ios - 仅获取协议(protocol)中定义的属性在修改对象的内部属性时导致编译错误

Question

考虑这样的代码:

protocol SomeProtocol {
    var something: Bool { get set }
}

class SomeProtocolImplementation: SomeProtocol {
    var something: Bool = false {
        didSet {
            print("something changed!")
        }
    }
}

protocol MyProtocol {
    var myProperty: SomeProtocol { get }
}

class MyClass: MyProtocol {
    var myProperty: SomeProtocol = SomeProtocolImplementation() {
        didSet {
            print("myProperty has changed")
        }
    }
}


var o: MyProtocol = MyClass()
o.myProperty.something = true

这段代码没有编译错误:

error: cannot assign to property: 'myProperty' is a get-only property
o.myProperty.something = true
~~~~~~~~~~~~           ^

为什么？我的属性是 SomeProtocolImplementation 类型，它是类类型，因此应该可以使用对 myProperty 的引用来修改它的内部属性。

更进一步，在修改 myProperty 定义之后，它看起来像这样:

var myProperty: SomeProtocol { get set }

奇怪的事情发生了。现在代码可以编译(不足为奇)，但输出是:

something changed!
myProperty has changed

所以此时 SomeProtocolImplementation 开始表现得像一个值类型 - 修改它的内部状态会导致触发 myProperty 的“didSet”回调。正如 SomeProtocolImplementation 将是结构化的...

我实际上找到了解决方案，但我也想了解发生了什么。解决方案是将 SomeProtocol 定义修改为:

protocol SomeProtocol: class {
    var something: Bool { get set }
}

它工作正常，但我试图理解为什么它会这样。谁能解释一下？

Answer 1

先读什么Class Only Protocol是。专注于说明部分:

Use a class-only protocol when the behavior defined by that protocol’s requirements assumes or requires that a conforming type has reference semantics rather than value semantics.

上面的引述应该让你明白了。

您正在尝试为您的 SomeProtocol 的符合类(即 SomeProtocolImplementation)获取引用类型的行为。您希望将来能够更改 something 的值。所以基本上你是在指向上面引用的句子。

如果您需要更多说明，请考虑以下更有意义的设计，为方便起见，我更改了命名:

protocol Base: class {
    var referenceTypeProperty: Bool { get set }
    // By now you are assuming: this property should be modifiable from any reference.
    // So, instantly make the protocol `Class-only`
}

class BaseImplementation: Base {
    var referenceTypeProperty: Bool = false {
        didSet {
            print("referenceTypeProperty did set")
        }
    }
}

protocol Child {
    var valueTypeProperty: Base { get }
    // This property shouldn't be modifiable from anywhere.
    // So, you don't need to declare the protocol as Class-only
}

class ChildImplementation: Child {
    var valueTypeProperty: Base = BaseImplementation() {
        didSet {
            print("valueTypeProperty did set")
        }
    }
}

let object: Child = ChildImplementation()
object.valueTypeProperty.referenceTypeProperty = true