Cocoa Programming for OS X: The Big Nerd Ranch Guide

Undo for Edits

It is time now for you to add undo support for editing of employees’s fields to RaiseMan. The first step is to register Document to observe changes to the Employee objects in its employees array. Add the following private variable near the top of Document.swift:

import Cocoa

private var KVOContext: Int = 0

class Document: NSDocument {

You will use the KVOContext variable to get a unique value to use as the KVO context pointer.

Next, add this section inside the Document implementation:

    // MARK: - Key Value Observing

    func startObservingEmployee(employee: Employee) {
        employee.addObserver(self,
                             forKeyPath: "name",
                             options: .Old,
                             context: &KVOContext)
        employee.addObserver(self,
                             forKeyPath: "raise",
                             options: .Old, ...