RFC: Slots

text/0000-slots.md

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+- Start Date: 2022-08-05
+- RFC PR: (leave this empty)
+- React Issue: (leave this empty)
+
+# Summary
+
+A way to support slots pattern in React, works similar to [Slots for Web Components](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_templates_and_slots)
+but more powerful as we can interpolate slots.
+
+# Basic example
+
+In this example we introduced two method called `createHost` and `createSlot`.
+
+`createSlot` creates a component won't be rendered to real DOM but collect props.
+
+`createHost` mounts the children included slotted components and read the
+collected slots props, and render the result to real DOM conditionally.
+
+```jsx
+import { createHost, createSlot } from "react-create-slots";
+
+const Tag = (props) => <span {...props} />;
+
+const TextField = (props) => {
+  const id = useId();
+
+  return createHost(props.children, (Slots) => {
+    const labelProps = Slots.get("label");
+    const inputProps = Slots.get("input");
+    const tagPropsList = Slots.getAll("tag");
+
+    return (
+      <div>
+        {labelProps && <label htmlFor={inputProps.id ?? id} {...labelProps} />}
+        <input id={id} {...label} />
+        {tagPropsList.length > 0 && (
+          <div>
+            {tagPropsList.map((tagProps, index) => (
+              <Tag data-index={index} {...tagProps} />
+            ))}
+          </div>
+        )}
+      </div>
+    );
+  });
+};
+
+const TextFieldLabel = createSlot("label");
+const TextFieldInput = createSlot("input");
+const TextFieldTag = createSlot("tag");
+
+export default function App() {
+  return (
+    <div>
+      <TextField>
+        <TextFieldInput id="input-id" />
+        <TextFieldLabel>It will be rendered above input</TextFieldLabel>
+        <TextFieldTag>Tag 1</TextFieldTag>
+        <TextFieldTag>Tag 2</TextFieldTag>
+      </TextField>
+    </div>
+  );
+}
+```
+
+# Motivation
+
+## Composite component in a configure manor
+
+When creating UI library, we have different approaches on the component
+API design. The most common way is the Configuration pattern used by
+most UI libraries, everything in one component and provides child props
+for composition, like `<TextField label helperText />`. It's easy to use,
+but when we need to to add some extra props to label like `data-testid`,
+then we have to introduce new props for that which will bloat the api
+very easily.
+
+Another way to solve this problem is Composition pattern, like
+`<TextField><TextFieldLabel /><TextFieldInput /></TextField>`,
+it provides the best flexibility, we are free to customise every
+part of our component, but it causes another problem: consistency,
+we have to organise your sub components exactly same order as expected,
+and the biggest problem is that it's harder to communicate between
+parent and children.
+
+For a dedicated Design System, we need consistent ui regardless how
+we compose it.
+
+Here the Slots pattern solve those problem perfectly, we can compose
+our component with both flexibility and consistency, and it's extreme
+easy to add A11y support thanks the ability of Inversion of Control.
+
+## Accessible List
+
+Quoted from [the comment](https://github.com/facebook/react/issues/24979#issuecomment-1193176328)
+by @devongovett
+
+> A bunch of libraries have a problem where they need to know about certain types of descendants. For example, a list component with keyboard navigation needs to know what elements exist in the collection in order to implement things like typeahead, arrow keys, selection, etc. Reach UI has a good [overview](https://github.com/reach/reach-ui/tree/dev/packages/descendants) of a bunch of different approaches to this. The most commonly used of them involve rendering all of the items to the DOM, and using some kind of context-based registration system to tell the parent about themselves, and the DOM to sort them into the correct order.  
+> This has the downside that all of the items must be in the DOM at all times. In some cases, like virtualized scrolling, or a combobox/select where users can set the item without showing the list, some or all of the items shouldn't be rendered to the DOM. In React Aria, we walk the JSX tree to do this, which makes for a more natural API than giving up JSX completely (info). But this breaks composition, because only certain known element types are allowed.
+
+Even with the Reach UI's solution, it doesn't work well with SSR.
+With the Slots pattern, we don't render the children to real DOM but
+only collect information, so virtualisation is supported by nature.
+
+## Why we want it built in core
+
+I've researched a lot different approaches, none of the current solutions
+work perfectly without any drawbacks, my [solution](https://github.com/nihgwu/create-slots)
+is the closest one. But as @devongovett pointed out [here](https://github.com/facebook/react/issues/24979#issuecomment-1205909188), `react-reconciler` is designed to do this job,
+It would be nice to see it in core, like `react-call-return`.
+
+# Detailed design
+
+The api is very similar to the [deleted](https://github.com/facebook/react/pull/12820)
+experimental package [`react-call-return`](https://github.com/facebook/react/pull/11364),
+but with a simpler mental model.
+
+`createSlot(slotName)` creates a slot component and won't be rendered to
+real DOM but only collect props, which will be used by `createHost`
+
+`createHost(children, callback)` mounts the children included slotted
+components and read the collected slots' props, and render the result to
+real DOM conditionally. The argument of `callback` provides the following
+methods:
+
+- `get(slotName: string)` returns the registered slot's props by name,
+  will return the last registered named slot if the host expect only one
+  but consumer provides more.
+- `getAll(slotName: string)` returns all the registered slots as array
+  this method is useful to create a list.
+- `getChildren()` returns the rest of children without slots, e.g.
+  `<Host>rest<Slot /></Host>` it will return `rest`.
+
+# Drawbacks
+
+Even we are going to support this feature in a separate package or entry,
+it will still increase the bundle size of React a bit as we need core support.
+
+# Alternatives
+
+- Bring back `react-call-return` which also could be used to implement this
+  feature, but the mental model is hard to understand.
+- Leave it to userspace to implement with current api, like [create-slots](https://github.com/nihgwu/create-slots), but not very efficient and have
+  some drawbacks, e.g. unable to catch children not wrapped in slots.
+
+# Adoption strategy
+
+There is no breaking changes, it's a new feature more for UI library authors
+instead of application developers.
+
+# How we teach this
+
+Slots pattern is a native feature of Web Components, other popular frameworks
+like Vue and Svelte also provide similar concepts. So the concept itself is
+easy to understand, we only have to document the api.
+
+# Unresolved questions
+
+- Finalise the namings
+- Do we need to provide internal key for list rendering, though I don't see it
+  in `react-call-return`
+- Adding to a new package or to `React` directly