vue-async-computed
With this plugin, you can have computed properties in Vue that are computed asynchronously.
Without using this plugin, you can't do this:
export default {
data () {
return {
userId: 1
}
},
computed: {
username () {
return fetch(`/get-username-by-id/${this.userId}`)
.then(response => response.json())
.then(user => user.username)
}
}
}
Or rather, you could, but it wouldn't do what you'd want it to do. But using this plugin, it works just like you'd expect:
export default {
data () {
return {
userId: 1
}
},
asyncComputed: {
username () {
return fetch(`/get-username-by-id/${this.userId}`)
.then(r => r.json())
.then(user => user.username)
}
}
}
This is especially useful with ES7 async functions:
export default {
asyncComputed: {
async someCalculation () {
const x = await someAsycFunction()
const y = await anotherAsyncFunction()
return x + y
}
}
}
Install
npm install --save vue-async-computed
And then install vue-async-computed
via app.use()
to make it available for all your components:
import { createApp } from 'vue'
import App from './App.vue'
import AsyncComputed from 'vue-async-computed'
const app = createApp(App)
app.use(AsyncComputed)
app.mount('#app')
Alternately, you can link it directly from a CDN:
<script src="https://unpkg.com/vue@3/dist/vue.global.js"></script>
<script src="https://unpkg.com/vue-async-computed@4.0.1"></script>
<div id="app">
<input type="number" v-model="x"> + <input type="number" v-model="y">
= {{sum == null ? 'Loading' : sum}}
</div>
<script>
const app = Vue.createApp({
data () {
return {
x: 2,
y: 3
}
},
asyncComputed: {
async sum () {
await new Promise(resolve => setTimeout(resolve, 1000))
return this.x + this.y
}
}
})
app.use(AsyncComputed)
app.mount('#app')
</script>
Usage example
export default {
data () {
return {
x: 2,
y: 3
}
},
asyncComputed: {
async sum () {
await new Promise(resolve => setTimeout(resolve, 1000))
return this.x + this.y
}
}
}
Like with regular synchronous computed properties, you can pass an object
with a get
method instead of a function, but unlike regular computed
properties, async computed properties are always getter-only. If the
object provided has a set
method it will be ignored.
Async computed properties can also have a custom default value, which will
be used until the data is loaded for the first time:
export default {
data () {
return {
postId: 1
}
},
asyncComputed: {
blogPostContent: {
async get () {
const post = await fetch(`/post/${this.postId}`)
.then(response => response.json())
return post.postContent
},
default: 'Loading...'
}
}
}
You can instead define the default value as a function, in order to depend on
props or on data:
export default {
data () {
return {
postId: 1
}
},
asyncComputed: {
blogPostContent: {
async get () {
const post = await fetch(`/post/${this.postId}`)
.then(response => response.json())
return post.postContent
},
default () {
return `Loading post ${this.postId}...`
}
}
}
}
You can also set a custom global default value in the options passed to app.use
:
app.use(AsyncComputed, {
default: 'Global default value'
})
Recalculation
Just like normal computed properties, async computed properties keep track of their dependencies, and are only
recalculated if those dependencies change. But often you'll have an async computed property you'll want to run again
without any of its (local) dependencies changing, such as for instance the data may have changed in the database.
You can set up a watch
property, listing the additional dependencies to watch.
Your async computed property will then be recalculated also if any of the watched
dependencies change, in addition to the real dependencies the property itself has:
export default {
data () {
return {
postId: 1,
timesPostHasBeenUpdated: 0
}
},
asyncComputed: {
blogPostContent: {
async get () {
const post = await fetch(`/post/${this.postId}`)
.then(response => response.json())
return post.postContent
},
watch: ['timesPostHasBeenUpdated']
}
}
}
Just like with Vue's normal watch
, you can use a dotted path in order to watch a nested property. For example, watch: ['a.b.c', 'd.e']
would declare a dependency on this.a.b.c
and on this.d.e
.
You can trigger re-computation of an async computed property manually, e.g. to re-try if an error occurred during evaluation. This should be avoided if you are able to achieve the same result using a watched property.
export default {
asyncComputed: {
blogPosts: {
async get () {
return fetch('/posts')
.then(response => response.json())
}
}
},
methods: {
refresh() {
this.$asyncComputed.blogPosts.update()
}
}
}
Conditional Recalculation
Using watch
it is possible to force the computed property to run again unconditionally.
If you need more control over when the computation should be rerun you can use shouldUpdate
:
export default {
data () {
return {
postId: 1,
pageType: 'index'
}
},
asyncComputed: {
blogPostContent: {
async get () {
const post = await fetch(`/post/${this.postId}`)
.then(response => response.json())
return post.postContent
},
shouldUpdate () {
return this.pageType !== 'index'
}
}
}
}
The main advantage over adding an if
statement within the get function is that the old value is still accessible even if the computation is not re-run.
Lazy properties
Normally, computed properties are both run immediately, and re-run as necessary when their dependencies change.
With async computed properties, you sometimes don't want that. With lazy: true
, an async computed
property will only be computed the first time it's accessed.
For example:
export default {
data () {
return {
id: 1
}
},
asyncComputed: {
mightNotBeNeeded: {
lazy: true,
async get () {
return fetch(`/might-not-be-needed/${this.id}`)
.then(response => response.json())
.then(response => response.value)
}
}
}
}
Computation status
For each async computed property, an object is added to $asyncComputed
that contains information about the current computation state of that object. This object contains the following properties:
{
state: 'updating',
updating: true,
success: false,
error: false,
exception: null
}
It is meant to be used in your rendering code to display update / error information:
<script>
export default {
asyncComputed: {
async posts() {
return fetch('/posts').then(r => r.json())
}
}
}
</script>
<template>
<template v-if="$asyncComputed.posts.updating">Loading...</template>
<template v-else-if="$asyncComputed.posts.error">
Error while loading posts: {{ $asyncComputed.posts.exception }}
<button @click="$asyncComputed.posts.update()">Retry</button>
</template>
<template v-else>
{{ posts }}
</template>
</template>
Note: If you want to display a special message the first time the posts load, you can use the fact that the default value is null:
<div v-if="$asyncComputed.posts.updating && posts === null"> Loading posts </div>
Global error handling
By default, in case of a rejected promise in an async computed property, vue-async-computed will take care of logging the error for you.
If you want to use a custom logging function, the plugin takes an errorHandler
option, which should be the function you want called with the error information.
By default, it will be called with only the error's stack trace as an argument,
but if you register the errorHandler
with useRawError
set to true
the
function will receive the raw error, a reference to the Vue
instance that
threw the error and the error's stack trace.
For example:
app.use(AsyncComputed, {
errorHandler (stack) {
console.log('Hey, an error!')
console.log('---')
console.log(stack)
}
})
app.use(AsyncComputed, {
useRawError: true,
errorHandler (err, vm, stack) {
console.log('An error occurred!')
console.log('The error message was: ' + err.msg)
console.log('And the stack trace was:')
console.log(stack)
}
})
You can pass false
as the errorHandler
in order to silently ignore rejected promises.
License
MIT © Benjamin Fox