title: Mutating of RxSwift Driver value
body: <p>In my application I have an array of notifications. Notification can be read and unread.</p>
  
  <p>When user clicks on the unread notification I need to change the model and reload data in my table view.</p>
  
  <p>In my ViewModel I have output stream:</p>
  
  <pre><code>let notifications: Driver&lt;[Notification]&gt;
  </code></pre>
  
  <p>And aslo I have an input stream with notification click:</p>
  
  <pre><code>let touchSingleNotificationIntent = PublishSubject&lt;Notification&gt;()
  </code></pre>
  
  <p>When I do something like this I get the error that it's let constant and I cannot mutate it.</p>
  
  <pre><code>touchSingleNotificationIntent
          .filter { !$0.isRead }
          .do(onNext: { notification in
              notification.isRead = true // I need to change state of the model immediately after user iteration
          })
          .map { $0.notificationID }
          .flatMap(markNotificationAsRead) // http request which doesn't reply with current notification model status
          .subscribe()
          .disposed(by: bag)
  </code></pre>
  
  <p>Do you have any ideas how to make it mutable? Thanks.</p>