Composition functions support and `this` value changes

[ktsn]

Summary

• Composition functions are available in class property initializers by wrapping setup helper.
  • Class property initializers are handled in setup function under the hood.
• Only $props (and its derived prop values), $attrs, $slots and $emit are available on this in class property initializers.

Example:

<template>
  <div>Count: {{ counter.count }}</div>
  <button @click="counter.increment()">+</button>
</template>

<script lang="ts">
import { ref, reactive, onMounted } from 'vue'
import { Vue, setup } from 'vue-class-component'

function useCounter () {
  const count = ref(0)

  function increment () {
    count.value++
  }

  onMounted(() => {
    console.log('onMounted')
  })

  return {
    count,
    increment
  }
}

export default class Counter extends Vue {
  counter = setup(() => useCounter())
}
</script>

Details

Prior to v7, class component constructor is initialized in data hook to collect class properties. In v8, it will be initialized in setup hook so that the users can use composition function in class property initializer.

The above class component definition is as same as following canonical component definition.

function useCounter () {
  const count = ref(0)

  function increment () {
    count.value++
  }

  onMounted(() => {
    console.log('onMounted')
  })

  return {
    count,
    increment
  }
}

export default {
  setup() {
    return { counter: useCounter() }
  }
}

setup helper

Wrapping a composition function with setup helper is needed because we need to delay the invocation of the composition function. Let's see the following example:

function usePost(postId) {
  const post = ref(null)

  watch(postId, async id => {
    post.value = await fetch('/posts/' + id)
  }, {
    immediate: true
  })

  return {
    post
  }
}

class App extends Vue {
  postId = '1'

  // DO NOT do this
  post = usePost(toRef(this, 'postId'))
}

In the above example, this.postId will be referred by watch helper to track reactive dependencies immediately but it is not reactive value at that moment. Then the watch callback won't be called when postId is changed.

setup helper will delay the invocation until this.postId become a proxy property to the actual reactive value.

setup unwrapping

As same as setup in the Vue core library, setup in Vue Class Component unwraps ref values. The unwrapping happens shallowly:

// The returned value is:
// { 
//    count: { value: 0 },
//    nested: {
//      anotherCount: { value: 1 }
//    }
// }
function useCount() {
  const count = ref(0)
  const anotherCount = ref(1)

  return {
    count,
    nested: {
      anotherCount
    }
  }
}

class Counter extends Vue {
  // counter will be:
  // { 
  //    count: 0, <-- unwrapped
  //    nested: {
  //      anotherCount: { value: 1 }
  //    }
  // }
  // The shallow ref (count) is unwrapped while the nested one (anotherCount) retains
  counter = setup(() => useCount())
}

In addition, if you return a single ref in setup helper, the ref will also be unwrapped:

// The returned value is: { value: 42 }
function useAnswer() {
  const answer = ref(42)
  return answer
}

class Answer extends Vue {
  // answer will be just 42 which is unwrapped
  answer = setup(() => useAnswer())
}

Available built in properties on this

Since the class constructor is used in setup hook, only following properties are available on this.

• $props
  • All props are proxied on this as well. (e.g. this.$props.foo -> this.foo)
• $emit
• $attrs
• $slots

Example using $props and $emit in a composition function.

function useCounter(props, emit) {
  function increment() {
    emit('input', props.count + 1)
  }

  return {
    increment
  }
}

export default class App extends Vue {
  counter = setup(() => {
    return useCounter(this.$props, this.$emit)
  })
}

Alternative Approach

Another possible approach is using super class and mixins.

import { ref } from 'vue'
import { setup } from 'vue-class-component'

const Super = setup((props, ctx) => {
  const count = ref(0)

  function increment() {
    count.value++
  }

  return {
    count,
    increment
  }
})

export default class App extends Super {}

Pros

• Can define properties directly on this.

Cons

• Need duplicated props type definition.

  // Props type definition for setup
  interface Props {
    foo: string
  }
  
  const Super = setup((props: Props) => { /* ... */ })
  
  export default class App extends Setup {
    // Another definition for foo prop to use it in the class
    @Prop foo!: string
  }

[kiaking]

I really like it. Well thoughts. One question is would it be possible to pass another class property to the composition function? This might not be the best example, but something like this. See useCounter taking argument, and here I'm padding in class property as the argument.

function useCounter (amount: number) {
  const count = ref(0)

  function increment () {
    count.value + amount
  }

  return {
    count,
    increment
  }
}

export default class Counter extends Vue {
  amount = 10

  get counter () {
    reactive(useCounter(this.amount))
  }
}

[ktsn]

@kiaking You mean like the following code? (passing primitive loses its reactivity and computed does not handle composition functions in this proposal)

function useCounter (amount: Ref<number>) {
  const count = ref(0)

  function increment () {
    count.value + amount.value
  }

  return {
    count,
    increment
  }
}

export default class Counter extends Vue {
  amount = 10

  counter = reactive(useCounter(
    toRef(this, 'amount')
  ))
}

I think we should discourage this usage because this in initializers is different from actual component constructor as same as v7, so amount will not be updated nor reactive in useCounter.

I would say the dependency direction between composition function and component instance should be composition <- instance. This is as same as the canonical Vue api (setup cannot access data, computed, methods etc.). If we want to define some value depended by a composition function, we can include it into the composition function.

function useCounter () {
  const amount = ref(10)
  const count = ref(0)

  function increment () {
    count.value + amount.value
  }

  return {
    count,
    amount,
    increment
  }
}

export default class Counter extends Vue {
  counter = reactive(useCounter())

  mounted() {
    this.counter.amount = 11
  }
}

[kiaking]

Ah OK good point. Yes I was afraid of that. I don't have good example here but I suspected there might be shared 3rd party composition function that might require argument to be passed.

But as you mentioned, we could always "wrap" such composition function to be used before assigning to the class component.

// 3rd party composition function. It requires argument.
function useCounter (amount: Ref<number>) {
  const count = ref(0)

  function increment () {
    count.value + amount.value
  }

  return { count, increment }
}

// Wrap the above composition function.
function useWrappedCounter () {
  const amount = ref(10)

  return {
    amount,
    ...useCounter(amount)
  }
}

// Use wrapped function.
export default class Counter extends Vue {
  counter = reactive(useWrappedCounter())

  mounted() {
    this.counter.amount = 11
  }
}

[ktsn]

An idea to allow such usecase would be to require wrapping a composition function and delay its initalization:

export default class Counter extends Vue {
  amount = 10

  counter = setup(() => {
    return useCounter(toRef(this, 'amount'))
  })
}

The current problems of this in initializers are

1. this is not an actual reactive data.
2. this.amount is still not reactive value in initializers.

To solve 1. we can just configure proxy getter / setter to actual reactive data for each initialized property. As for 2. we need to delay the invocation of composition function to make sure accessed properties are converted reactive value. We can solve 2. with setup function on the above example.

I noticed that mandating setup helper could also reduce the confusion of manual unwrapping as the helper always does it for us.

[ktsn]

I have updated the proposal regarding the above setup helper idea.

[kiaking]

Nice! I really dig the new setup function. It also shows that property is more "special" interns of composition api. 👍

[ktsn]

Since Vue core changed the unwrapping behavior of setup (from deep to shallow), Vue Class Component follows that direction. I've updated the original post to add setup unwrapping section. You can see the detailed unwrapping behavior of setup in Vue Class Component there.

[LifeIsStrange]

Since setup() for variables used on the state add a bunch a unwanted boilerplate I am asking similarly as for props, would there be ways to define (state) variables used on the template inside the class ?
related: how can vue class component leverage https://github.com/vuejs/rfcs/blob/sfc-improvements/active-rfcs/0000-sfc-script-setup.md ? (maybe that you could influence the design of the RFC while its active btw)

[ktsn]

@LifeIsStrange I'm afraid I don't understand what you mean. Could you elaborate "ways to define (state) variables used on the template inside the class"?

I'm not sure how we can integrate class component with the new <script setup> because it will not probably be class component anymore?