How to position a Widget at the bottom of a SingleChildScrollView?

The chosen answer doesn't really work, as using a SizedBox restrict the max size of the Column to the height of the screen, so you can't put as many widgets as you want in it (or they will go off-screen without beeing scrollable).

This solution will work no matter of many widgets are in the column: https://github.com/flutter/flutter/issues/18711#issuecomment-505791677

Important note: It will only work if the widgets in the column have a fixed height (for example TextInputField does NOT have a fixed height). If they have variable a height, wrap them with a Container of fixed height.

I just copied and pasted here the best solution I found, quoted by @Quentin, elaborated by @NikitaZhelonkin in issue 18711 of Flutter on Github. Simply the perfect solution!

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('AppBar'),
        ),
        body: LayoutBuilder(builder: (context, constraints) {
          return SingleChildScrollView(
              child: ConstrainedBox(
                  constraints: BoxConstraints(minWidth: constraints.maxWidth, minHeight: constraints.maxHeight),
                  child: IntrinsicHeight(
                    child: Column(
                        mainAxisSize: MainAxisSize.max,
                        children: [
                          Text('header'),
                          Expanded(
                            child: Container(
                              color: Colors.green,
                              child: Text('body'),
                            ),
                          ),
                          Text('footer'),
                        ]
                    ),
                  )
              )
          );
        })
    );
  }
}

The issue with SingleChildScrollView is that it shrikwrap it's children. So to have auto size widget in between - we need to use MediaQuery to get the screen height & SizedBox to expand - SingleChildScrollView.

Here Button will be at bottom of screen.

working Code:

double height = MediaQuery.of(context).size.height;

    SingleChildScrollView(
          child: SizedBox(
              height: height,
              child: Column(
                  crossAxisAlignment: CrossAxisAlignment.stretch,
                  mainAxisSize: MainAxisSize.max,
                  children: <Widget>[
                    Column(
                      children: <Widget>[
                        Text('Dummy'),
                        Text('Dummy'),
                        Text('Dummy'),
                      ],
                    ),
                    Spacer(),
                    FlatButton(
                      onPressed: () {},
                      child: Text('Demo'),
                    )
                  ])),
        )