Custom Pull-to-refresh

With your current implementation, it seems that if I have a ListView inside a Column for instance (with a footer), the pull-to-refresh doesn't work.

             Column(
                children: <Widget>[
                  Expanded(
                    child: ListView(
                      children: List.generate(20, (index) => Text('$index')),
                      itemExtent: 100,
                    ),
                  ),
                  Text('Footer'),
                ],
              );

And if I set hasScrollBody: false, it throws.

I guess I would have to use the PullToRefersh directly above the ListView, inside the Expanded.
Maybe the pull-to-refresh should be optionnal or external, so we can use our custom behaviour ?
For instance I could want to implement a custom PullToRefresh component like this.

Thanks.

Start by reproducing the provided Column, Expanded, ListView, and footer example, then inspect the current pull-to-refresh integration and the hasScrollBody false path. Done should include a non-throwing layout with pull-to-refresh and a supported way to supply custom pull-to-refresh behavior, including the nested-scroll use case described in the issue.