属性 具有通用（可选）用户默认值的包装器

Question

With due reference:

https://github.com/apple/swift-evolution/blob/master/proposals/0258-property-wrappers.md#user-defaults

我们已经开始为 UserDefaults 使用 属性 包装器，它可以与非可选属性无缝配合。

但是，设置可选 属性 的 nil 会崩溃：

[User Defaults] Attempt to set a non-property-list object as an NSUserDefaults/CFPreferences value for key "someKeyThatWeSet"

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to insert non-property list object null for key "someKeyThatWeSet"'

下面的代码可以直接在Playground上测试：

@propertyWrapper
struct C2AppProperty<T> {
    let key: String
    let defaultValue: T

    init(_ key: String, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }

    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key)
        }
    }
}

struct C2User {
    @C2AppProperty("userID", defaultValue: nil)
    public static var publicUserID: String?
}

print(C2User.publicUserID)
C2User.publicUserID = "edusta"
print(C2User.publicUserID)
C2User.publicUserID = nil
print(C2User.publicUserID)

Expected:

nil

Optional<"edusta">

nil

Found:

nil

Optional<"edusta">

libc++abi.dylib: terminating with uncaught exception of type NSException

到目前为止我尝试过的：

set {
    // Comparing non-optional value of type 'T' to nil always returns false.
    if newValue == nil {
        UserDefaults.standard.removeObject(forKey: combinedKey)
    } else {
        UserDefaults.standard.set(newValue, forKey: combinedKey)
    }
}

这里需要什么样的检查才能发现 newValue 是 nil？或者 Optional<nil>?

Answer 1

这段代码对我有用：

@propertyWrapper
struct UserDefault<T> {
    private let key: String
    private let defaultValue: T
    private let userDefaults: UserDefaults

    init(_ key: String, defaultValue: T, userDefaults: UserDefaults = .standard) {
        self.key = key
        self.defaultValue = defaultValue
        self.userDefaults = userDefaults
    }

    var wrappedValue: T {
        get {
            guard let value = userDefaults.object(forKey: key) else {
                return defaultValue
            }

            return value as? T ?? defaultValue
        }
        set {
            if let value = newValue as? OptionalProtocol, value.isNil() {
                userDefaults.removeObject(forKey: key)
            } else {
                userDefaults.set(newValue, forKey: key)
            }
        }
    }
}

fileprivate protocol OptionalProtocol {
    func isNil() -> Bool
}

extension Optional : OptionalProtocol {
    func isNil() -> Bool {
        return self == nil
    }
}