Method _getDirectChildren

src/cdk/tree/tree.ts:744–780

* Gets the direct children of a node; used for compatibility between the old tree and the * new tree.

(dataNode: T)

   * new tree.
   */
  _getDirectChildren(dataNode: T): Observable<T[]> {
    const levelAccessor = this._getLevelAccessor();
    const expansionModel = this._expansionModel ?? this.treeControl?.expansionModel;
    if (!expansionModel) {
      return observableOf([]);
    }

    const key = this._getExpansionKey(dataNode);

    const isExpanded = expansionModel.changed.pipe(
      switchMap(changes => {
        if (changes.added.includes(key)) {
          return observableOf(true);
        } else if (changes.removed.includes(key)) {
          return observableOf(false);
        }
        return EMPTY;
      }),
      startWith(this.isExpanded(dataNode)),
    );

    if (levelAccessor) {
      return combineLatest([isExpanded, this._flattenedNodes]).pipe(
        map(([expanded, flattenedNodes]) => {
          if (!expanded) {
            return [];
          }
          return this._findChildrenByLevel(levelAccessor, flattenedNodes, dataNode, 1);
        }),
      );
    }
    const childrenAccessor = this._getChildrenAccessor();
    if (childrenAccessor) {
      return coerceObservable(childrenAccessor(dataNode) ?? []);
    }
    throw getTreeControlMissingError();
  }

  /**
   * Given the list of flattened nodes, the level accessor, and the level range within