title: Android LiveData - switchMap is not triggered on second update
body: <p>I have a <code>LiveData</code> object that depends on another <code>LiveData</code>. As I understand, <code>Transformations.switchMap</code> should allow to chain them. But <code>switchMap</code> handler is triggered only once and it doesn't react on further updates. If instead I use <code>observe</code> on the first object and, when it's ready, retrieve the second, it works fine but in this case I have to do it in <code>Activity</code> rather than <code>ViewModel</code>. Is it possible to chain <code>LiveData</code> objects, like <code>Transformations.switchMap</code>, but receive all updates, not only the first one?</p>
  
  <p>Here is an attempt to use <code>switchMap</code>:</p>
  
  <pre><code>LiveData&lt;Resource&lt;User&gt;&gt; userLiveData = usersRepository.get();
  return Transformations.switchMap(userLiveData, resource -&gt; {
      if (resource.status == Status.SUCCESS &amp;&amp; resource.data != null) {
          return apiService.cartItems("Bearer " + resource.data.token);
      } else {
          return AbsentLiveData.create();
      }
  });
  </code></pre>
  
  <p>Here is an approach with <code>observe</code> in activity (works but requires to keep logic in activity):</p>
  
  <pre><code>viewModel.user().observe(this, x -&gt; {
      if (x != null &amp;&amp; x.data != null) {
          viewModel.items(x.data.token).observe(this, result -&gt; {
              // use result
          });
      }
  });
  </code></pre>