Storing items of generic type - swift

Question

So I'm writing an app that uses the Argo library to parse our JSON. Works pretty well. However, I'm trying to find a way to cache the parsed types we get back. They're all implemented as structs. I tried something like this:
struct CachedItem <T: Decodable where T == T.DecodedType> {
let value: T
let cachedTime: NSTimeInterval
init(value: T) {
self.value = value
cachedTime = NSDate().timeIntervalSince1970
}
func isExpired() -> Bool {
let currentTime = NSDate().timeIntervalSince1970
return ((currentTime - cachedTime) > 20.minutes)
}
}
However, attempting to create a cache like this:
var cache: [NSURL : CachedItem]
fails with the error: "reference to generic type 'CachedItem' requires arguments in <...>", which I understand to mean I need to do something like this:
var cache: [NSURL : CachedItem<Item>]
Is there any way I can get what I want here? Or any other suggestions for caching generic types that aren't Objective-C based classes.
Edit:
For posterity, here's the Cache and CacheItem types I came up with after Rob's answer.
struct Cache {
private var cache: [String : CachedValue] = [:]
private let queue = dispatch_queue_create("Cache Queue", DISPATCH_QUEUE_SERIAL)
mutating func setValue(value: Any?, forType type: String) {
dispatch_sync(queue) {
guard let value = value else {
return
}
self.cache[type] = CachedValue(value: value)
}
}
func valueForType<T>(type: String) -> T? {
var result: T?
dispatch_sync(queue) {
guard let cachedValue = self.cache[type] where !cachedValue.isExpired() else {
result = .None
return
}
result = cachedValue.value as? T
}
return result
}
}
struct CachedValue {
let value: Any
private let cachedTime: NSTimeInterval
init(value: Any) {
self.value = value
cachedTime = NSDate().timeIntervalSince1970
}
func isExpired() -> Bool {
let currentTime = NSDate().timeIntervalSince1970
return ((currentTime - cachedTime) > 1.minutes)
}
}

Answer 1

Basically, all you can say about the types in this cache are that they're Decodable, and that really isn't telling you anything useful since they're already decoded. That leaves AnyObject. If this really is a "cache of anything", then [NSURL: AnyObject] or even [NSURL: Any] is appropriate. This puts a lot of ugliness on the consuming side to figure out its type, but that's basically the identical ugliness that you had to have used to parse the JSON (which fundamentally works with AnyObject).
Seldom do I suggest AnyObject, but in this case, it's probably appropriate if you really want one big cache (rather than a cache for each type, which would be preferable if possible).