Generic in-memory cache implementation for Swift 3.0

Example.swift

// create a chache with 60 seconds expiration time
var cache: InMemoryCache = InMemoryCache<LocalUrl>(expirationInSeconds: 60)

// data will either be returned from cache or created via callback and then returned
let data = cache.cachedData {
  // do the heavy object creation here
  return MyHeavyObject()
}

// invalidate the cache, next call will have to re-create the heavy object
cache.expire()

InMemoryCache.swift

class InMemoryCache<T> {
    private var expirationInSeconds: TimeInterval!
    private var currentExpirationTimestamp: TimeInterval?
    private var data: T?

    init(expirationInSeconds: TimeInterval) {
        self.expirationInSeconds = expirationInSeconds
    }

    func expire() {
        currentExpirationTimestamp = nil
        data = nil
    }

    func cachedData(callback: () -> T?) -> T? {
        if let cacheExpirationTimestamp = currentExpirationTimestamp, cacheExpirationTimestamp > Date().timeIntervalSince1970 {
            return data
        } else {
            data = callback()
            currentExpirationTimestamp = Date().timeIntervalSince1970 + expirationInSeconds
            return data
        }
    }
}