RN: Implements EventSubscription

Summary:
Changes the to-be-legacy `_EventSubscription` and `_EmitterSubscription` classes to implement the event-agnostic `EventSubscription` interface.

This interface is valuable because it abstracts away the event definitions.

Changelog:
[Internal]

Reviewed By: lunaleaps

Differential Revision: D26155911

fbshipit-source-id: f94280976d4f9219f4780ac8fae0d5fbc1865386

Libraries/vendor/emitter/EventEmitter.js

@@ -12,8 +12,8 @@
 
 const EventEmitter = require('./_EventEmitter');
 
+import type {EventSubscription} from './EventSubscription';
+
 export default EventEmitter;
 
-export interface EventSubscription {
-  remove(): void;
-}
+export type {EventSubscription};

Libraries/vendor/emitter/EventSubscription.js

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) Facebook, Inc. and its 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
+ */
+
+'use strict';
+
+// This exists as a separate file only to avoid circular dependencies from
+// using this in `_EmitterSubscription`. Combine this back into `EventEmitter`
+// after migration and cleanup is done.
+
+export interface EventSubscription {
+  remove(): void;
+}

Libraries/vendor/emitter/_EmitterSubscription.js

@@ -11,16 +11,16 @@
 'use strict';
 
 import type EventEmitter from './EventEmitter';
-import EventSubscription from './_EventSubscription';
+import _EventSubscription from './_EventSubscription';
 import type EventSubscriptionVendor from './_EventSubscriptionVendor';
+import {type EventSubscription} from './EventSubscription';
 
 /**
  * EmitterSubscription represents a subscription with listener and context data.
  */
-class EmitterSubscription<
-  EventDefinitions: {...},
-  K: $Keys<EventDefinitions>,
-> extends EventSubscription<EventDefinitions, K> {
+class EmitterSubscription<EventDefinitions: {...}, K: $Keys<EventDefinitions>>
+  extends _EventSubscription<EventDefinitions, K>
+  implements EventSubscription {
   emitter: EventEmitter<EventDefinitions>;
   listener: ?(...$ElementType<EventDefinitions, K>) => mixed;
   context: ?$FlowFixMe;
@@ -49,7 +49,7 @@ class EmitterSubscription<
 
   /**
    * Removes this subscription from the emitter that registered it.
-   * Note: we're overriding the `remove()` method of EventSubscription here
+   * Note: we're overriding the `remove()` method of _EventSubscription here
    * but deliberately not calling `super.remove()` as the responsibility
    * for removing the subscription lies with the EventEmitter.
    */

Libraries/vendor/emitter/_EventSubscription.js

@@ -10,13 +10,15 @@
 
 'use strict';
 
+import {type EventSubscription} from './EventSubscription';
 import type EventSubscriptionVendor from './_EventSubscriptionVendor';
 
 /**
  * EventSubscription represents a subscription to a particular event. It can
  * remove its own subscription.
  */
-class EventSubscription<EventDefinitions: {...}, K: $Keys<EventDefinitions>> {
+class _EventSubscription<EventDefinitions: {...}, K: $Keys<EventDefinitions>>
+  implements EventSubscription {
   eventType: K;
   key: number;
   subscriber: EventSubscriptionVendor<EventDefinitions>;
@@ -39,4 +41,4 @@ class EventSubscription<EventDefinitions: {...}, K: $Keys<EventDefinitions>> {
   }
 }
 
-module.exports = EventSubscription;
+module.exports = _EventSubscription;