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我正在使用属性包装器来保存我的用户默认值。在 iOS 13 设备上，此解决方案效果很好。但是在 iOS 11 和 iOS 12 上，这些值不会保存到用户默认值中。我读到属性包装器是向后兼容的，所以我不知道为什么这在旧 iOS 版本上不起作用。

这是属性包装器：

@propertyWrapper
struct UserDefaultWrapper<T: Codable> {
    private let key: String
    private let defaultValue: T

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

    var wrappedValue: T {
        get {
            guard let data = UserDefaults.standard.object(forKey: key) as? Data else {
                // Return defaultValue when no data in UserDefaults
                return defaultValue
            }

            // Convert data to the desire data type
            let value = try? JSONDecoder().decode(T.self, from: data)
            return value ?? defaultValue
        }
        set {
            // Convert newValue to data
            let data = try? JSONEncoder().encode(newValue)

            UserDefaults.standard.set(data, forKey: key)
            UserDefaults.standard.synchronize()
        }
    }
}

struct UserDefault {
    @UserDefaultWrapper(key: "userIsSignedIn", defaultValue: false)
    static var isSignedIn: Bool
}

然后我可以像这样设置值：

UserDefault.isSignedIn = true

我是否使用了错误的属性包装器？是否还有其他人在旧 iOS 版本上遇到属性包装器问题？

4

1 回答 1

13

Nothing to do with property wrappers! The problem is that in iOS 12 and before, a simple value like a Bool (or String, etc.), though Codable as a property of a Codable struct (for example), cannot itself be JSON encoded. The error (which you are throwing away) is quite clear about this:

Top-level Bool encoded as number JSON fragment.

To see this, just run this code:

    do {
        _ = try JSONEncoder().encode(false)
        print("succeeded")
    } catch {
        print(error)
    }

On iOS 12, we get the error. On iOS 13, we get "succeeded".

But if we wrap our Bool (or String, etc.) in a Codable struct, all is well:

    struct S : Codable { let prop : Bool }
    do {
        _ = try JSONEncoder().encode(S(prop:false))
        print("succeeded")
    } catch {
        print(error)
    }

That works fine on both iOS 12 and iOS 13.

And that fact suggests a solution! Redefine your property wrapper so that it wraps its value in a generic Wrapper struct:

struct UserDefaultWrapper<T: Codable> {

    struct Wrapper<T> : Codable where T : Codable {
        let wrapped : T
    }

    private let key: String
    private let defaultValue: T

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

    var wrappedValue: T {
        get {
            guard let data = UserDefaults.standard.object(forKey: key) as? Data 
                else { return defaultValue }
            let value = try? JSONDecoder().decode(Wrapper<T>.self, from: data)
            return value?.wrapped ?? defaultValue
        }
        set {
            do {
                let data = try JSONEncoder().encode(Wrapper(wrapped:newValue))
                UserDefaults.standard.set(data, forKey: key)
            } catch {
                print(error)
            }
        }
    }
}

Now it works on iOS 12 and iOS 13.

By the way, I actually think you would do better to save as a property list rather than JSON. But that makes no difference to the question generally. You can’t encode a bare Bool as a property list either. You’d still need the Wrapper approach.

于 2019-12-25T03:52:43.087 回答