ViewGroup的android:descendantFocusability
<!-- Defines the relationship between the ViewGroup and its descendants
when looking for a View to take focus. -->
<attr name="descendantFocusability">
<!-- The ViewGroup will get focus before any of its descendants. -->
<enum name="beforeDescendants" value="0" />
<!-- The ViewGroup will get focus only if none of its descendants want it. -->
<enum name="afterDescendants" value="1" />
<!-- The ViewGroup will block its descendants from receiving focus. -->
<enum name="blocksDescendants" value="2" />
</attr>