feat: EventEmitter#eject, #inject

packages/puppeteer-core/src/common/EventEmitter.ts

@@ -15,6 +15,7 @@
  */
 
 import mitt, {
+    WildcardHandler,
   type Emitter,
   type EventHandlerMap,
 } from '../../third_party/mitt/mitt.js';
@@ -69,6 +70,23 @@ export type EventsWithWildcard<Events extends Record<EventType, unknown>> =
     '*': Events[keyof Events];
   };
 
+/**
+ * An ejected set of {@link EventEmitter} listeners.
+ * 
+ * @public
+ */
+export class EmitterState<Events extends Record<EventType, unknown>> {
+  #map: EventHandlerMap<Events>;
+
+  constructor(map: EventHandlerMap<Events>) {
+    this.#map = map
+  }
+
+  toMap(): EventHandlerMap<Events> {
+    return this.#map;
+  }
+}
+
 /**
  * The EventEmitter class that many Puppeteer classes extend.
  *
@@ -94,6 +112,50 @@ export class EventEmitter<Events extends Record<EventType, unknown>>
     this.#emitter = mitt(this.#handlers);
   }
 
+  /**
+   * Returns all event listeners without modifying them.
+   *
+   * @public
+   */
+  eject(): EmitterState<EventsWithWildcard<Events>> {
+    const handlers = new Map();
+    this.#handlers.forEach((v, k) => handlers.set(k, v))
+    return new EmitterState(handlers)
+  }
+
+  /**
+   * Remove all listeners & import a new {@link state}.
+   * 
+   * @param state - A previously {@link EventEmitter.eject | ejected} {@link EmitterState}.
+   * @returns `this` to enable you to chain method calls.
+   *
+   * @public
+   */
+  inject(state: EmitterState<EventsWithWildcard<Events>>): this {
+    this.#handlers.clear()
+
+    const map = state.toMap()
+
+    const assertWildcardHandler: (_: unknown) => asserts _ is WildcardHandler<EventsWithWildcard<Events>> = _ => {}
+    const assertHandler: (_: unknown) => asserts _ is Handler<Events[keyof Events]> = _ => {}
+
+    map.forEach((hs, ev) => {
+      if (ev === '*') {
+        hs.forEach((h: unknown) => {
+          assertWildcardHandler(h)
+          this.#emitter.on(ev, h)
+        })
+      } else {
+        hs.forEach((h: unknown) => {
+          assertHandler(h)
+          this.#emitter.on(ev, h)
+        })
+      }
+    })
+
+    return this
+  }
+
   /**
    * Bind an event listener to fire when an event occurs.
    * @param type - the event type you'd like to listen to. Can be a string or symbol.