Overriding a stored property in Swift

问题

I noticed that the compiler won't let me override a stored property with another stored value (which seems odd):

class Jedi {
    var lightSaberColor = "Blue"
}


class Sith: Jedi {
    override var lightSaberColor = "Red" // Cannot override with a stored property lightSaberColor
}

However, I'm allowed to do this with a computed property:

class Jedi {
    let lightSaberColor = "Blue"
}


class Sith: Jedi {
    override var lightSaberColor : String{return "Red"}

}

Why am I not allowed to give it another value?

Why is overriding with a stored property an abomination and doing it with a computed one kosher? What where they thinking?

回答1:

Why am I not allowed to just give it another value?

You are definitely allowed to give an inherited property a different value. You can do it if you initialize the property in a constructor that takes that initial value, and pass a different value from the derived class:

class Jedi {
    // I made lightSaberColor read-only; you can make it writable if you prefer.
    let lightSaberColor : String
    init(_ lsc : String = "Blue") {
        lightSaberColor = lsc;
    }
}

class Sith : Jedi {
    init() {
        super.init("Red")
    }
}

let j1 = Jedi()
let j2 = Sith()

println(j1.lightSaberColor)
println(j2.lightSaberColor)

Overriding a property is not the same as giving it a new value - it is more like giving a class a different property. In fact, that is what happens when you override a computed property: the code that computes the property in the base class is replaced by code that computes the override for that property in the derived class.

[Is it] possible to override the actual stored property, i.e. lightSaberColor that has some other behavior?

Apart from observers, stored properties do not have behavior, so there is really nothing there to override. Giving the property a different value is possible through the mechanism described above. This does exactly what the example in the question is trying to achieve, with a different syntax.

回答2:

For me, your example does not work in Swift 3.0.1.

I entered in the playground this code:

class Jedi {
    let lightsaberColor = "Blue"
}

class Sith: Jedi {
    override var lightsaberColor : String {
        return "Red"
    }
}

Throws error at compile time in Xcode:

cannot override immutable 'let' property 'lightsaberColor' with the getter of a 'var'

No, you can not change the type of stored property. The Liskov Substitution Principle forces you to allow that a subclass is used in a place where the superclass is wanted.

But if you change it to var and therefore add the set in the computed property, you can override the stored property with a computed property of the same type.

class Jedi {
    var lightsaberColor = "Blue"
}


class Sith: Jedi {
    override var lightsaberColor : String {
        get {
            return "Red"
        }
        set {
            // nothing, because only red is allowed
        }
    }
}

This is possible because it can make sense to switch from stored property to computed property.

But override a stored var property with a stored var property does not make sense, because you can only change the value of the property.

You can, however, not override a stored property with a stored property at all.

---

I would not say Sith are Jedi :-P. Therefore it is clear that this can not work.

回答3:

You probably want to assing another value to the property:

class Jedi {
    var lightSaberColor = "Blue"
}


class Sith: Jedi {
    override init() {
        super.init()
        self.lightSaberColor = "Red"
    }
}

回答4:

class SomeClass {
    var hello = "hello"
}
class ChildClass: SomeClass {
    override var hello: String {
        set {
            super.hello = newValue
        }
        get {
            return super.hello
        }    
    }
}

回答5:

For Swift 4, from Apple's documentation:

You can override an inherited instance or type property to provide your own custom getter and setter for that property, or to add property observers to enable the overriding property to observe when the underlying property value changes.

回答6:

In Swift, this is unfortunately not possible to do. The best alternative is the following:

class Jedi {
    private(set) var lightsaberColor = "Blue"
}


class Sith: Jedi {
    override var lightsaberColor : String {
        get {
            return "Red"
        }
    }
}

回答7:

I had the same problem to set a constant for a view controller.

As I'm using interface builder to manage the view, I cannot use init(), so my workaround was similar to other answers, except I used a read-only computed variable on both base and inherited classes.

class Jedi {
    var type: String {
        get { return "Blue" }
    }
}

class Sith: Jedi {
    override var type: String {
        get { return "Red" }
    }
}

回答8:

You also can use a function to override. It's not direct answer, but can enrich this topic)

Class A

override func viewDidLoad() {
    super.viewDidLoad()

    if shouldDoSmth() {
       // do
    }
}

public func shouldDoSmth() -> Bool {
    return true
}

Class B: A

public func shouldDoSmth() -> Bool {
    return false
}

来源：https://stackoverflow.com/questions/26691935/overriding-a-stored-property-in-swift