y to onListen when
the returned stream stops having listener. If it later gets a new
listener, the onListen function is called again.</p>
<p>Use the callbacks, for example, for pausing the underlying subscription
while having no subscribers to prevent losing events, or canceling the
subscription when there are no listeners.</p>
    </section>
    
    <section class="summary source-code" id="source">
      <h2><span>Implementation</span></h2>
      <pre class="language-dart"><code class="language-dart">@ove