[swift-users] UserDefaults with generic keys

[Kim Burgestrand]

Here's yet another alternative. I read an article doing this very thing a
while back, it might be interesting to you:
http://radex.io/swift/nsuserdefaults/static/. It makes the key type a class
instead, and inherits from a non-generic parent class to which it adds the
static properties.

The gist of it is roughly like this (although the article uses subscript,
which does not allow for a generic implementation so I use a get/set
approach here for brevity):

```
class DefaultsKeys {}
final class DefaultsKey<T>: DefaultsKeys {
    let value: String

    init(_ value: String) {
        self.value = value
    }
}

extension UserDefaults {
    func get<T>(_ key: DefaultsKey<T>) -> T? {
        return object(forKey: key.value) as? T
    }

    func set<T>(_ key: DefaultsKey<T>, to value: T) {
        set(value, forKey: key.value)
    }
}

extension DefaultsKeys {
    static let version = DefaultsKey<String>("version")
}

let defaults = UserDefaults.standard
defaults.set(.version, to: "1.0")
let version = defaults.get(.version)
print(version ?? "N/A")
```

On Fri, 7 Jul 2017 at 14:00 Vladimir.S via swift-users <
swift-users at swift.org> wrote:

> On 07.07.2017 14:02, Thierry Passeron via swift-users wrote:
> > Hi Everyone,
> >
> > Using Swift 3.1, I was wondering if I could come up with something
> largely inspired by Notification.Name to help me deal with UserDefaults so
> I started by doing something like:
>
> The only kind of solution I was able to implement, is with calculated
> properties in
> extension. Hope this have any sense and most likely can be improved(code
> from swift
> sandbox):
>
> struct DefaultsKey<T> {
>         var rawValue : String
>
>         init(_ name: String) {
>                 rawValue = name
>         }
> }
>
> extension DefaultsKey {
>         static var version : DefaultsKey<String> { return
> DefaultsKey<String>("version") }
>         static var code : DefaultsKey<Int> { return
> DefaultsKey<Int>("code") }
> }
>
> func UserDefaults_standard_object(forKey: String) -> Any? {
>         switch forKey {
>                 case "version" : return "1.0.0"
>                 case "code" : return 12345
>                 default : return nil
>         }
> }
>
> func Defaults<T>(_ key: DefaultsKey<T>) -> T? {
>    return UserDefaults_standard_object(forKey: key.rawValue) as? T
> }
>
> let version = Defaults(.version)
> let code = Defaults(.code)
>
> print(version ?? "-no value-", type(of: version)) // 1.0.0 Optional<String>
> print(code ?? "-no value-", type(of: code))       // 12345 Optional<Int>
>
>
>
> >
> > public struct DefaultsKey: RawRepresentable, Equatable, Hashable,
> Comparable {
> >
> >    public var rawValue: String
> >    public var hashValue: Int { return rawValue.hash }
> >
> >    public init(_ rawValue: String) { self.rawValue = rawValue }
> >    public init(rawValue: String) { self.rawValue = rawValue }
> >
> >    /* Protocols implementation .. */
> > }
> >
> > Now I can make extensions like:
> >
> > extension DefaultsKey {
> >    static let version = DefaultsKey("version »)
> > }
> >
> > And use it to query the UserDefaults.
> >
> > public func Defaults<T>(_ key: DefaultsKey) -> T? {
> >    return UserDefaults.standard.object(forKey: key.rawValue) as? T
> > }
> >
> > let version: String? = Defaults(.version)
> >
> > Nice, concise, I love it…
> >
> > But It could be even better to let the compiler check the return type of
> the UserDefault for the DefaultKey that I ask if only I could create the
> key and bind it to a type. So I tried this:
> >
> > public struct DefaultsKey<T>: RawRepresentable, Equatable, Hashable,
> Comparable {
> > …
> > }
> >
> > extension DefaultsKey {
> >    static let version = DefaultsKey<String>("version »)
> > }
> >
> > But this doesn’t compile:
> >     error: static stored properties not supported in generic types
> >
> > I guess I could keep all the keys outside an extension scope but then it
> would not be as concise as with Notification.Name
> >
> > Please let me know if there is indeed a generic way to solve this. Any
> help would be greatly appreciated.
> > Thanks in advance.
> >
> > Thierry.
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
> >
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
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