T extends State< class StatefulWidget>>
Provides non-leaking access to a BuildContext.
A BuildContext is only valid if it is pointing to an active Element.
Element.dispose method is called, the BuildContext should not
be accessed further. This class makes it possible for a StatefulWidget to
share its build context safely with other objects.
Creators of this object must guarantee the following:
- They create this object at or after State.initState but before State.dispose. In particular, do not attempt to create this from the constructor of a state.
- They call dispose from State.dispose.
This object will not hold on to the State after disposal.