In order to use a FocusTraversalPolicy, you give one to a
FocusTraversalGroup, which determines the widget subtree in which the policy
will be effective. The member functions of the class are rarely called directly:
they are meant to be used by the focus system.

The focus manager

The FocusManager maintains the current primary focus for the system. It
only has a few pieces of API that are useful to users of the focus system. One
is the FocusManager.instance.primaryFocus property, which contains the
currently focused focus node and is also accessible from the global
primaryFocus field.