Can I fetch a list of all childless descendants of a given tree item?

[alexdarling]

Hi all, I'm trying to create a tree with indeterminate checks based on whether a descendent is checked, like in this CSS Tricks tutorial. This may be an X/Y problem so I'll describe my thought process. I'm looking at a 2-part logic that keys off of which tree items are selected:

1. All childless tree items are checked if they are selected, or unchecked otherwise
2. A tree item with children should be indeterminately checked if ANY of its descendants (i.e. it's children, or it's children's children, or it's children's children's children's, etc) are selected, and should be fully checked if ALL of is descendants are selected.

Here's some rough code I put together for this calculation:

enum CheckState {
  None,
  Partial,
  Full,
}

const deriveCheckedState = (item : TreeItem, context: TreeItemRenderContext) => {
  // Leaf items are either checked or not
  if (!item.hasChildren) {
    return context.isSelected ? CheckState.Full : CheckState.None;
  }
  const childlessDescendants : TreeItemIndex = // derive this somehow
  const selectedItems : Set<TreeItemIndex> = // derive this somehow
  // Set intersection
  const delta = childlessDescendants.length - childlessDescendants.filter(i => selectedItems.has(i)).length;
  // Full intersection = normal check
  if (delta == 0) {
    return CheckState.Full;
  }
  // No intersection = no check
  if (delta == descendants.length) {
    return CheckState.None;
  }
  // Otherwise, indeterminate check
  return CheckState.Partial;
}

I can derive selectedItems with something like this:

// get an environment ref with useRef() / createRef()
const selectedItems = new Set(treeEnvironmentRef.current.viewState[MY_TREE_ID].selectedItems);

What I'm stuck on now is determining all the descendants of a given tree item. All I need is a list of TreeItemIndex and I can feed them into my selectedItems, but I'm not sure how to fetch descendant information. Is there an API for this, or a way to query a tree ref by tree item index?