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Swift 4 Programming Cookbook

You may often find that you would like to know when a property changes its value; maybe you want to update the value of another property, or inform a delegate. In Objective-C, this was often accomplished by writing your own getter and setter, or using Key-Value observing (KVO), but we have native support for property observers in Swift.
To examine property observers, we should create an object with a property that we want to observe. Let's create an object to manage users and a property to hold the current user's name:
class UserManager { var currentUserName: String = "Emmanuel Goldstein" }
We want to present some friendly messages when the current user changes, and we'll use property observers to do it.
Amend the currentUserName
property definition to be the following:
class UserManager { var currentUserName: String = "Emmanuel Goldstein" { willSet (newUserName) { print...