Expose API (#343)

Author: yyx990803

active-rfcs/0042-expose-api.md

@@ -0,0 +1,235 @@
+- Start Date: 2020-09-06
+- Target Major Version: 3.x
+- Reference Issues: #135, #210
+
+# Summary
+
+Provide the ability for components to control what is publicly exposed on its component instance. This proposal unifies #135 and #210 with additional details.
+
+# Basic example
+
+## Options API
+
+```js
+export default {
+  expose: ['increment'],
+
+  data() {
+    return {
+      count: 0
+    }
+  },
+  methods: {
+    increment() {
+      this.count++
+    }
+  }
+}
+```
+
+## Composition API
+
+```javascript
+import { ref } from 'vue'
+
+export default {
+  setup(props, { expose }) {
+    const count = ref(0)
+
+    function increment() {
+      count.value++
+    }
+
+    // public
+    expose({
+      increment
+    })
+
+    // private
+    return { increment, count }
+  }
+}
+```
+
+Here in the both cases, other components would only be able to access the `increment` method from this component, and nothing else.
+
+# Motivation
+
+In Vue, we have a few ways to retrieve the "public instance" of a component:
+
+- Via template refs
+- Via the `$parent` or `$root` properties
+
+Up until now, the concept of the "public instance" is equivalent to the `this` context inside a component. However, this creates a number of issues:
+
+1. A component's public and internal interface isn't always the same. A component may have internal properties that it doesn't want to expose to other components. In other cases a component may want to expose methods that are specifically meant to be used by other components.
+
+2. A component returning render function from `setup()` encloses all its state inside the `setup()` closure, so nothing is exposed on the public instance, and there's currently no way to selectively expose properties while using this pattern.
+
+3. `<script setup>` is also compiled into (2) so has the same issue.
+
+The proposed APIs solve these issues by giving components the ability to explicitly declare publicly exposed properties.
+
+# Detailed design
+
+## Options API
+
+A new option, `expose` is introduced. It expects an array of property keys to be exposed:
+
+```js
+// Child.vue
+export default {
+  expose: ['increment'],
+
+  data() {
+    return {
+      count: 0
+    }
+  },
+  methods: {
+    increment() {
+      this.count++
+    }
+  }
+}
+```
+
+In a parent component:
+
+```html
+<Child ref="child" />
+```
+
+```js
+this.$refs.child.increment() // ok
+this.$refs.child.count // undefined
+```
+
+If `expose` is an empty array, then the component would be considered "closed" and no properties would be exposed.
+
+> **Note:** `expose` option is only respected in the base component and ignored when used in mixins.
+
+## Composition API
+
+In Composition API, the 2nd argument of `setup` (aka the "setup context") now also provides the `expose` method:
+
+```js
+import { ref } from 'vue'
+
+export default {
+  setup(props, { expose }) {
+    const count = ref(0)
+
+    function increment() {
+      count.value++
+    }
+
+    // public
+    expose({
+      increment
+    })
+
+    // private
+    return { increment, count }
+  }
+}
+```
+
+The `expose` method expects an object of the actual values to be exposed.
+
+### In `<script setup>`
+
+The `defineExpose()` macro compiles into runtime `expose()` call.
+
+```html
+<script setup>
+  const count = ref(0)
+
+  function increment() {
+    count.value++
+  }
+
+  defineExpose({
+    increment
+  })
+</script>
+```
+
+See [relevant section](https://github.com/vuejs/rfcs/blob/master/active-rfcs/0040-script-setup.md#exposing-components-public-interface) in the `<script setup>` RFC.
+
+### Exposed Ref Unwrapping Behavior
+
+Note the exposed instance unwraps refs similar to the normal public instance:
+
+```js
+// in child
+const count = ref(0)
+expose({
+  count
+})
+```
+
+```js
+// in parent
+this.$refs.child.count // 0
+```
+
+## Limiting Expose Control to Base Component
+
+As raised in the discussions for a previous draft (#210), it would be very confusing if mixins and external composition functions are able to expose arbitrary properties. Therefore, the API is designed to restrict the capability to the base component only:
+
+- The `expose` option is ignored when encountered in `mixins` or `extends` sources.
+- The Composition API `expose` function is provided via the setup context instead of a global API (so that it cannot be imported in external composition functions) and can only be called once.
+
+## Additionally Exposed Properties
+
+An experimental implementation has been shipped in prior versions and we found in some cases there are patterns or tools (e.g. `vue-test-utils`) that relies on the presence of Vue built-in instance properties like `$el` or `$refs`.
+
+The public instance with explicit `expose` thus still exposes these built-in instance properties.
+
+# Drawbacks
+
+N/A
+
+# Alternatives
+
+N/A
+
+# Adoption strategy
+
+This feature is additive and doesn't affect existing usage.
+
+# Unresolved Questions
+
+## Type Inference
+
+Currently no Vue tooling has the ability to infer types of child component instances obtained from a ref.
+
+A workaround is to explicitly export the public interface from the child component:
+
+```html
+<!-- Child.vue -->
+<script lang="ts">
+  export interface Api {
+    // ...
+  }
+</script>
+```
+
+In parent:
+
+```html
+<script setup lang="ts">
+  import { ref } from 'vue'
+  import Child from './Child.vue'
+  import type { Api } from './Child.vue'
+
+  const childRef = ref<Api>()
+</script>
+
+<template>
+  <Child ref="childRef" />
+</template>
+```
+
+The above does not affect the runtime API design and therefore should not block this RFC from being merged.