Swift 可选只读变量的协议默认实现

Question

我有以下代码，协议 MyDisplayable 有三个可选的 String，我有一个通过扩展协议的默认实现。我的问题是，因为我确定扩展名 returns 是三个字符串，有没有一种方法可以将它们用作非可选的，如果其他实现覆盖它会有任何风险吗？（见下面代码中的问题点1和2）

非常感谢！

protocol MyDisplayable {
    var displayName: String? { get }
    var shortDescription: String? { get }
    var longDescription: String? { get }
}

protocol MyObject : MyDisplayable, CustomStringConvertible {
}

extension MyObject {
    var displayName: String? {
        return "noname"
    }

    var shortDescription: String? {
        return "something can't be described"
    }

    var longDescription: String? {
        return "no way to describe it further"
    }

    var description: String {
        // **1. is there a way to use the strings as if they are non-optional?**
        // **2. is it a problem if another class implements the protocol and returns `nil` for any of the strings, but here they are force unwrapped?**
        return "\(displayName!): \(shortDescription!)\n\(longDescription!)"
    }
}

class Something : MyObject {
}

let something = Something()
print("Something: \(something)")

Answer 1

在您的默认实现中有条件展开怎么样？

return "\(displayName ?? "" ): \(shortDescription ?? "" )\n\(longDescription ?? "")"

Answer 2

遗憾的是，无法将已声明的可选项视为非可选项。您已在协议中将这些字符串声明为可选，因此当您实施该协议时，它们仍然是可选的。

但是，您可以使用 getter-setter 来确保您的变量始终存储一些值，即使它们被声明为可选。

我会用一些代码详细说明：

protocol MyDisplayable {
    var displayName: String? { get set }
    var shortDescription: String? { get set }
    var longDescription: String? { get set }
}

protocol MyObject : MyDisplayable, CustomStringConvertible {
}

extension MyObject {
    var displayName: String? {
        get {
            return "noname"
        }
        set {
            newValue ?? ""
        }
    }

    var shortDescription: String? {
        get {
            return "something can't be described"
        }
        set {
            newValue ?? ""
        }
    }

    var longDescription: String? {
        get {
            return "no way to describe it further"
        }
        set {
            newValue ?? ""
        }
    }

    var description: String {
        // **1. is there a way to use the strings as if they are non-optional?**
        // **2. is it a problem if another class implements the protocol and returns `nil` for any of the strings, but here they are force unwrapped?**
        return "\(displayName!): \(shortDescription!)\n\(longDescription!)"
    }
}

class Something : MyObject {
}

let something = Something()
print("Something: \(something)")

现在，即使其他 class 将 nil 值覆盖到这些字符串，它们也会 return 空字符串“”。它们仍然是可选的，因为它们被声明为可选的，但现在它们将始终具有非零值。

Swift 可选只读变量的协议默认实现

Swift protocol default implementation for optional readonly variables

optional

ios

swift

swift-extensions

swift-protocols