• Start Date: 2021-11-26
• Target Major Version: 3.x
• Reference Issues:SFC Improvements #182,Ref sugar #228,Ref sugar (take 2) #368,Reactivity Transform Unification #413

This Proposal has been Dropped

Seereasoning here.

Summary

Introduce a set of compiler transforms to improve ergonomics when using Vue's reactivity APIs, specifically to be able to use refs without.value.

Basic example

// declaring a reactive variable backed by an underlying refletcount=$ref(1)// log count, and automatically log again whenever it changes.// no need for .value anymore!watchEffect(()=>console.log(count))functioninc(){// assignments are reactivecount++}

import{watchEffect,ref}from'vue'constcount=ref(1)watchEffect(()=>console.log(count.value))functioninc(){count.value++}

Motivation

Ever since the introduction of the Composition API, one of the primary unresolved questions is the use of refs vs. reactive objects. It can be cumbersome to use.value everywhere, and it is easy to miss if not using a type system. Some users specifically lean towards usingreactive() exclusively so that they don't have to deal with refs.

This proposal aims to improve the ergonomics of refs with a set of compile-time macros.

Detailed design

Overview

• Every ref-creating API has a$-prefixed macro version, e.g.$ref(), that createsreactive variables instead of refs.
• Destructure objects or convert existing refs to reactive variables with$()
• DestructuringdefineProps() in<script setup> also creates reactive variables.
• Retrieve the underlying refs from reactive variables with$$()

Reactive Variables

To understand the issue we are trying solve, let's start with an example. Here's a normal ref, created by the originalref() API:

import{ref,watchEffect}from'vue'constcount=ref(0)watchEffect(()=>{// access value (track)console.log(count.value)})// mutate value (trigger)count.value=1

The above code works without any compilation, but is constrained by how JavaScript works: we need to use the.value property, so that Vue can intercept its get/set operations in order to perform dependency tracking and effect triggering.

Now let's look at a version usingreactive variables:

import{watchEffect}from'vue'letcount=$ref(0)watchEffect(()=>{// access value (track)// compiles to `console.log(count.value)`console.log(count)})// mutate value (trigger)// compiles to `count.value = 1`count=1

This version behaves exactly the same as the original version, but notice that we no longer need to use.value. In fact, this makes our JS/TS code work the same way as in Vue templates where root-level refs are automatically unwrapped.

The$ref() function is acompile-time macro that creates areactive variable. It serves as a hint to the compiler: whenever the compiler encounters thecount variable, it will automatically append.value for us! Under the hood, the reactive variable version is compiled into the normal ref version.

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

Optional Import

Because$ref() is a macro and not a runtime API, it doesn't need to be imported fromvue. However, if you want to be more explicit, you can import it fromvue/macros:

import{$ref}from'vue/macros'letcount=$ref(0)

Bind existing ref as 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. Therefore, we also provide a more generic$() macro that can convert any existing refs into reactive variables:

functionmyCreateRef(){returnref(0)}letcount=$(myCreateRef())

Destructuring objects of refs with$()

It is common for a composition function to return an object of refs, and use destructuring to retrieve these refs.$() can also be used in this case:

import{useMouse}from'@vueuse/core'const{ x, y}=$(useMouse())console.log(x,y)

Compiled output:

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 ifx 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 into a ref so the rest of the code work as expected.

$() destructure works on both reactive objects AND plain objects containing refs.

Reactive props destructure

There are two pain points with the currentdefineProps() usage in<script setup>:

1. 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.

2. When using thetype-only props declaration, there is no easy way to declare default values for the props. We introduced thewithDefaults() API for this exact purpose, but it's still clunky to use.

We can address these issues by applying the same logic for reactive variables destructure todefineProps:

<scriptsetuplang="ts">interfaceProps{    msg:stringcount?:numberfoo?: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 changeconsole.log(msg,count,bar)})</script>

the above will be compiled into the following runtime declaration equivalent:

exportdefault{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 with$$()

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 object as argument, e.g.:

functiontrackChange(x:Ref<number>){watch(x,(x)=>{console.log('x changed!')})}letcount=$ref(0)trackChange(count)// doesn't work!

The above case will not work as expected because it compiles to:

letcount=ref(0)trackChange(count.value)

Herecount.value is passed as a number wheretrackChange expects an actual ref. This can be fixed by wrappingcount with$$() before passing it:

let count = $ref(0)- trackChange(count)+ trackChange($$(count))

The above compiles to:

import{ref}from'vue'letcount=ref(0)trackChange(count)

As we can see,$$() is a macro that serves as anescape hint: reactive variables inside$$() will not get.value appended.

Returning inside function scope

Reactivity can also be lost if reactive variables are used directly in a returned expression:

functionuseMouse(){letx=$ref(0)lety=$ref(0)// listen to mousemove...// doesn't work!return{    x,    y}}

The above return statement compiles to:

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 be retained as reference to their underlying refs:

functionuseMouse(){letx=$ref(0)lety=$ref(0)// listen to mousemove...// fixedreturn$$({    x,    y})}

$$() Usage on destructured props

$$() works on destructured props since they are reactive variables as well. The compiler will convert it withtoRef for efficiency:

const{ count}=defineProps<{count:number}>()passAsRef($$(count))

compiles to:

setup(props){const__props_count=toRef(props,'count')passAsRef(__props_count)}

TypeScript & Tooling Integration

Vue will provide typings for these macros (available globally) and all types will work as expected. There are no incompatibilities with standard TypeScript semantics so the syntax would 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 aenv.d.ts file):

/// <reference types="vue/macros-global" />

When explicitly importing the macros fromvue/macros, the type will work without declaring the globals.

Implementation Status

You can try the transform in theVue SFC Playground (works in both.vue and.(js|ts) files).

Vue 3.2.25+ ships an implementation of this RFC as an experimental feature under the package@vue/reactivity-transform. The package can be used standalone as a low-level library. It is also integrated (with its APIs re-exported) in@vue/compiler-sfc so most userland projects won't need to explicitly install it.

Higher-level tools like@vitejs/plugin-vue andvue-loader can be configured to apply the transform to vue, js(x) and ts(x) files. SeeAppendix for how to enable the transform in specific tools.

Experimental features are unstable and may change between any release types (including patch releases). By explicitly enabling an experimental feature, you are taking on the risk of potentially having to refactor into updated syntax, or even refactor away from the usage if the feature ends up being removed.

Unresolved Questions

defineProps destructure details

1. ShoulddefineProps destructure require additional hints?

   Some may have the concern that reactive destructure ofdefineProps isn't obvious enough because it doesn't have the$() indication, which may confuse new users.

   An alternative of making it more explicit would be requiring$() to enable the reactive behavior:

   const{ foo}=$(defineProps(['foo']))

   However, the only benefit of this is for a new user to more easily notice thatfoo is reactive. If this change lands, the documentation would mention the destructure reactivity when introducingdefineProps. Assuming all users learn about this on inital onboarding, the extra wrapping doesn't really serve any real purpose (similar to$(ref(0))).

2. The proposed reactive destructure fordefineProps is technically a small breaking change, because previously the same syntax also worked, just without the reactivity. This could technically break the case where the user intentionally destructures the props object to get a non-reactive initial value of a prop:

   const{ foo}=defineProps(['foo'])

   However, this should be extremely rare because without reactive destructure, doing so meantall props retrieved this way are non-reactive. A more realistic example would be:

   constprops=defineProps(['foo','bar'])const{ foo}=propsprops.bar// reactive access to `bar`

   A simple workaround after this RFC:

   const{ foo, bar}=defineProps(['foo','bar'])constinitialFoo=foo

Alternatives

Other related proposals

This RFC is a revised version of#368 which also includes feedback from discussions in#413 and#394.

Earlier proposals

The whole discussion traces all the way back to the first draft of ref sugar, but most are outdated now. They are listed here for the records.

• Alternative idea to simplify reactive code #223 by@jods4
• Ref sugar #228 using label syntax
• Cleaner Composition API #213
• Script refs #214

Adoption strategy

This feature is opt-in. Existing code is unaffected.

Appendix

Enabling the Macros

• All setups requirevue@^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.
• NoterefTransform is now a plugin root-level option instead of nested asscript.refSugar, since it affects not just SFCs.

// vite.config.jsexportdefault{plugins:[vue({reactivityTransform:true})]}

vue-cli

• Currently only affects SFCs
• requiresvue-loader@^17.0.0

// vue.config.jsmodule.exports={chainWebpack:(config)=>{config.module.rule('vue').use('vue-loader').tap((options)=>{return{          ...options,reactivityTransform:true}})}}

Plainwebpack +vue-loader

• Currently only affects SFCs
• requiresvue-loader@^17.0.0

// webpack.config.jsmodule.exports={module:{rules:[{test:/\.vue$/,loader:'vue-loader',options:{reactivityTransform:true}}]}}