title: Retry after delay if signal doesn't send next in ReactiveCocoa 4
body: <p>I am using ReactiveCocoa 4.0 with Swift. I have a signal called <code>startedSignal</code> of type <code>Signal&lt;String, NoError&gt;</code> that I subscribe to in a view controller.</p>
  
  <pre><code>startedSignal.observeNext { _ in
    // Do stuff
  }
  </code></pre>
  
  <p>I basically want to wait a number of seconds, then do something else if <code>startedSignal</code> doesn't send in any next values. I looked at the docs and saw things like <code>retry</code> on <code>SignalProducer</code> but I am not sure how that can be used to achieve this, given that <code>startedSignal</code> doesn't complete or send errors.</p>