ios - 为什么默认情况下符合 Swift 协议(protocol)的值被视为值类型？

Question

我有一个协议(protocol)，它只描述一个接口(interface)。

protocol SampleProtocol {
var message: String? { get set }}

为什么编译器总是将符合标准的值/对象视为值类型？

示例示例，

 // Protocol value usage
import Foundation


protocol SampleProtocol {
    var message: String? { get set }
}

final class SampleObject: SampleProtocol {
    var message: String?
}


final class Controller {

    var sampleValue: SampleProtocol! {
        didSet {
            print("New value has been set")
        }
    }
}


let controller = Controller()
controller.sampleValue = AlphaObject() // Correctly prints "New value has been set"
controller.sampleValue.message = "New message" // Prints again "New value has been set", like for value types

Answer 1

您的协议(protocol) SampleProtocol可以被 class 采用或 struct . Swift 使用的是限制性更强的值类型的行为，直到您告诉它该协议(protocol)将仅由 class 使用。引用类型。

添加符合 AnyObject到您的协议(protocol)以获取引用类型行为:

protocol SampleProtocol: AnyObject {
    var message: String? { get set }
}

见 The Swift 5.1 Programming Guide - Class-Only Protocols更多细节。

该指南指出:

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.

历史注释:使用 class关键字 :

在 Swift 4.0 之前，这是使用 class 编写的。关键词:

protocol SampleProtocol: class {
    var message: String? { get set }
}

这暂时仍然有效，但它目前只是 AnyObject 的类型别名。并且可能会在更高版本的 Swift 中被删除。