expo-modules-core/src/LegacyEventEmitter.ts

The core of Expo Modules architecture

import invariant from 'invariant';
import { NativeEventEmitter, Platform } from 'react-native';

import { EventSubscription } from './EventEmitter';

const nativeEmitterSubscriptionKey = '@@nativeEmitterSubscription@@';

type NativeModule = {
  __expo_module_name__?: string;
  startObserving?: () => void;
  stopObserving?: () => void;

  // Erase these types as they would conflict with the new NativeModule type.
  // This EventEmitter is deprecated anyway.
  addListener?: any;
  removeListeners?: any;
};

/**
 * @deprecated Deprecated in favor of `EventEmitter`.
 */
export class LegacyEventEmitter {
  _listenerCount = 0;

  // @ts-expect-error
  _nativeModule: NativeModule;

  // @ts-expect-error
  _eventEmitter: NativeEventEmitter;

  constructor(nativeModule: NativeModule) {
    // If the native module is a new module, just return it back as it's already an event emitter.
    // This is for backwards compatibility until we stop using this legacy class in other packages.
    if (nativeModule.__expo_module_name__) {
      // @ts-expect-error
      return nativeModule;
    }
    this._nativeModule = nativeModule;
    this._eventEmitter = new NativeEventEmitter(nativeModule as any);
  }

  addListener<T>(eventName: string, listener: (event: T) => void): EventSubscription {
    if (!this._listenerCount && Platform.OS !== 'ios' && this._nativeModule.startObserving) {
      this._nativeModule.startObserving();
    }

    this._listenerCount++;
    const nativeEmitterSubscription = this._eventEmitter.addListener(eventName, listener);
    const subscription = {
      [nativeEmitterSubscriptionKey]: nativeEmitterSubscription,
      remove: () => {
        this.removeSubscription(subscription);
      },
    };
    return subscription;
  }

  removeAllListeners(eventName: string): void {
    // @ts-ignore: the EventEmitter interface has been changed in react-native@0.64.0
    const removedListenerCount = this._eventEmitter.listenerCount
      ? // @ts-ignore: this is available since 0.64
        this._eventEmitter.listenerCount(eventName)
      : // @ts-ignore: this is available in older versions
        this._eventEmitter.listeners(eventName).length;
    this._eventEmitter.removeAllListeners(eventName);
    this._listenerCount -= removedListenerCount;
    invariant(
      this._listenerCount >= 0,
      `EventEmitter must have a non-negative number of listeners`
    );

    if (!this._listenerCount && Platform.OS !== 'ios' && this._nativeModule.stopObserving) {
      this._nativeModule.stopObserving();
    }
  }

  removeSubscription(subscription: EventSubscription): void {
    const nativeEmitterSubscription = subscription[nativeEmitterSubscriptionKey];
    if (!nativeEmitterSubscription) {
      return;
    }

    if ('remove' in nativeEmitterSubscription) {
      nativeEmitterSubscription.remove();
    }
    this._listenerCount--;

    // Ensure that the emitter's internal state remains correct even if `removeSubscription` is
    // called again with the same subscription
    delete subscription[nativeEmitterSubscriptionKey];

    // Release closed-over references to the emitter
    subscription.remove = () => {};

    if (!this._listenerCount && Platform.OS !== 'ios' && this._nativeModule.stopObserving) {
      this._nativeModule.stopObserving();
    }
  }

  emit(eventName: string, ...params: any[]): void {
    this._eventEmitter.emit(eventName, ...params);
  }
}