swift - 是否可以在 Swift 中将变量或常量的类型设置为 Protocol Equatable

Question

我是 Swift 的新手。我正在尝试做类似的事情:

let e:Equatable? where Equatable.Item == Int = nil

这可能吗？

问候，

谢兹

Answer 1

简短的回答是您不能基于协议(protocol)创建变量。

正如官方文档所述:

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

因此，开发人员所需的最少工作量是创建一个符合特定协议(protocol)的结构或类，比方说 Equatable。

我们再从官方文档中举个例子。 Equatable 协议(protocol)允许比较两个相同类型的元素，因此要符合它 ==(lhs: Type, rhs: Type,)方法需要实现。

class StreetAddress {
    let number: String
    let street: String
    let unit: String?

    init(_ number: String, _ street: String, unit: String? = nil) {
        self.number = number
        self.street = street
        self.unit = unit
    }
}

extension StreetAddress: Equatable {
    static func == (lhs: StreetAddress, rhs: StreetAddress) -> Bool {
        return
            lhs.number == rhs.number &&
            lhs.street == rhs.street &&
            lhs.unit == rhs.unit
    }
}

只有现在才能根据您的谓词创建和比较 StreetAddress 的两个实例。