OperatorSubscriber.ts

import { Subscriber } from '../Subscriber';

/**
 * A generic helper for allowing operators to be created with a Subscriber and
 * use closures to capture necessary state from the operator function itself.
 */
export class OperatorSubscriber<T> extends Subscriber<T> {
  /**
   * Creates an instance of an `OperatorSubscriber`.
   * @param destination The downstream subscriber.
   * @param onNext Handles next values, only called if this subscriber is not stopped or closed. Any
   * error that occurs in this function is caught and sent to the `error` method of this subscriber.
   * @param onError Handles errors from the subscription, any errors that occur in this handler are caught
   * and send to the `destination` error handler.
   * @param onComplete Handles completion notification from the subscription. Any errors that occur in
   * this handler are sent to the `destination` error handler.
   * @param onFinalize Additional teardown logic here. This will only be called on teardown if the
   * subscriber itself is not already closed. This is called after all other teardown logic is executed.
   */
  constructor(
    destination: Subscriber<any>,
    onNext?: (value: T) => void,
    onComplete?: () => void,
    onError?: (err: any) => void,
    private onFinalize?: () => void
  ) {
    // It's important - for performance reasons - that all of this class's
    // members are initialized and that they are always initialized in the same
    // order. This will ensure that all OperatorSubscriber instances have the
    // same hidden class in V8. This, in turn, will help keep the number of
    // hidden classes involved in property accesses within the base class as
    // low as possible. If the number of hidden classes involved exceeds four,
    // the property accesses will become megamorphic and performance penalties
    // will be incurred - i.e. inline caches won't be used.
    //
    // The reasons for ensuring all instances have the same hidden class are
    // further discussed in this blog post from Benedikt Meurer:
    // https://benediktmeurer.de/2018/03/23/impact-of-polymorphism-on-component-based-frameworks-like-react/
    super(destination);
    this._next = onNext
      ? function (this: OperatorSubscriber<T>, value: T) {
          try {
            onNext(value);
          } catch (err) {
            destination.error(err);
          }
        }
      : super._next;
    this._error = onError
      ? function (this: OperatorSubscriber<T>, err: any) {
          try {
            onError(err);
          } catch (err) {
            // Send any errors that occur down stream.
            destination.error(err);
          } finally {
            // Ensure teardown.
            this.unsubscribe();
          }
        }
      : super._error;
    this._complete = onComplete
      ? function (this: OperatorSubscriber<T>) {
          try {
            onComplete();
          } catch (err) {
            // Send any errors that occur down stream.
            destination.error(err);
          } finally {
            // Ensure teardown.
            this.unsubscribe();
          }
        }
      : super._complete;
  }

  unsubscribe() {
    const { closed } = this;
    super.unsubscribe();
    // Execute additional teardown if we have any and we didn't already do so.
    !closed && this.onFinalize?.();
  }
}