Swift属性覆盖不起作用 [英] Swift property override not working

问题描述

当我尝试覆盖属性时，我收到错误无法覆盖具有只读属性的可变属性

When I try to override a property I get an error "can not override mutable property with read-only property"

我提供了get和set in super class。

I have provided get and set in the super class.

class Card {
    var contents:String {
        get {
            return self.contents
        }
        set {
            self.contents = newValue
        }
    }
    init() {
        self.contents = ""
    }
}

这是我的子类，我试图覆盖content属性。

Here is my Subclass where I am trying to override the "contents" property.

class PlayingCard: Card {
    override var contents:String { //<-- this is where I get the build error
        get {
            var rankStrings:Array<String> = PlayingCard.rankStrings()
            return rankStrings[Int(self.rank)] + self.suit
        }
    }
}

我究竟做错了什么？

推荐答案

如果您要覆盖的属性同时具有getter和setter，则还需要在子类中同时提供这两个属性。这是相关部分来自Swift语言指南（强调我的）：

If the property you're overriding has both a getter and a setter, you need to provide both in your subclass as well. Here's the relevant part from the Swift language guide (emphasis mine):

您可以将继承的只读属性呈现为读写通过在子类
属性覆盖中提供getter和setter来
属性。 但是，您不能将继承的
读写属性作为只读属性。

如果你没有对该值做任何特殊处理，那么你通常希望将设置的值传递给基类：

If you're not doing anything special with the value, then you'll typically want to pass the value being set on to the base class:

set {
    super.contents = newValue
}

你也可以用一个空的setter来丢弃这个值（尽管我不能想到一个很好的理由去做这个）：

You could also just discard the value with an empty setter (although I can't think of a good reason to do this offhand):

set { }

---

我也想要指出您的卡类中的内容属性中存在无限循环。当你这样做时：

---

I also wanted to point out that you have an infinite loop in the contents property in your Card class. When you you do this:

get {
    return self.contents
}

你实际上只是再次调用相同的getter，创建一个无限循环;你对二传手做同样的事情。 Swift不像Objective-C那样自动为您的属性创建ivars，因此您需要自己创建它们。创建该属性的更合适的方法是执行以下操作：

You're actually just calling that same getter again, creating an infinite loop; you're doing the same with the setter. Swift doesn't create ivars for your properties automatically like Objective-C did, so you need to create them yourself. A more appropriate way to create that property would be to do something like this:

class Card {
    private var _contents: String
    var contents: String {
        get {
            return _contents
        }
        set {
            _contents = newValue
        }
    }
    init() {
        _contents = ""
    }
}

但是，由于除了在setter和getter中设置并返回 _contents 之外没有做任何其他事情，你可以将其简化为：

However, since you're not doing anything other than setting and returning _contents in your setter and getter, you can simplify it down to this:

class Card {
    var contents: String = ""
    init() {

    }
}

注意： contents 也可能是使用可选（ String？）并将其设置为的理想选择nil 而不是将其初始化为空字符串。

Note: contents might also be a good candidate for using an optional (String?) and setting it to nil rather than initializing it to an empty string.

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