Method _findChildrenByLevel

src/cdk/tree/tree.ts:789–816

* Given the list of flattened nodes, the level accessor, and the level range within * which to consider children, finds the children for a given node. * * For example, for direct children, `levelDelta` would be 1. For all descendants, * `levelDelta` would be Infinity.

(
    levelAccessor: (node: T) => number,
    flattenedNodes: readonly T[],
    dataNode: T,
    levelDelta: number,
  )

   * `levelDelta` would be Infinity.
   */
  private _findChildrenByLevel(
    levelAccessor: (node: T) => number,
    flattenedNodes: readonly T[],
    dataNode: T,
    levelDelta: number,
  ): T[] {
    const key = this._getExpansionKey(dataNode);
    const startIndex = flattenedNodes.findIndex(node => this._getExpansionKey(node) === key);
    const dataNodeLevel = levelAccessor(dataNode);
    const expectedLevel = dataNodeLevel + levelDelta;
    const results: T[] = [];

    // Goes through flattened tree nodes in the `flattenedNodes` array, and get all
    // descendants within a certain level range.
    //
    // If we reach a node whose level is equal to or less than the level of the tree node,
    // we hit a sibling or parent's sibling, and should stop.
    for (let i = startIndex + 1; i < flattenedNodes.length; i++) {
      const currentLevel = levelAccessor(flattenedNodes[i]);
      if (currentLevel <= dataNodeLevel) {
        break;
      }
      if (currentLevel <= expectedLevel) {
        results.push(flattenedNodes[i]);
      }
    }
    return results;
  }

  /**
   * Adds the specified node component to the tree's internal registry.