Method transform

packages/common/src/pipes/async_pipe.ts:178–200

(obj: Observable<T> | Subscribable<T> | PromiseLike<T> | null | undefined)

  transform<T>(obj: null | undefined): null;
  transform<T>(obj: Observable<T> | Subscribable<T> | PromiseLike<T> | null | undefined): T | null;
  transform<T>(obj: Observable<T> | Subscribable<T> | PromiseLike<T> | null | undefined): T | null {
    if (!this._obj) {
      if (obj) {
        try {
          // Only call `markForCheck` if the value is updated asynchronously.
          // Synchronous updates _during_ subscription should not wastefully mark for check -
          // this value is already going to be returned from the transform function.
          this.markForCheckOnValueUpdate = false;
          this._subscribe(obj);
        } finally {
          this.markForCheckOnValueUpdate = true;
        }
      }
      return this._latestValue;
    }

    if (obj !== this._obj) {
      this._dispose();
      return this.transform(obj);
    }

    return this._latestValue;
  }

  private _subscribe(obj: Subscribable<any> | PromiseLike<any> | EventEmitter<any>): void {
    this._obj = obj;