Add JS stub for PerformanceObserver specs

Summary:
This stubs all the type definitions for the [PerformanceObserver web perf APIs](https://www.w3.org/TR/performance-timeline/#the-performanceobserver-interface).

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D40892974

fbshipit-source-id: 31fe972fc069eb62e51bca82e9cd42ca65811753

Author: rshest

Libraries/Performance/PerformanceObserver.js

@@ -0,0 +1,124 @@
+/**
+ * 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.
+ *
+ * @format
+ * @flow strict
+ */
+
+'use strict';
+
+export type Timestamp = number;
+export type PerformanceEntryType = 'event';
+
+export class PerformanceEntry {
+  name: string;
+  entryType: PerformanceEntryType;
+  startTime: Timestamp;
+  duration: number;
+
+  // For "event" entries only:
+  processingStart: ?Timestamp;
+  processingEnd: ?Timestamp;
+  interactionId: ?number;
+
+  // $FlowIgnore: Flow(unclear-type)
+  toJSON(): Object {
+    return {
+      name: this.name,
+      entryType: this.entryType,
+      startTime: this.startTime,
+      duration: this.duration,
+    };
+  }
+}
+
+export type PerformanceEntryList = $ReadOnlyArray<PerformanceEntry>;
+
+export class PerformanceObserverEntryList {
+  _entries: PerformanceEntryList;
+
+  constructor(entries: PerformanceEntryList) {
+    this._entries = entries;
+  }
+
+  getEntries(): PerformanceEntryList {
+    return this._entries;
+  }
+
+  getEntriesByType(type: PerformanceEntryType): PerformanceEntryList {
+    return this._entries.filter(entry => entry.entryType === type);
+  }
+
+  getEntriesByName(
+    name: string,
+    type?: PerformanceEntryType,
+  ): PerformanceEntryList {
+    if (type === undefined) {
+      return this._entries.filter(entry => entry.name === name);
+    } else {
+      return this._entries.filter(
+        entry => entry.name === name && entry.entryType === type,
+      );
+    }
+  }
+}
+
+export type PerformanceObserverCallback = (
+  list: PerformanceObserverEntryList,
+  observer: PerformanceObserver,
+) => void;
+
+export type PerformanceObserverInit =
+  | {
+      entryTypes: PerformanceEntryType[],
+      buffered?: boolean,
+    }
+  | {
+      type: PerformanceEntryType,
+      buffered?: boolean,
+    };
+
+/**
+ * Implementation of the PerformanceObserver interface for RN,
+ * corresponding to the standard in https://www.w3.org/TR/performance-timeline/
+ *
+ * @example
+ * const observer = new PerformanceObserver((list, _observer) => {
+ *   const entries = list.getEntries();
+ *   entries.forEach(entry => {
+ *     reportEvent({
+ *       eventName: entry.name,
+ *       startTime: entry.startTime,
+ *       endTime: entry.startTime + entry.duration,
+ *       processingStart: entry.processingStart,
+ *       processingEnd: entry.processingEnd,
+ *       interactionId: entry.interactionId,
+ *     });
+ *   });
+ * });
+ * observer.observe({ type: "event" });
+ */
+export default class PerformanceObserver {
+  _callback: PerformanceObserverCallback;
+
+  constructor(callback: PerformanceObserverCallback) {
+    this._callback = callback;
+  }
+
+  observe(options: PerformanceObserverInit) {
+    console.log('PerformanceObserver: started observing');
+  }
+
+  disconnect(): void {
+    console.log('PerformanceObserver: stopped observing');
+  }
+
+  takeRecords(): PerformanceEntryList {
+    return [];
+  }
+
+  static supportedEntryTypes: PerformanceEntryType[] = ['event'];
+}