foo.xml

<object class="EggStateMachine">
 <state id="default">
  <object id="my_revealer">
    <property name="reveal-child">false</property>
  </object>
 </state>
 <state id="search">
  <object id="my_revealer">
   <property name="reveal-child">true</property>
  </object>
  <object id="my_header_bar">
   <style>
    <class name="selection-mode"/>
   </style>
  </object>
 </state>
</object>