How To Create Loading Progress Indicator Using Flutter Android App

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      home: new MyHomePage(
          title: 'Loading Progress Indicator'
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool _loading = false;
  void _onLoading() {
    setState(() {
      _loading = true;
      new Future.delayed(new Duration(seconds: 3), _login);
    });
  }
  Future _login() async{
    setState((){
      _loading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    var body = new Column(
      children: <Widget>[
        new Container(
          height: 40.0,
          padding: const EdgeInsets.all(10.0),
          margin: const EdgeInsets.fromLTRB(15.0, 150.0, 15.0, 0.0),
          decoration: new BoxDecoration(
            color: Colors.white,
          ),
          child: new TextField(
            decoration: new InputDecoration.collapsed(hintText: "username"),
          ),
        ),
        new Container(
          height: 40.0,
          padding: const EdgeInsets.all(10.0),
          margin: const EdgeInsets.all(15.0),
          decoration: new BoxDecoration(
            color: Colors.white,
          ),
          child: new TextField(
            decoration: new InputDecoration.collapsed(hintText: "password"),
          ),
        ),
      ],
    );
    var bodyProgress = new Container(
      child: new Stack(
        children: <Widget>[
          body,
          new Container(
            alignment: AlignmentDirectional.center,
            decoration: new BoxDecoration(
              color: Colors.white70,
            ),
            child: new Container(
              decoration: new BoxDecoration(
                  color: Colors.blue[200],
                  borderRadius: new BorderRadius.circular(10.0)
              ),
              width: 300.0,
              height: 200.0,
              alignment: AlignmentDirectional.center,
              child: new Column(
                crossAxisAlignment: CrossAxisAlignment.center,
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  new Center(
                    child: new SizedBox(
                      height: 50.0,
                      width: 50.0,
                      child: new CircularProgressIndicator(
                        value: null,
                        strokeWidth: 7.0,
                      ),
                    ),
                  ),
                  new Container(
                    margin: const EdgeInsets.only(top: 25.0),
                    child: new Center(
                      child: new Text(
                        "loading.. wait...",
                        style: new TextStyle(
                            color: Colors.white
                        ),
                      ),
                    ),
                  ),
                ],
              ),
            ),
          ),
        ],
      ),
    );
    return new Scaffold(
      appBar: new AppBar(
        backgroundColor:  Color(0xFF4A148C),
        title: new Text(widget.title),
      ),
      body: new Container(
          decoration: new BoxDecoration(
              color: Colors.blue[200]
          ),
          child: _loading ? bodyProgress : body
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _onLoading,
        tooltip: 'Loading',
        backgroundColor: Color(0xFF4A148C),
        child: new Icon(Icons.check),
      ),
    );
  }
}