How to make a full screen dialog in flutter?

Well here is my implementation which is quite straightforward.

from first screen

Navigator.of(context).push(PageRouteBuilder(
    opaque: false,
    pageBuilder: (BuildContext context, _, __) =>
        RedeemConfirmationScreen()));

at 2nd screen

class RedeemConfirmationScreen extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
  backgroundColor: Colors.white.withOpacity(0.85), // this is the main reason of transparency at next screen. I am ignoring rest implementation but what i have achieved is you can see.
.....
  );
 }
}

and here are the results.

You can use the Navigator to push a semi-transparent ModalRoute:

import 'package:flutter/material.dart';

class TutorialOverlay extends ModalRoute<void> {
  @override
  Duration get transitionDuration => Duration(milliseconds: 500);

  @override
  bool get opaque => false;

  @override
  bool get barrierDismissible => false;

  @override
  Color get barrierColor => Colors.black.withOpacity(0.5);

  @override
  String get barrierLabel => null;

  @override
  bool get maintainState => true;

  @override
  Widget buildPage(
      BuildContext context,
      Animation<double> animation,
      Animation<double> secondaryAnimation,
      ) {
    // This makes sure that text and other content follows the material style
    return Material(
      type: MaterialType.transparency,
      // make sure that the overlay content is not cut off
      child: SafeArea(
        child: _buildOverlayContent(context),
      ),
    );
  }

  Widget _buildOverlayContent(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          Text(
            'This is a nice overlay',
            style: TextStyle(color: Colors.white, fontSize: 30.0),
          ),
          RaisedButton(
            onPressed: () => Navigator.pop(context),
            child: Text('Dismiss'),
          )
        ],
      ),
    );
  }

  @override
  Widget buildTransitions(
      BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation, Widget child) {
    // You can add your own animations for the overlay content
    return FadeTransition(
      opacity: animation,
      child: ScaleTransition(
        scale: animation,
        child: child,
      ),
    );
  }
}


// Example application:
void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Playground',
      home: TestPage(),
    );
  }
}

class TestPage extends StatelessWidget {
  void _showOverlay(BuildContext context) {
    Navigator.of(context).push(TutorialOverlay());
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Test')),
      body: Padding(
        padding: EdgeInsets.all(16.0),
        child: Center(
          child: RaisedButton(
            onPressed: () => _showOverlay(context),
            child: Text('Show Overlay'),
          ),
        ),
      ),
    );
  }
}

Screenshot (Flutter's native dialog)

Call this method to show the dialog in fullscreen.

showGeneralDialog(
  context: context,
  barrierColor: Colors.black12.withOpacity(0.6), // Background color
  barrierDismissible: false,
  barrierLabel: 'Dialog',
  transitionDuration: Duration(milliseconds: 400),
  pageBuilder: (_, __, ___) {
    return Column(
      children: <Widget>[
        Expanded(
          flex: 5,
          child: SizedBox.expand(child: FlutterLogo()),
        ),
        Expanded(
          flex: 1,
          child: SizedBox.expand(
            child: ElevatedButton(
              onPressed: () => Navigator.pop(context),
              child: Text('Dismiss'),
            ),
          ),
        ),
      ],
    );
  },
);