example flutter

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  Page progressDialog = Page.getProgressDialog();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Stack(
        children: <Widget>[
          Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                RaisedButton(
                  onPressed: (){
                    progressDialog.showProgress();
                  },
                  child: Text('Show dialog'),
                )
              ],
            ),
          ),
          progressDialog,
        ],
      ),
    );
  }
}


//---------------------------------------
class Page extends StatefulWidget {

  PageState seguimientoDialogState;

  @override
  createState() => seguimientoDialogState = new PageState();

  void showProgress() {
    seguimientoDialogState.showProgress();
  }
  static Widget getProgressDialog() {
    return new Page();
  }
}

class PageState extends State <Page> {

  bool _opacity = false;

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return new Container(
      child: !_opacity
        ? null
        : new Opacity(
          opacity: _opacity ? 1.0 : 0.0,
          child: new Container(
            color: Colors.black54,
            child: new Stack(
              children: <Widget>[
                new Center(
                  child: Stack(
                    children: <Widget>[
                      Padding(
                        padding: EdgeInsets.all(4.0),
                        child: Card(
                          shape: RoundedRectangleBorder(
                            borderRadius: BorderRadius.circular(10.0),
                          ),
                          child: Container(
                            width: MediaQuery.of(context).size.width-50.0,
                            height: MediaQuery.of(context).size.height-100.0,
                            padding: EdgeInsets.all(10.0),
                            child: Column(
                              crossAxisAlignment: CrossAxisAlignment.center,
                              children: <Widget>[
                                Padding(
                                  padding: EdgeInsets.all(5.0),
                                  child: Container(
                                    child: Text('test'),
                                    color: Colors.green[100],
                                  )
                                ),
                              ],
                            ),
                          ),
                        ),
                      ),
                      Positioned(
                        top: .0,
                        right: .0,
                        child: InkWell(
                          //child: Image.asset('assets/close_alt.png',width: 25.0,),
                          child: Icon(Icons.close, color:Colors.black, size: 25.0),
                          onTap: () {
                            setState(() {
                              _opacity=false;
                            });
                          },
                        ),
                      )
                    ],
                  ),
                ),
              ],
            ),
          ),
        )
    );
  }
  void showProgress() {
    setState(() {
      _opacity = true;
    });
  }
}