SafeDB.swift

/// Protocol that defines the entities in our database.
/// They must have an identifier that is used to retrieve them.
protocol Entity {
  /// The identifier of the resource
  var id: String { get }
}

extension Entity {
  /// Helper to retrieve the table name from the Type of the entity.
  static var tableName: String { return String(reflecting: Self.self) }
}

/// Our in memory DB. It can store any object that conforms to Entity.
class InMemoryDB {
  typealias Table = [String: Entity]

  /// The storage of our database. 
  /// The key is a String obtained by the type using the `String(reflecting:)` API.
  private var storage: [String: Table] = [:]

  private var queue: OperationQueue

  init() {
    self.queue = OperationQueue()
    self.queue.name = "com.concurrency.app.in-memory-db"
    self.queue.maxConcurrentOperationCount = 1
  }

  /// Retrieves the entity from the storage.
  func get<T: Entity>(for id: String, callback: @escaping (T?) -> Void) {
    // Adds an operation to the queue.
    self.queue.addOperation {
      let tableName = T.tableName

      let table = self.storage[tableName]
      let entity = table?[id]
      let typedEntity = entity as? T
      // Invokes the callback so the client can use the value.
      callback(typedEntity)
    }
  }

  /// Sets the entity into the storage, eventually creating a new table for it.
  func update<T: Entity>(for key: String, updateClosure: @escaping (T?) -> T?) {
    // Adds an operation to the queue.
    self.queue.addOperation {
      let tableName = T.tableName

      // Retrieves the entity from the table.
      var table: [String: Entity] = self.storage[tableName] ?? [:]
      let oldValue = self.storage[key] as? T

      // Runs the code that will update the value.
      let newValue = updateClosure(oldValue)

      // Sets the new value in the database.
      table[key] = newValue
      self.storage[tableName] = table
    }
  }
}