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.
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