override.swift

class A {
    private var underlying: Int = 0
    open var x: Int {
        set {
            print("A.set \(newValue)")
            underlying = newValue
        }
        get { underlying }
    }
}

class B: A {
    override open var x: Int {
        didSet {
            print("B.didSet \(oldValue) -> \(x)")
        }
    }
}

class C: B {
    override open var x: Int {
        didSet {
            print("C.didSet \(oldValue) -> \(x)")
        }
    }
}

C().x = 4
// A.set 4
// B.didSet 0 -> 4
// C.didSet 0 -> 4