import Foundation

/// A protocol used for adding and removing storage observations
public protocol StorageObservationRegistry {
  associatedtype S: StorageAware

  /**
   Registers observation closure which will be removed automatically
   when the weakly captured observer has been deallocated.
   - Parameter observer: Any object that helps determine if the observation is still valid
   - Parameter closure: Observation closure
   - Returns: Token used to cancel the observation and remove the observation closure
   */
  @discardableResult
  func addStorageObserver<O: AnyObject>(
    _ observer: O,
    closure: @escaping (O, S, StorageChange) -> Void
  ) -> ObservationToken

  /// Removes all registered key observers
  func removeAllStorageObservers()
}

// MARK: - StorageChange

public enum StorageChange: Equatable {
  case add(key: String)
  case remove(key: String)
  case removeAll
  case removeExpired
}

public func == (lhs: StorageChange, rhs: StorageChange) -> Bool {
  switch (lhs, rhs) {
  case (.add(let key1), .add(let key2)), (.remove(let key1), .remove(let key2)):
    return key1 == key2
  case (.removeAll, .removeAll), (.removeExpired, .removeExpired):
    return true
  default:
    return false
  }
}