What is the purpose of willSet and didSet in Swift?

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The point seems to be that sometimes, you need a property that has automatic storage and some behavior, for instance to notify other objects that the property just changed. When all you have is get/set, you need another field to hold the value. With willSet and didSet, you can take action when the value is modified without needing another field. For instance, in that example:

class Foo {
    var myProperty: Int = 0 {
        didSet {
            print("The value of myProperty changed from \(oldValue) to \(myProperty)")
        }
    }
}

myProperty prints its old and new value every time it is modified. With just getters and setters, I would need this instead:

class Foo {
    var myPropertyValue: Int = 0
    var myProperty: Int {
        get { return myPropertyValue }
        set {
            print("The value of myProperty changed from \(myPropertyValue) to \(newValue)")
            myPropertyValue = newValue
        }
    }
}

So willSet and didSet represent an economy of a couple of lines, and less noise in the field list.

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