didSet is called on each update of Array

didSet_Array.swift

class Node: CustomStringConvertible {
    let title: String
    var parent: Node?
    var children: [Node] = [] {
        didSet {
            children.forEach({ $0.parent = self })
        }
    }
    
    var description: String {
        return title
    }
    
    init(_ title: String) {
        self.title = title
    }
}