Overriding getter in Swift

Question

I have a situation where I need to override the getter of a property.

Let's say we have:

public class MyBaseClass {     private var _name: String     public internal(set) var name: String {         get {             return self._name         }         set {             self._name = newValue         }     } } 

Nothing really fancy, I guess.

Now, if I try to override the getter in a derived class:

public class MyDerivedClass: MyBaseClass {     public var name: String {         get {             return "Derived - \(super.name)"         }     } } 

I get the compile error: Cannot override mutable property with read-only property 'name'.

If I try to add the setter and overriding it:

public class MyDerivedClass: MyBaseClass {     public internal(set) var name: String {         get {             return "Derived - \(super.name)"         }         set {             super.name = newValue         }     } } 

I get the error: Setter of overriding var must be as accessible as the declaration it overrides.

And if I try the following:

public class MyDerivedClass: MyBaseClass {     public internal(set) var name: String {         get {             return "Derived - \(super.name)"         }     } } 

Then, the compiler crashes...

How can I achieve to override only the getter ?

Answer 1

This works for me:

public class MyBaseClass {     private var _name: String = "Hi"     public internal(set) var name: String {         get {             return self._name         }         set {             self._name = newValue         }     } }  public class MyDerivedClass:MyBaseClass {     override public var name: String {         get {             return "Derived - \(super.name)"         }         set {             super._name = newValue         }     } }  MyDerivedClass().name 

EDIT

This code works for me in a playground, placing it in the Sources -> SupportCode.swift file

public class MyBaseClass { private var _name: String = "Hi" public internal(set) var name: String {     get {         return self._name     }     set {         self._name = newValue     } } public init() {  }  }  public class MyDerivedClass:MyBaseClass {     override public var name: String {         get {             return "Derived - \(super.name)"         }         set {            // do nothing         }     }    public override init() {      } } 

It's a bit of a bodge because I get the same warning as you that internal(set) cannot be placed before the overridden subclass variable. It may well be a bug. And also I'm cheating to make sure the setter of the derived class does nothing.

A more common use of internal(set) or private(set) is to have code like this, which is similar to that in the documentation:

public class MyBaseClass {     public private(set) var _name: String = "Hi"     public var name: String {         get {             return self._name         }         set {             self._name = newValue         }     }     public init() {      }  }  public class MyDerivedClass:MyBaseClass {     override public var name: String {         get {             return "Derived - \(super.name)"         }         set {            super._name = newValue         }     }    public override init() {      } } 

Here the setter can be read directly with MyDerivedClass()._name but it cannot be altered, e.g. this MyDerivedClass()._name = "Fred" would raise an error but MyDerivedClass().name = "Fred" would be OK.

Answer 2

Problem 1

MyBaseClass does not compile because:

1. it has a stored property (_name)
2. this stored property is non optional so it cannot be nil
3. there is no initializer to populate it

So first of all we need to add a proper initializer to MyBaseClass

public class MyBaseClass {     private var _name: String     public internal(set) var name: String {         get { return self._name }         set { self._name = newValue }     }     init(name : String){         _name = name     } } 

Problem 2

Now we can declare MyDerivedClass that overrides the computed property:

1. we need to use the magic keyword override
2. we need to provide both the setter and the getter

Here's the code:

public class MyDerivedClass: MyBaseClass {     public override var name: String {         get { return "Derived - \(super.name)" }         set { super.name = newValue }     } } 

Test

From my playground:

let somethingWithAName = MyDerivedClass(name: "default name") println(somethingWithAName.name) // > "Derived - default name" somethingWithAName.name = "another name" println(somethingWithAName.name) // > "Derived - another name"