Add ref to Fragment (facebook#32465)

*This API is experimental and subject to change or removal.*

This PR is an alternative to
facebook#32421 based on feedback:
facebook#32421 (review)
. The difference here is that we traverse from the Fragment's fiber at
operation time instead of keeping a set of children on the
`FragmentInstance`. We still need to handle newly added or removed child
nodes to apply event listeners and observers, so we treat those updates
as effects.

**Fragment Refs**

This PR extends React's Fragment component to accept a `ref` prop. The
Fragment's ref will attach to a custom host instance, which will provide
an Element-like API for working with the Fragment's host parent and host
children.

Here I've implemented `addEventListener`, `removeEventListener`, and
`focus` to get started but we'll be iterating on this by adding
additional APIs in future PRs. This sets up the mechanism to attach refs
and perform operations on children. The FragmentInstance is implemented
in `react-dom` here but is planned for Fabric as well.

The API works by targeting the first level of host children and proxying
Element-like APIs to allow developers to manage groups of elements or
elements that cannot be easily accessed such as from a third-party
library or deep in a tree of Functional Component wrappers.

```javascript
import {Fragment, useRef} from 'react';

const fragmentRef = useRef(null);

<Fragment ref={fragmentRef}>
  <div id="A" />
  <Wrapper>
    <div id="B">
      <div id="C" />
    </div>
  </Wrapper>
  <div id="D" />
</Fragment>
```

In this case, calling `fragmentRef.current.addEventListener()` would
apply an event listener to `A`, `B`, and `D`. `C` is skipped because it
is nested under the first level of Host Component. If another Host
Component was appended as a sibling to `A`, `B`, or `D`, the event
listener would be applied to that element as well and any other APIs
would also affect the newly added child.

This is an implementation of the basic feature as a starting point for
feedback and further iteration.

Author: jackpope

packages/react-art/src/ReactFiberConfigART.js

@@ -318,6 +318,27 @@ export function cloneMutableTextInstance(textInstance) {
   return textInstance;
 }
 
+export type FragmentInstanceType = null;
+
+export function createFragmentInstance(fiber): null {
+  return null;
+}
+
+export function updateFragmentInstanceFiber(fiber, instance): void {
+  // Noop
+}
+
+export function commitNewChildToFragmentInstance(
+  child,
+  fragmentInstance,
+): void {
+  // Noop
+}
+
+export function deleteChildFromFragmentInstance(child, fragmentInstance): void {
+  // Noop
+}
+
 export function finalizeInitialChildren(domElement, type, props) {
   return false;
 }

packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -34,6 +34,7 @@ import {getCurrentRootHostContainer} from 'react-reconciler/src/ReactFiberHostCo
 import hasOwnProperty from 'shared/hasOwnProperty';
 import {checkAttributeStringCoercion} from 'shared/CheckStringCoercion';
 import {REACT_CONTEXT_TYPE} from 'shared/ReactSymbols';
+import {OffscreenComponent} from 'react-reconciler/src/ReactWorkTags';
 
 export {
   setCurrentUpdatePriority,
@@ -2159,6 +2160,235 @@ export function subscribeToGestureDirection(
   }
 }
 
+type EventListenerOptionsOrUseCapture =
+  | boolean
+  | {
+      capture?: boolean,
+      once?: boolean,
+      passive?: boolean,
+      signal?: AbortSignal,
+      ...
+    };
+
+type StoredEventListener = {
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture: void | EventListenerOptionsOrUseCapture,
+};
+
+export type FragmentInstanceType = {
+  _fragmentFiber: Fiber,
+  _eventListeners: null | Array<StoredEventListener>,
+  addEventListener(
+    type: string,
+    listener: EventListener,
+    optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+  ): void,
+  removeEventListener(
+    type: string,
+    listener: EventListener,
+    optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+  ): void,
+  focus(): void,
+};
+
+function FragmentInstance(this: FragmentInstanceType, fragmentFiber: Fiber) {
+  this._fragmentFiber = fragmentFiber;
+  this._eventListeners = null;
+}
+// $FlowFixMe[prop-missing]
+FragmentInstance.prototype.addEventListener = function (
+  this: FragmentInstanceType,
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+): void {
+  if (this._eventListeners === null) {
+    this._eventListeners = [];
+  }
+
+  const listeners = this._eventListeners;
+  // Element.addEventListener will only apply uniquely new event listeners by default. Since we
+  // need to collect the listeners to apply to appended children, we track them ourselves and use
+  // custom equality check for the options.
+  const isNewEventListener =
+    indexOfEventListener(listeners, type, listener, optionsOrUseCapture) === -1;
+  if (isNewEventListener) {
+    listeners.push({type, listener, optionsOrUseCapture});
+    traverseFragmentInstanceChildren(
+      this,
+      this._fragmentFiber.child,
+      addEventListenerToChild,
+      type,
+      listener,
+      optionsOrUseCapture,
+    );
+  }
+  this._eventListeners = listeners;
+};
+function addEventListenerToChild(
+  child: Instance,
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+): boolean {
+  child.addEventListener(type, listener, optionsOrUseCapture);
+  return false;
+}
+// $FlowFixMe[prop-missing]
+FragmentInstance.prototype.removeEventListener = function (
+  this: FragmentInstanceType,
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+): void {
+  const listeners = this._eventListeners;
+  if (listeners === null) {
+    return;
+  }
+  if (typeof listeners !== 'undefined' && listeners.length > 0) {
+    traverseFragmentInstanceChildren(
+      this,
+      this._fragmentFiber.child,
+      removeEventListenerFromChild,
+      type,
+      listener,
+      optionsOrUseCapture,
+    );
+    const index = indexOfEventListener(
+      listeners,
+      type,
+      listener,
+      optionsOrUseCapture,
+    );
+    if (this._eventListeners !== null) {
+      this._eventListeners.splice(index, 1);
+    }
+  }
+};
+function removeEventListenerFromChild(
+  child: Instance,
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture?: EventListenerOptionsOrUseCapture,
+): boolean {
+  child.removeEventListener(type, listener, optionsOrUseCapture);
+  return false;
+}
+// $FlowFixMe[prop-missing]
+FragmentInstance.prototype.focus = function (this: FragmentInstanceType) {
+  traverseFragmentInstanceChildren(
+    this,
+    this._fragmentFiber.child,
+    setFocusIfFocusable,
+  );
+};
+
+function traverseFragmentInstanceChildren<A, B, C>(
+  fragmentInstance: FragmentInstanceType,
+  child: Fiber | null,
+  fn: (Instance, A, B, C) => boolean,
+  a: A,
+  b: B,
+  c: C,
+): void {
+  while (child !== null) {
+    if (child.tag === HostComponent) {
+      if (fn(child.stateNode, a, b, c)) {
+        return;
+      }
+    } else if (
+      child.tag === OffscreenComponent &&
+      child.memoizedState !== null
+    ) {
+      // Skip hidden subtrees
+    } else {
+      traverseFragmentInstanceChildren(
+        fragmentInstance,
+        child.child,
+        fn,
+        a,
+        b,
+        c,
+      );
+    }
+    child = child.sibling;
+  }
+}
+
+function normalizeListenerOptions(
+  opts: ?EventListenerOptionsOrUseCapture,
+): string {
+  if (opts == null) {
+    return '0';
+  }
+
+  if (typeof opts === 'boolean') {
+    return `c=${opts ? '1' : '0'}`;
+  }
+
+  return `c=${opts.capture ? '1' : '0'}&o=${opts.once ? '1' : '0'}&p=${opts.passive ? '1' : '0'}`;
+}
+
+function indexOfEventListener(
+  eventListeners: Array<StoredEventListener>,
+  type: string,
+  listener: EventListener,
+  optionsOrUseCapture: void | EventListenerOptionsOrUseCapture,
+): number {
+  for (let i = 0; i < eventListeners.length; i++) {
+    const item = eventListeners[i];
+    if (
+      item.type === type &&
+      item.listener === listener &&
+      normalizeListenerOptions(item.optionsOrUseCapture) ===
+        normalizeListenerOptions(optionsOrUseCapture)
+    ) {
+      return i;
+    }
+  }
+  return -1;
+}
+
+export function createFragmentInstance(
+  fragmentFiber: Fiber,
+): FragmentInstanceType {
+  return new (FragmentInstance: any)(fragmentFiber);
+}
+
+export function updateFragmentInstanceFiber(
+  fragmentFiber: Fiber,
+  instance: FragmentInstanceType,
+): void {
+  instance._fragmentFiber = fragmentFiber;
+}
+
+export function commitNewChildToFragmentInstance(
+  childElement: Instance,
+  fragmentInstance: FragmentInstanceType,
+): void {
+  const eventListeners = fragmentInstance._eventListeners;
+  if (eventListeners !== null) {
+    for (let i = 0; i < eventListeners.length; i++) {
+      const {type, listener, optionsOrUseCapture} = eventListeners[i];
+      childElement.addEventListener(type, listener, optionsOrUseCapture);
+    }
+  }
+}
+
+export function deleteChildFromFragmentInstance(
+  childElement: Instance,
+  fragmentInstance: FragmentInstanceType,
+): void {
+  const eventListeners = fragmentInstance._eventListeners;
+  if (eventListeners !== null) {
+    for (let i = 0; i < eventListeners.length; i++) {
+      const {type, listener, optionsOrUseCapture} = eventListeners[i];
+      childElement.removeEventListener(type, listener, optionsOrUseCapture);
+    }
+  }
+}
+
 export function clearContainer(container: Container): void {
   const nodeType = container.nodeType;
   if (nodeType === DOCUMENT_NODE) {