Method _getAllChildrenRecursively

src/cdk/tree/tree.ts:934–951

* Gets all children and sub-children of the provided node. * * This will emit multiple times, in the order that the children will appear * in the tree, and can be combined with a `reduce` operator.

(dataNode: T)

   * in the tree, and can be combined with a `reduce` operator.
   */
  private _getAllChildrenRecursively(dataNode: T): Observable<T[]> {
    if (!this.childrenAccessor) {
      return observableOf([]);
    }

    return coerceObservable(this.childrenAccessor(dataNode)).pipe(
      take(1),
      switchMap(children => {
        // Here, we cache the parents of a particular child so that we can compute the levels.
        for (const child of children) {
          this._parents.set(this._getExpansionKey(child), dataNode);
        }
        return observableOf(...children).pipe(
          concatMap(child => concat(observableOf([child]), this._getAllChildrenRecursively(child))),
        );
      }),
    );
  }

  private _getExpansionKey(dataNode: T): K {
    // In the case that a key accessor function was not provided by the