swift - 为什么我不能在通知中传递带有枚举键的字典

Question

我尝试在 Notification ([TestEnum: String]) 中传递带有 enum 的字典作为键。不幸的是，在收到通知后将字典类型转换为 [TestEnum: String] 失败。

enum TestEnum {
    case test
}

class NotificationTest {

    var sender: String = "" {
        didSet {
            NotificationCenter.default.post(name: Notification.Name(rawValue: "Test"), object: nil, userInfo: [TestEnum.test: "test"])
        }
    }

    init() {
        NotificationCenter.default.addObserver(self, selector: #selector(notificationReceived(_:)), name: Notification.Name(rawValue: "Test"), object: nil)
    }

    @objc func notificationReceived(_ notification: Notification) {
        print("Notification Received")
        guard let userInfo = notification.userInfo as? [TestEnum: String] else { return } // Type casting fails here even though userInfo shows a TestEnum key in debugger
        print(userInfo[.test])
    }

}

let test = NotificationTest()
test.sender = "blah"

但是，如果我使用 TestEnum 的 rawValue 作为键，当 notification.userInfo 被转换为 [String : 字符串].

Answer 1

刚刚看过AnyHashable的源代码，当你将Hashable(你的枚举)转换为AnyHashable时，它被包裹了在 AnyHashable 内的属性 base 中。因此，它不能直接转换回您的枚举。这里使用reduce将[AnyHashable:Any]转换为[TestEnum:String]:

@objc func notificationReceived(_ notification: Notification) {
    print("Notification Received")
    guard let userInfo = notification.userInfo?.reduce(into: [TestEnum:String](), { (result, entry) in
        if let key = entry.key.base as? TestEnum
        {
            result[key] = entry.value as? String
        }
    }) else {
        print(notification.userInfo)
        return
    } // Type casting fails here even though userInfo shows a TestEnum key in debugger
    print(userInfo[.test])
}

而且因为AnyHashable符合CustomStringConvertible，所以可以直接转换为String。