Method _flattenNestedNodesWithExpansion

src/cdk/tree/tree.ts:1004–1048

* Given a set of root nodes and the current node level, flattens any nested * nodes into a single array. * * If any nodes are not expanded, then their children will not be added into the array. * This will still traverse all nested children in order to build up our internal data * mod

(nodes: readonly T[], level = 0)

   * models, but will not include them in the returned array.
   */
  private _flattenNestedNodesWithExpansion(nodes: readonly T[], level = 0): Observable<T[]> {
    const childrenAccessor = this._getChildrenAccessor();
    // If we're using a level accessor, we don't need to flatten anything.
    if (!childrenAccessor) {
      return observableOf([...nodes]);
    }

    return observableOf(...nodes).pipe(
      concatMap(node => {
        const parentKey = this._getExpansionKey(node);
        if (!this._parents.has(parentKey)) {
          this._parents.set(parentKey, null);
        }
        this._levels.set(parentKey, level);

        const children = coerceObservable(childrenAccessor(node));
        return concat(
          observableOf([node]),
          children.pipe(
            take(1),
            tap(childNodes => {
              this._ariaSets.set(parentKey, [...(childNodes ?? [])]);
              for (const child of childNodes ?? []) {
                const childKey = this._getExpansionKey(child);
                this._parents.set(childKey, node);
                this._levels.set(childKey, level + 1);
              }
            }),
            switchMap(childNodes => {
              if (!childNodes) {
                return observableOf([]);
              }
              return this._flattenNestedNodesWithExpansion(childNodes, level + 1).pipe(
                map(nestedNodes => (this.isExpanded(node) ? nestedNodes : [])),
              );
            }),
          ),
        );
      }),
      reduce((results, children) => {
        results.push(...children);
        return results;
      }, [] as T[]),
    );
  }

  /**
   * Converts children for certain tree configurations.