didSet_children_updates.swift

#!/usr/bin/swift

class Node {

    var children: [Node] = []

    var depth: Int = 0 {
        didSet {
            for child in children {
                child.depth = depth + 1
            }
        }
    }

}

let node = Node()
let child = Node()
let grandchild = Node()

node.children.append(child)
child.children.append(grandchild)
node.depth = 1

print("Node: \(node.depth)")
print("Child: \(child.depth)")
print("Grandchild: \(grandchild.depth)")