Swift 中协议的只读属性 [英] Read-only properties of protocols in Swift

问题描述

来自Learn the Essentials of Swift"游乐场，有一个示例协议:

From the "Learn the Essentials of Swift" playground, there's an example protocol:

protocol ExampleProtocol {
    var simpleDescription: String { get }
    func adjust()
}

在这个例子之后有一段短文，内容如下:

There's a short passage after this example that reads:

注意:simpleDescription 属性后面的 { get } 表示它是只读的，意味着该属性的值可以查看，但永远不会改变.

Note: The { get } following the simpleDescription property indicates that it is read-only, meaning that the value of the property can be viewed, but never changed.

另外给出了一个符合这个协议的类的例子:

Additionally an example is given of a class conforming to this protocol:

class SimpleClass: ExampleProtocol {
    var simpleDescription: String = "A very simple class."
    var anotherProperty: Int = 69105
    func adjust() {
        simpleDescription += "  Now 100% adjusted."
    }
}

var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription

但是这个类是如何符合协议的呢?是什么阻止我改变 simpleDescription?什么我不明白?

However how does this class conform to the protocol? What's stopping me from mutating simpleDescription? What don't I understand?

推荐答案

无法在协议中指定您必须具有只读只读属性.您的协议要求 simpleDescription 属性，并且允许但不要求一个 setter.

There's no way to specify in a protocol that you must have a read-only property. Your protocol asks for a simpleDescription property, and allows but does not require a setter.

另请注意，您可能改变 simpleDescription 的唯一原因是您知道您的 a 属于 SimpleClass 类型.如果我们有一个 ExampleProtocol 类型的变量......

Note also that the only reason you may mutate simpleDescription is because you know your a is of type SimpleClass. If we have a variable of type ExampleProtocol instead...

var a: ExampleProtocol = SimpleClass()
a.simpleDescription = "newValue" //Not allowed!

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