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>