packages/node_modules/pouchdb-replication/src/replication.js

import EE from 'events';

// We create a basic promise so the caller can cancel the replication possibly
// before we have actually started listening to changes etc
class Replication extends EE {
  constructor() {
    super();
    this.cancelled = false;
    this.state = 'pending';
    const promise = new Promise((fulfill, reject) => {
      this.once('complete', fulfill);
      this.once('error', reject);
    });
    this.then = function (resolve, reject) {
      return promise.then(resolve, reject);
    };
    this.catch = function (reject) {
      return promise.catch(reject);
    };
    // As we allow error handling via "error" event as well,
    // put a stub in here so that rejecting never throws UnhandledError.
    this.catch(function () {});
  }

  cancel() {
    this.cancelled = true;
    this.state = 'cancelled';
    this.emit('cancel');
  }

  ready(src, target) {
    if (this._readyCalled) {
      return;
    }
    this._readyCalled = true;
  
    const onDestroy = () => {
      this.cancel();
    };
    src.once('destroyed', onDestroy);
    target.once('destroyed', onDestroy);
    function cleanup() {
      src.removeListener('destroyed', onDestroy);
      target.removeListener('destroyed', onDestroy);
    }
    this.once('complete', cleanup);
    this.once('error', cleanup);
  }
}

export default Replication;