sample.dart

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> {
  bool showFirst = true;
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Stack(
        overflow: Overflow.visible,
        children: <Widget>[
          AnimatedPositioned(
            duration: Duration(seconds: 1),
            bottom: 10,
            height: 200,
            right: showFirst ? 0 : MediaQuery.of(context).size.width,
            left: showFirst ? 0 : -MediaQuery.of(context).size.width,
            child: Container(
              width: MediaQuery.of(context).size.width - 100,
              child: Padding(
                padding: const EdgeInsets.all(8.0),
                child: Container(
                  color: Colors.red,
                  child: Center(
                    child: FlatButton(
                      onPressed: () {
                        setState(() {
                          showFirst = !showFirst;
                        });
                      },
                      child: Text("Next"),
                    ),
                  ),
                ),
              ),
            ),
          ),
          AnimatedPositioned(
            duration: Duration(seconds: 1),
            bottom: 10,
            height: 200,
            right: showFirst ? -MediaQuery.of(context).size.width : 0,
            left: showFirst ? MediaQuery.of(context).size.width : 0,
            child: Container(
              width: MediaQuery.of(context).size.width - 100,
              child: Padding(
                padding: const EdgeInsets.all(8.0),
                child: Container(
                  color: Colors.blue,
                  child: Center(
                    child: FlatButton(
                      onPressed: () {
                        setState(() {
                          showFirst = !showFirst;
                        });
                      },
                      child: Text("Previous"),
                    ),
                  ),
                ),
              ),
            ),
          )
        ],
      ),
    );
  }
}