Optional props declaration (#25)

Author: yyx990803

active-rfcs/0010-optional-props-declaration.md

@@ -0,0 +1,92 @@
+- Start Date: 2019-04-08
+- Target Major Version: 2.x & 3.x
+- Reference Issues: N/A
+- Implementation PR: N/A
+
+# Summary
+
+Make component `props` declaration optional.
+
+# Basic example
+
+``` html
+<template>
+  <div>{{ $props.foo }}</div>
+</template>
+
+<script>
+export default {}
+</script>
+```
+
+# Motivation
+
+In simple use cases where there is no need for runtime props type checking (especially in functional components), making props optional could result in simpler code.
+
+# Detailed design
+
+## Stateful Components
+
+When a component has no `props` declarations, all attributes passed by the parent are exposed in `this.$props`. Unlike declared props, they will NOT be exposed directly on `this`. In addition, in this case `this.$attrs` and `this.$props` will be pointing to the same object.
+
+``` html
+<template>
+  <div>{{ $props.foo }}</div>
+</template>
+
+<script>
+export default {}
+</script>
+```
+
+If the component has no other options, the entire `<script>` block can be dropped, so the following would be a valid SFC:
+
+``` html
+<template>
+  <div>{{ $props.foo }}</div>
+</template>
+```
+
+## Functional Components
+
+This is based on plain-function functional components proposed in [Functional and Async Component API Change](https://github.com/vuejs/rfcs/pull/27).
+
+``` js
+const FunctionalComp = props => {
+  return h('div', props.foo)
+}
+```
+
+To declare props for plain-function functional components, attach it to the function itself:
+
+``` js
+FunctionalComp.props = {
+  foo: Number
+}
+```
+
+Similar to stateful components, when props are declared, the `props` arguments will only contain the declared props - attributes received but not declared as props will be in the 3rd argument (`attrs`):
+
+``` js
+const FunctionalComp = (props, slots, attrs) => {
+  // `attrs` contains all received attributes except declared `foo`
+}
+
+FunctionalComp.props = {
+  foo: Number
+}
+```
+
+For mode details on the new functional component signature, see [Render Function API Change](https://github.com/vuejs/rfcs/pull/28).
+
+# Drawbacks
+
+N/A
+
+# Alternatives
+
+N/A
+
+# Adoption strategy
+
+The behavior is fully backwards compatible.