builder method
Implementation
List<Widget> builder(BuildContext context, BoxConstraints constraints) {
// left pane
view0 ??= view(0);
view1 ??= view(1);
// ratio box1:box2. if 1, box 1 is 100% size
var ratio = widget.model.ratio;
if (ratio < 0) ratio = 0;
if (ratio > 1) ratio = 1;
var flex = (ratio * 1000).ceil();
List<Widget> children = [];
// left/top view
view0 ??= view(0);
view0!.model.setFlex(flex);
view0!.model.needsLayout = true;
children.add(view0!);
// handle
Widget handle = _buildHandle(constraints);
children.add(handle);
// right/bottom view
view1 ??= view(1);
view1!.model.setFlex(1000 - flex);
view1!.model.needsLayout = true;
children.add(view1!);
return children;
}