/// This class is completely thread-safe (yay!).
final class MyCache<Value> {
    private let entries: Atomic<[String: Value]> = Atomic([:])

    func valueForKey(key: String) -> Value? {
        return entries.withValue { $0[key] }
    }

    func setValue(value: Value, forKey: Key) {
        entries.modify { (var dict) in
            dict[key] = value
            return dict
        }
    }

    func clear() {
        entries.value = [:]
    }

    func copy() -> [String: Value] {
        return entries.value
    }
}

/// Thread-safe manager for the `networkActivityIndicator` on iOS.
final class NetworkActivityIndicatorManager {
    static let shared = NetworkActivityIndicatorManager()

    private let count = Atomic(0)

    func incrementActivityCount() {
        let oldValue = count.modify { $0 + 1 }
        if oldValue == 0 {
            updateUI(true)
        }
    }

    func decrementActivityCount() {
        let oldValue = count.modify { $0 - 1 }
        if oldValue == 1 {
            updateUI(false)
        }
    }

    private func updateUI(on: Bool) {
        dispatch_async(dispatch_get_main_queue()) {
            UIApplication.sharedApplication().networkActivityIndicatorVisible = true
        }
    }
}