Implement Event and EventTarget

Summary:
Changelog: [internal]

This implements a (mostly) spec-compliant version of the [`Event`](https://dom.spec.whatwg.org/#interface-event) and [`EventTarget`](https://dom.spec.whatwg.org/#interface-eventtarget) Web interfaces.

It does not implement legacy methods in either of the interfaces, and ignores the parts of the spec that are related to Web-specific quirks (shadow roots, re-mapping of animation events with webkit prefixes, etc.).

IMPORTANT: This only creates the interfaces and does not expose them externally yet (no `Event` or `EventTarget` in the global scope).

Differential Revision: D67738145

packages/react-native/src/private/webapis/dom/events/Event.js

@@ -0,0 +1,177 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @format
+ */
+
+/**
+ * This module implements the `Event` interface from the DOM.
+ * See https://dom.spec.whatwg.org/#interface-event.
+ */
+
+// flowlint unsafe-getters-setters:off
+
+import type EventTarget from './EventTarget';
+
+import {
+  COMPOSED_PATH_KEY,
+  CURRENT_TARGET_KEY,
+  EVENT_PHASE_KEY,
+  IN_PASSIVE_LISTENER_FLAG_KEY,
+  IS_TRUSTED_KEY,
+  STOP_IMMEDIATE_PROPAGATION_FLAG_KEY,
+  STOP_PROPAGATION_FLAG_KEY,
+  TARGET_KEY,
+  getComposedPath,
+  getCurrentTarget,
+  getEventPhase,
+  getInPassiveListenerFlag,
+  getIsTrusted,
+  getTarget,
+  setStopImmediatePropagationFlag,
+  setStopPropagationFlag,
+} from './internals/EventInternals';
+
+type EventInit = {
+  bubbles?: boolean,
+  cancelable?: boolean,
+  composed?: boolean,
+};
+
+export default class Event {
+  static NONE: 0 = 0;
+  static CAPTURING_PHASE: 1 = 1;
+  static AT_TARGET: 2 = 2;
+  static BUBBLING_PHASE: 3 = 3;
+
+  #bubbles: boolean;
+  #cancelable: boolean;
+  #composed: boolean;
+  #type: string;
+
+  #defaultPrevented: boolean = false;
+  #timeStamp: number = performance.now();
+
+  // $FlowExpectedError[unsupported-syntax]
+  [COMPOSED_PATH_KEY]: boolean = [];
+
+  // $FlowExpectedError[unsupported-syntax]
+  [CURRENT_TARGET_KEY]: EventTarget | null = null;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [EVENT_PHASE_KEY]: boolean = Event.NONE;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [IN_PASSIVE_LISTENER_FLAG_KEY]: boolean = false;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [IS_TRUSTED_KEY]: boolean = false;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [STOP_IMMEDIATE_PROPAGATION_FLAG_KEY]: boolean = false;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [STOP_PROPAGATION_FLAG_KEY]: boolean = false;
+
+  // $FlowExpectedError[unsupported-syntax]
+  [TARGET_KEY]: EventTarget | null = null;
+
+  constructor(type: string, options?: ?EventInit) {
+    if (arguments.length < 1) {
+      throw new TypeError(
+        "Failed to construct 'Event': 1 argument required, but only 0 present.",
+      );
+    }
+
+    if (options != null && typeof options !== 'object') {
+      throw new TypeError(
+        "Failed to construct 'Event': The provided value is not of type 'EventInit'.",
+      );
+    }
+
+    this.#type = String(type);
+    this.#bubbles = Boolean(options?.bubbles);
+    this.#cancelable = Boolean(options?.cancelable);
+    this.#composed = Boolean(options?.composed);
+  }
+
+  get bubbles(): boolean {
+    return this.#bubbles;
+  }
+
+  get cancelable(): boolean {
+    return this.#cancelable;
+  }
+
+  get composed(): boolean {
+    return this.#composed;
+  }
+
+  get currentTarget(): EventTarget | null {
+    return getCurrentTarget(this);
+  }
+
+  get defaultPrevented(): boolean {
+    return this.#defaultPrevented;
+  }
+
+  get eventPhase(): EventPhase {
+    return getEventPhase(this);
+  }
+
+  get isTrusted(): boolean {
+    return getIsTrusted(this);
+  }
+
+  get target(): EventTarget | null {
+    return getTarget(this);
+  }
+
+  get timeStamp(): number {
+    return this.#timeStamp;
+  }
+
+  get type(): string {
+    return this.#type;
+  }
+
+  composedPath(): $ReadOnlyArray<EventTarget> {
+    return getComposedPath(this).slice();
+  }
+
+  preventDefault(): void {
+    if (!this.#cancelable) {
+      return;
+    }
+
+    if (getInPassiveListenerFlag(this)) {
+      console.error(
+        new Error(
+          'Unable to preventDefault inside passive event listener invocation.',
+        ),
+      );
+      return;
+    }
+
+    this.#defaultPrevented = true;
+  }
+
+  stopImmediatePropagation(): void {
+    setStopPropagationFlag(this, true);
+    setStopImmediatePropagationFlag(this, true);
+  }
+
+  stopPropagation(): void {
+    setStopPropagationFlag(this, true);
+  }
+}
+
+export type EventPhase =
+  | (typeof Event)['NONE']
+  | (typeof Event)['CAPTURING_PHASE']
+  | (typeof Event)['AT_TARGET']
+  | (typeof Event)['BUBBLING_PHASE'];