Override property observer

Question

When I override the function noise, the function gets replaced by the new one. But when I override a property with an observer, the old and new value gets both executed.

In playground:

class Vehicle {     func noise(sound: String) {         println("Vehicle sound sounds like \(sound)")     } }  class Train: Vehicle {     override func noise(sound: String) {         println("A train does: \(sound)")     } } 

Output:

var oldTrain = Train() bulletTrain.noise("tjoek tjoek") // Prints: "A train does: tjoek tjoek" 

But when I do the same with an property with an observer:

In playground:

class Foo {     var something: Int! {         didSet {             println("vroom")         }     } }  class Bar: Foo {     override var something: Int! {         didSet {             println("toot toot")         }     } } 

Output:

var foobar = Bar() foobar.something = 3 // Prints: "vroom" & "toot toot" 

So how am I supposed to override a property with an observer and how to prevent the old values to be executed as well?

Answer 1

You can override the set and get part of the property and move your println there. This way Swift won't call the original code -- unless you call super.

class Foo {     private var _something: Int!      var something: Int! {         get {             return _something         }         set {             _something = newValue             println("vroom")         }     } }  class Bar: Foo {     override var something: Int! {         get {             return _something         }         set {             _something = newValue             println("toot toot")         }     } } 

That's not pretty, though.

Here's a better -- and simpler -- solution:

class Foo {     var something: Int! {         didSet {             somethingWasSet()         }     }      func somethingWasSet() {         println("vroom")     } }  class Bar: Foo {     override func somethingWasSet() {         println("toot toot")     } } 

Since there is no way to "override" the didSet, what remains is overriding a secondary function especially created for that purpose.