title: Cannot refer to other View ID in Android data binding
body: <p>I just finished watching <a href="https://youtu.be/DAmMN7m3wLU?t=13m59s" rel="noreferrer">Advanced Data Binding - Google I/O 2016</a> and would like to apply the following to reduce repetition of my expression used in different views.</p>
  
  <p><a href="https://i.stack.imgur.com/ZbuFW.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ZbuFW.png" alt="enter image description here"></a></p>
  
  <p>But I cannot make it work in my case:</p>
  
  <pre><code>&lt;ImageButton
                  android:id="@+id/btn_list"
                  android:layout_width="48dp"
                  android:layout_height="48dp"
                  android:layout_gravity="start"
                  android:background="@drawable/btn_s01_list"
                  android:visibility="@{bean.shouldHideControls? View.GONE: View.VISIBLE}"/&gt;
  
              &lt;ToggleButton
                  android:id="@+id/btn_radar"
                  android:layout_width="48dp"
                  android:layout_height="48dp"
                  android:background="@drawable/btn_radar_selector"
                  android:checked="false"
                  android:gravity="end"
                  android:text=""
                  android:textOff=""
                  android:textOn=""
                  android:visibility="@{btn_list.visibility}"/&gt;
  </code></pre>
  
  <p>and I got </p>
  
  <blockquote>
    <p>Error:(426, 39) Identifiers must have user defined types from the XML
    file. btn_list is missing it</p>
  </blockquote>
  
  <p><strong>Edit:</strong></p>
  
  <p>I missed an important point in the same talk... The View IDs are camel-casified.</p>
  
  <p><a href="https://i.stack.imgur.com/qYMCd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/qYMCd.png" alt="enter image description here"></a></p>