可以在 Swift 中创建泛型类型检查功能吗？

Question

我正在向我继承的 Web API 框架添加更好的错误处理。目前它做了一堆强制转换，当数据与预期不匹配时会导致崩溃。

我想将 as! 的所有用法替换为检查类型并在失败时抛出异常（带有详细说明）的函数。

这就是我到目前为止所做的：

func checkType<T>(type: AnyClass, value: T?, name: String) throws -> T {
    guard let value = value else {
        let message = "[\(name)] Expected \(type), but value was nil"
        throw PlaidsterError.InvalidType(message)
    }

    guard value.dynamicType.self == type else {
        let message = "[\(name)] Expected \(type), but it was an \(value.dynamicType.self) with value: \(value)"
        throw PlaidsterError.InvalidType(message)
    }

    return value
}

但这有多个问题。它只能接受对象类型，如果类不完全匹配则失败（例如 NSString 失败，因为它实际上是一个 __NSCFString），并且它不能用于任何类型，即 String、Int、Double...

我似乎无法为Any 值类型找到AnyClass 的同义词，这是第一个问题。此外，似乎不可能做类似value.dynamicType.self is type 的事情，因为它说type 不是一种类型。

---

有可能做我想做的事吗？有没有更好的方法来进行这种类型检查，而不需要大量的样板代码分散在解析代码中？

我的目标是得到这样甚至更简单的东西：

public struct PlaidCategory {

    // MARK: Properties
    public let id: String
    public let hierarchy: [String]
    public let type: String

    // MARK: Initialization
    public init(category: [String: Any]) throws {
        id = try checkType(String.self, value: category["id"], name: "id")
        hierarchy = try checkType([String].self, value: category["hierarchy"], name: "hierarchy")
        type = try checkType(String.self, value: category["type"], name: "type")
}

}

Answer 1

您可以使用另一个泛型类型参数编写类似的内容。

func checkType<T, U>(type: U.Type, value: T?, name: String) throws -> U {
    guard let value = value else {
        let message = "[\(name)] Expected \(type), but value was nil"
        throw PlaidsterError.InvalidType(message)
    }

    guard let result = value as? U else {
        let message = "[\(name)] Expected \(type), but it was an \(value.dynamicType) with value: \(value)"
        throw PlaidsterError.InvalidType(message)
    }

    return result
}

并将其用作：

do {
    let n: Any = 3
    let t = try checkType(Int.self, value: n, name: "n")
    print(t) //->3
} catch let error {
    print(error)
}
do {
    let n: Any = "x"
    let t = try checkType(Int.self, value: n, name: "n")
    print(t)
} catch let error {
    print(error) //->InvalidType("[n] Expected Int, but it was an _NSContiguousString with value: x")
}

我希望这适用于您的 PlaidCategory.init(category:)，但尚未经过测试。

---

还有一个。如果仅在返回类型可推断的情况下使用，则无需传递type 参数。

func checkType<T, U>(value: T?, name: String) throws -> U {
    guard let value = value else {
        let message = "[\(name)] Expected \(U.self), but value was nil"
        throw PlaidsterError.InvalidType(message)
    }

    guard let result = value as? U else {
        let message = "[\(name)] Expected \(U.self), but it was an \(value.dynamicType) with value: \(value)"
        throw PlaidsterError.InvalidType(message)
    }

    return result
}

并将其用作：

do {
    let n: Any = "x"
    let t: Int = try checkType(n, name: "n")
    print(t)
} catch let error {
    print(error) //->InvalidType("[n] Expected Int, but it was an String with value: x")
}

或：

public struct PlaidCategory {

    // MARK: Properties
    public let id: String
    public let hierarchy: [String]
    public let type: String

    // MARK: Initialization
    public init(category: [String: Any]) throws {
        id = try checkType(category["id"], name: "id")
        hierarchy = try checkType(category["hierarchy"], name: "hierarchy")
        type = try checkType(category["type"], name: "type")
    }
}