RN: Rewrite EventEmitter

Summary:
Rewrites `EventEmitter` as a simple, type-safe abstraction with a minimal interface.

The public interface of `EventEmitter` is unchanged. This rewrite was made possible only after deprecating and removing public methods that imposed restrictions on implementation details (e.g. deleting `removeListener`).

However, this includes a subtle breaking change that makes it behave the same as `EventEmitter` in Node.js and `EventTarget` in the DOM. The set of listeners being notified by `emit` will no longer be influenced by changes made during the course of notifying the existing listeners.

Changelog:
[General][Changed] - `EventEmitter#emit` now freezes the set of listeners before iterating over them, meaning listeners that are added or removed will not affect that iteration.

Reviewed By: javache

Differential Revision: D22153962

fbshipit-source-id: 81b87113590dee0296eff61374bf732171855453

Libraries/vendor/emitter/EventEmitter.js

@@ -8,46 +8,139 @@
  * @format
  */
 
-'use strict';
+export interface EventSubscription {
+  remove(): void;
+}
+
+export interface IEventEmitter<TEventToArgsMap: {...}> {
+  addListener<TEvent: $Keys<TEventToArgsMap>>(
+    eventType: TEvent,
+    listener: (...args: $ElementType<TEventToArgsMap, TEvent>) => mixed,
+    context?: mixed,
+  ): EventSubscription;
 
-const EventEmitter = require('./_EventEmitter');
+  emit<TEvent: $Keys<TEventToArgsMap>>(
+    eventType: TEvent,
+    ...args: $ElementType<TEventToArgsMap, TEvent>
+  ): void;
 
-import type {EventSubscription} from './EventSubscription';
+  removeAllListeners<TEvent: $Keys<TEventToArgsMap>>(eventType?: ?TEvent): void;
+
+  listenerCount<TEvent: $Keys<TEventToArgsMap>>(eventType: TEvent): number;
+}
 
-export default EventEmitter;
+interface Registration<TArgs> {
+  +context: mixed;
+  +listener: (...args: TArgs) => mixed;
+  +remove: () => void;
+}
 
-export type {EventSubscription};
+type Registry<TEventToArgsMap: {...}> = $ObjMap<
+  TEventToArgsMap,
+  <TArgs>(TArgs) => Set<Registration<TArgs>>,
+>;
 
 /**
- * Essential interface for an EventEmitter.
+ * EventEmitter manages listeners and publishes events to them.
+ *
+ * EventEmitter accepts a single type parameter that defines the valid events
+ * and associated listener argument(s).
+ *
+ * @example
+ *
+ *   const emitter = new EventEmitter<{
+ *     success: [number, string],
+ *     error: [Error],
+ *   }>();
+ *
+ *   emitter.on('success', (statusCode, responseText) => {...});
+ *   emitter.emit('success', 200, '...');
+ *
+ *   emitter.on('error', error => {...});
+ *   emitter.emit('error', new Error('Resource not found'));
+ *
  */
-export interface IEventEmitter<TEventToArgsMap: {...}> {
+export default class EventEmitter<TEventToArgsMap: {...}>
+  implements IEventEmitter<TEventToArgsMap>
+{
+  _registry: Registry<TEventToArgsMap> = {};
+
   /**
    * Registers a listener that is called when the supplied event is emitted.
    * Returns a subscription that has a `remove` method to undo registration.
    */
   addListener<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
     listener: (...args: $ElementType<TEventToArgsMap, TEvent>) => mixed,
-    context?: mixed,
-  ): EventSubscription;
+    context: mixed,
+  ): EventSubscription {
+    const registrations = allocate(this._registry, eventType);
+    const registration: Registration<$ElementType<TEventToArgsMap, TEvent>> = {
+      context,
+      listener,
+      remove(): void {
+        registrations.delete(registration);
+      },
+    };
+    registrations.add(registration);
+    return registration;
+  }
 
   /**
    * Emits the supplied event. Additional arguments supplied to `emit` will be
    * passed through to each of the registered listeners.
+   *
+   * If a listener modifies the listeners registered for the same event, those
+   * changes will not be reflected in the current invocation of `emit`.
    */
   emit<TEvent: $Keys<TEventToArgsMap>>(
     eventType: TEvent,
     ...args: $ElementType<TEventToArgsMap, TEvent>
-  ): void;
+  ): void {
+    const registrations: ?Set<
+      Registration<$ElementType<TEventToArgsMap, TEvent>>,
+    > = this._registry[eventType];
+    if (registrations != null) {
+      for (const registration of [...registrations]) {
+        registration.listener.apply(registration.context, args);
+      }
+    }
+  }
 
   /**
    * Removes all registered listeners.
    */
-  removeAllListeners<TEvent: $Keys<TEventToArgsMap>>(eventType?: ?TEvent): void;
+  removeAllListeners<TEvent: $Keys<TEventToArgsMap>>(
+    eventType?: ?TEvent,
+  ): void {
+    if (eventType == null) {
+      this._registry = {};
+    } else {
+      delete this._registry[eventType];
+    }
+  }
 
   /**
    * Returns the number of registered listeners for the supplied event.
    */
-  listenerCount<TEvent: $Keys<TEventToArgsMap>>(eventType: TEvent): number;
+  listenerCount<TEvent: $Keys<TEventToArgsMap>>(eventType: TEvent): number {
+    const registrations: ?Set<Registration<mixed>> = this._registry[eventType];
+    return registrations == null ? 0 : registrations.size;
+  }
+}
+
+function allocate<
+  TEventToArgsMap: {...},
+  TEvent: $Keys<TEventToArgsMap>,
+  TEventArgs: $ElementType<TEventToArgsMap, TEvent>,
+>(
+  registry: Registry<TEventToArgsMap>,
+  eventType: TEvent,
+): Set<Registration<TEventArgs>> {
+  let registrations: ?Set<Registration<TEventArgs>> = registry[eventType];
+  if (registrations == null) {
+    registrations = new Set();
+    registry[eventType] = registrations;
+  }
+  return registrations;
 }