".

Now that we have our prop set up, we can check the modelModifiers object keys and write a handler to change the emitted value. In the code below we will capitalize the string whenever the <input /> element fires an input event.

<script setup>
const props = defineProps({
  modelValue: String,
  modelModifiers: { default: () => ({}) }
})

const emit = defineEmits(['update:modelValue'])

function emitValue(e) {
  let value = e.target.value
  if (props.modelModifiers.capitalize) {
    value = value.charAt(0).toUpperCase() + value.slice(1)
  }
  emit('update:modelValue', value)
}
</script>

<template>
  <input type="text" :value="modelValue" @input="emitValue" />
</template>

<script>
export default {
  props: {
    modelValue: String,
    modelModifiers: {
      default: () => ({})
    }
  },
  emits: ['update:modelValue'],
  methods: {
    emitValue(e) {
      let value = e.target.value
      if (this.modelModifiers.capitalize) {
        value = value.charAt(0).toUpperCase() + value.slice(1)
      }
      this.$emit('update:modelValue', value)
    }
  }
}
</script>

<template>
  <input type="text" :value="modelValue" @input="emitValue" />
</template>

For k-model bindings with both argument and modifiers, the generated prop name will be arg + "Modifiers". For example:

<MyComponent k-model:title.capitalize="myText">

The corresponding declarations should be:

const props = defineProps(['title', 'titleModifiers'])
defineEmits(['update:title'])

console.log(props.titleModifiers) // { capitalize: true }

export default {
  props: ['title', 'titleModifiers'],
  emits: ['update:title'],
  created() {
    console.log(this.titleModifiers) // { capitalize: true }
  }
}