Cannot Center Column Widget

Question

I am new to flutter and i am trying to centre all the widgets inside column widget but its not working. I tried encapsulating column into centre widget but its still the same and aligned with the top of screen. Below is my code, please have a look and let me know what i am doing wrong. Thanks :)

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

class SignIn extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Material(
      child: Padding(
        padding: EdgeInsets.all(15.0),
        child: Column(
          children: <Widget>[
            Text(
              'SignIn Screen',
              style: TextStyle(fontSize: 26.0),
            ),
            Padding(
              padding: EdgeInsets.only(top: 30.0),
            ),
            TextField(
              maxLength: 25,
              decoration: InputDecoration(
                hintText: 'Enter username',
              ),
            ),
            TextField(
              maxLength: 25,
              decoration: InputDecoration(
                hintText: 'Enter password',
              ),
            ),
            Expanded(
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: <Widget>[
                  RaisedButton(
                    child: Text('SignIn'),
                    onPressed: _signIn,
                  ),
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }

  Future _signIn() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    await prefs.setBool('isLoggedIn', true);
  }
}

Answer 1

Add MainAxisAlignment.center and remove Expanded

class SignIn extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Material(
      child: Padding(
        padding: EdgeInsets.all(15.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          crossAxisAlignment: CrossAxisAlignment.stretch,
          children: <Widget>[
            Text(
              'SignIn Screen',
              style: TextStyle(fontSize: 26.0),
            ),
            Padding(
              padding: EdgeInsets.only(top: 30.0),
            ),
            TextField(
              maxLength: 25,
              decoration: InputDecoration(
                hintText: 'Enter username',
              ),
            ),
            TextField(
              maxLength: 25,
              decoration: InputDecoration(
                hintText: 'Enter password',
              ),
            ),
            RaisedButton(
                    child: Text('SignIn'),
                    onPressed: _signIn,
                  ),
          ],
        ),
      ),
    );
  }

Answer 2

Center vertically

Column(
  mainAxisAlignment: MainAxisAlignment.center,
  children: <Widget>[ ... ],
)

Center horizontally

Column(
  crossAxisAlignment: CrossAxisAlignment.center,
  children: <Widget>[ ... ],
)

But if the column is hugging its content then just wrap it in a Center widget

Center(
  child: Column(
    children: <Widget>[ ... ],
  ),
)

Answer 3

For centering vertically,

Center(
   child:Column(
      mainAxisAlignment: MainAxisAlignment.start,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[ ... ],
   )
)