NoMoreCycle.swift

class Parent {
  var child: Child? {
    didSet {
      child.callback = weak(self)?.delegateCallback
    }
  }
  
  func delegateCallback() {
    // ...
  }
}

class Child {
  var callback: (Void -> Void)?
  
  private func triggerCallback() {
    callback?()
  }
}