Property observers respond to changes to a property’s value.

var myProperty = 5 {
    willSet {
        print("Will set to \\(newValue). It was previously \\(myProperty)")
    }
    didSet {
        print("Did set to \\(myProperty). It was previously \\(oldValue)")
    }
}
myProperty = 6
// prints: Will set to 6, It was previously 5
// prints: Did set to 6. It was previously 5

• willSet is called before myProperty is set. The new value is available as newValue, and the old value is still available as myProperty.
• didSet is called after myProperty is set. The old value is available as oldValue, and the new value is now available as myProperty .

Note: didSet and willSet will not be called in the following cases:

• Assigning an initial value
• Modifying the variable within its own didSet or willSet
• The parameter names for oldValue and newValue of didSet and willSet can also be declared to increase readability:

var myFontSize = 10 {
    willSet(newFontSize) {
        print("Will set font to \\(newFontSize), it was \\(myFontSize)")
    }
    didSet(oldFontSize) {
        print("Did set font to \\(myFontSize), it was \\(oldFontSize)")
    }
}

Caution: While it is supported to declare setter parameter names, one should be cautious not to mix names up:

• willSet(oldValue) and didSet(newValue) are entirely legal, but will considerably confuse readers of your code.