Reactivity Transform
Experimental Feature
Reactivity Transform is currently an experimental feature. It is disabled by default and requires explicit opt-in. It may also change before being finalized. To stay up-to-date, keep an eye on its proposal and discussion on GitHub.
Composition-API-specific
Reactivity Transform is a Composition-API-specific feature and requires a build step.
Refs vs. Reactive Variables
Ever since the introduction of the Composition API, one of the primary unresolved questions is the use of refs vs. reactive objects. It's easy to lose reactivity when destructuring reactive objects, while it can be cumbersome to use .value
everywhere when using refs. Also, .value
is easy to miss if not using a type system.
Vue Reactivity Transform is a compile-time transform that allows us to write code like this:
vue
<script setup>
let count = $ref(0)
console.log(count)
function increment() {
count++
}
</script>
<template>
<button @click="increment">{{ count }}</button>
</template>
The $ref()
method here is a compile-time macro: it is not an actual method that will be called at runtime. Instead, the Vue compiler uses it as a hint to treat the resulting count
variable as a reactive variable.
Reactive variables can be accessed and re-assigned just like normal variables, but these operations are compiled into refs with .value
. For example, the <script>
part of the above component is compiled into:
js
import { ref } from 'vue'
let count = ref(0)
console.log(count.value)
function increment() {
count.value++
}
Every reactivity API that returns refs will have a $
-prefixed macro equivalent. These APIs include:
ref
->$ref
computed
->$computed
shallowRef
->$shallowRef
customRef
->$customRef
toRef
->$toRef
These macros are globally available and do not need to be imported when Reactivity Transform is enabled, but you can optionally import them from vue/macros
if you want to be more explicit:
js
import { $ref } from 'vue/macros'
let count = $ref(0)
Destructuring with $()
It is common for a composition function to return an object of refs, and use destructuring to retrieve these refs. For this purpose, reactivity transform provides the $()
macro:
js
import { useMouse } from '@vueuse/core'
const { x, y } = $(useMouse())
console.log(x, y)
Compiled output:
js
import { toRef } from 'vue'
import { useMouse } from '@vueuse/core'
const __temp = useMouse(),
x = toRef(__temp, 'x'),
y = toRef(__temp, 'y')
console.log(x.value, y.value)
Note that if x
is already a ref, toRef(__temp, 'x')
will simply return it as-is and no additional ref will be created. If a destructured value is not a ref (e.g. a function), it will still work - the value will be wrapped in a ref so the rest of the code works as expected.
$()
destructure works on both reactive objects and plain objects containing refs.
Convert Existing Refs to Reactive Variables with $()
In some cases we may have wrapped functions that also return refs. However, the Vue compiler won't be able to know ahead of time that a function is going to return a ref. In such cases, the $()
macro can also be used to convert any existing refs into reactive variables:
js
function myCreateRef() {
return ref(0)
}
let count = $(myCreateRef())
Reactive Props Destructure
There are two pain points with the current defineProps()
usage in <script setup>
:
Similar to
.value
, you need to always access props asprops.x
in order to retain reactivity. This means you cannot destructuredefineProps
because the resulting destructured variables are not reactive and will not update.When using the type-only props declaration, there is no easy way to declare default values for the props. We introduced the
withDefaults()
API for this exact purpose, but it's still clunky to use.
We can address these issues by applying a compile-time transform when defineProps
is used with destructuring, similar to what we saw earlier with $()
:
html
<script setup lang="ts">
interface Props {
msg: string
count?: number
foo?: string
}
const {
msg,
// default value just works
count = 1,
// local aliasing also just works
// here we are aliasing `props.foo` to `bar`
foo: bar
} = defineProps<Props>()
watchEffect(() => {
// will log whenever the props change
console.log(msg, count, bar)
})
</script>
The above will be compiled into the following runtime declaration equivalent:
js
export default {
props: {
msg: { type: String, required: true },
count: { type: Number, default: 1 },
foo: String
},
setup(props) {
watchEffect(() => {
console.log(props.msg, props.count, props.foo)
})
}
}
Retaining Reactivity Across Function Boundaries
While reactive variables relieve us from having to use .value
everywhere, it creates an issue of "reactivity loss" when we pass reactive variables across function boundaries. This can happen in two cases:
Passing into function as argument
Given a function that expects a ref as an argument, e.g.:
ts
function trackChange(x: Ref<number>) {
watch(x, (x) => {
console.log('x changed!')
})
}
let count = $ref(0)
trackChange(count) // doesn't work!
The above case will not work as expected because it compiles to:
ts
let count = ref(0)
trackChange(count.value)
Here count.value
is passed as a number, whereas trackChange
expects an actual ref. This can be fixed by wrapping count
with $$()
before passing it:
diff
let count = $ref(0)
- trackChange(count)
+ trackChange($$(count))
The above compiles to:
js
import { ref } from 'vue'
let count = ref(0)
trackChange(count)
As we can see, $$()
is a macro that serves as an escape hint: reactive variables inside $$()
will not get .value
appended.
Returning inside function scope
Reactivity can also be lost if reactive variables are used directly in a returned expression:
ts
function useMouse() {
let x = $ref(0)
let y = $ref(0)
// listen to mousemove...
// doesn't work!
return {
x,
y
}
}
The above return statement compiles to:
ts
return {
x: x.value,
y: y.value
}
In order to retain reactivity, we should be returning the actual refs, not the current value at return time.
Again, we can use $$()
to fix this. In this case, $$()
can be used directly on the returned object - any reference to reactive variables inside the $$()
call will retain the reference to their underlying refs:
ts
function useMouse() {
let x = $ref(0)
let y = $ref(0)
// listen to mousemove...
// fixed
return $$({
x,
y
})
}
Using $$()
on destructured props
$$()
works on destructured props since they are reactive variables as well. The compiler will convert it with toRef
for efficiency:
ts
const { count } = defineProps<{ count: number }>()
passAsRef($$(count))
compiles to:
js
setup(props) {
const __props_count = toRef(props, 'count')
passAsRef(__props_count)
}
TypeScript Integration
Vue provides typings for these macros (available globally) and all types will work as expected. There are no incompatibilities with standard TypeScript semantics, so the syntax will work with all existing tooling.
This also means the macros can work in any files where valid JS / TS are allowed - not just inside Vue SFCs.
Since the macros are available globally, their types need to be explicitly referenced (e.g. in a env.d.ts
file):
ts
/// <reference types="vue/macros-global" />
When explicitly importing the macros from vue/macros
, the type will work without declaring the globals.
Explicit Opt-in
Reactivity Transform is currently disabled by default and requires explicit opt-in. In addition, all of the following setups require vue@^3.2.25
.
Vite
- Requires
@vitejs/plugin-vue@>=2.0.0
- Applies to SFCs and js(x)/ts(x) files. A fast usage check is performed on files before applying the transform so there should be no performance cost for files not using the macros.
- Note
reactivityTransform
is now a plugin root-level option instead of nested asscript.refSugar
, since it affects not just SFCs.
js
// vite.config.js
export default {
plugins: [
vue({
reactivityTransform: true
})
]
}
vue-cli
- Currently only affects SFCs
- Requires
vue-loader@>=17.0.0
js
// vue.config.js
module.exports = {
chainWebpack: (config) => {
config.module
.rule('vue')
.use('vue-loader')
.tap((options) => {
return {
...options,
reactivityTransform: true
}
})
}
}
Plain webpack
+ vue-loader
- Currently only affects SFCs
- Requires
vue-loader@>=17.0.0
js
// webpack.config.js
module.exports = {
module: {
rules: [
{
test: /\.vue$/,
loader: 'vue-loader',
options: {
reactivityTransform: true
}
}
]
}
}