从 Swift 中的非泛型函数调用泛型函数

Question

MMCondition 是在 Swift 中定义的协议，但与 Objective-C 互操作（用 @objc 注释）。

@objc public protocol MMCondition {
    static var name: String { get }
    static var isMutuallyExclusive: Bool { get }
}

我有以下代码：

// addCondition cannot be generic as I want it to be accessible from Objective-C as well.
public func addCondition(condition: MMCondition) {
    // How do I initialize OperationConditionImplementer here?
   let operationCondition = OperationConditionImplementer(condition: condition) // doesn't compile
   // Error: Cannot invoke initializer for type 'OperationConditionImplementer<T>' with an argument list of type '(condition: MMCondition)'
   // Can I use condition.dynamicType to init OperationConditionImplementer somehow?
}

struct OperationConditionImplementer<T: MMCondition> {
    let condition: T

    static var name: String {
        return "Silent<\(T.name)>"
    }

    static var isMutuallyExclusive: Bool {
        return T.isMutuallyExclusive
    }

    init(condition: T) {
        self.condition = condition
    }
}

Answer 1

从 Objective-C 开始，您不能使用 documentation 中所述的泛型。

You’ll have access to anything within a class or protocol that’s marked with the @objc attribute as long as it’s compatible with Objective-C. This excludes Swift-only features such as those listed here:

Generics

...

因此您需要完全删除泛型代码。一种可能的解决方案可能是：

@objc protocol MMCondition {
    static var name: String { get }
    static var isMutuallyExclusive: Bool { get }
}

struct OperationConditionImplementer {
    let condition: MMCondition

    var name: String {
        return "Silent<\(condition.dynamicType.name)>"
    }

    var isMutuallyExclusive: Bool {
        return condition.dynamicType.isMutuallyExclusive
    }

    init(condition: MMCondition) {
        self.condition = condition

        // Here decide comparing types
        if condition.dynamicType === ExampleCondition.self {
            print(condition.dynamicType.name)
        }
    }
}

例如，如果您在操场上尝试一下：

class ExampleCondition: NSObject, MMCondition {
    static var name: String  = "ExampleCondition"
    static var isMutuallyExclusive: Bool = false
}

let example = OperationConditionImplementer(condition: ExampleCondition())

您会看到 "ExampleCondition" 打印。

如果你最终切换到纯Swift，你需要在初始化一个OperationConditionImplementer时指定T。

您可以将 addCondition 方法定义为：

func addCondition<T: MMCondition>(condition: T) {
   let a = OperationConditionImplementer<T>(condition: condition)
}

Answer 2

因为 Swift 通用类的 2.0 实例可以实现 Objective-C 协议。我认为不可能有结构实现协议。事实上，我希望您的协议可能需要从 NSObjectProtocol 继承才能在 Objective-C 中使用，这将阻止您使用结构或枚举实现协议。

您还正确地提到您无法从 Objective-C 访问泛型函数。

有关使用泛型实现 Objective-C 协议的具体示例，请查看 this blog post。

从 Swift 中的非泛型函数调用泛型函数

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