Swift: Cannot override mutable property with read-only property ‘xxx’

swift

I don't understand the meaning of this error. This is what I am trying to do:

protocol P {
    var testTitle: String? { get }
}

class MyViewController: UIViewController, P {
    var testTitle: String? {
        didSet {
            self.title = testTitle
        }
    }
}

final class TestViewController: MyViewController {
    var testTitle: String? { // Error occurs here
        return "Test" 
    }
}

And I'm getting:

Cannot override mutable property with read-only property 'testTitle'

Could you help me understanding what's going on? Maybe it's trivial for some people but not for me.

Thanks for your help.

Best Answer

You cannot implement a subclass to be more restrictive than the base class (it would only work the other way round). So if the base class implements a read/write property, you cannot make it read only in your derived class.

Therefore:

final class TestViewController: MyViewController {
    override var testTitle: String? { // No error occurs here 
        get {
            return "Test" 
        }
        set {
            super.testTitle = newValue
        }
    }
}
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