Signature for the buildWhen function which takes the previous state and
the current state and is responsible for returning a bool which
determines whether to rebuild StoreBuilder with the current state.
Signature for the listenWhen function which takes the previous state
and the current state and is responsible for returning a bool which
determines whether or not to call StoreWidgetListener of StoreListener
with the current state.
Signature for the builder function which takes the BuildContext and
state and is responsible for returning a widget which is to be rendered.
This is analogous to the builder function in StreamBuilder.