AtomicDictionary.swift

/** 
Gets and sets to this dictionary-like type are atomic.

Does not fully implement correct value semantics for copy 
operations, since the contained queue is a reference type
and will be shared among copies of this object

*/
struct AtomicDictionary<KeyType:Hashable,ValueType>
{
  private var internalDictionary:Dictionary<KeyType,ValueType> = [:]
  private let queue = dispatch_queue_create("access queue", DISPATCH_QUEUE_CONCURRENT)
  
  /* provide subscript accessors */
  subscript(key: KeyType) -> ValueType? {
    get {
      var value : ValueType?
      dispatch_sync(self.queue) { () -> Void in
        value = self.internalDictionary[key]
      }
      return value
    }
    
    set {
      setValue(newValue, forKey: key)
    }
  }
  
  mutating func setValue(value: ValueType?, forKey key: KeyType) {
    // use a dispatch barrier so writes are serialized
    dispatch_barrier_sync(self.queue) { () -> Void in
      self.internalDictionary[key] = value
    }
  }
}