samples/animations/lib/src/basics/09_animated_positioned.dart

// Copyright 2020 The Flutter team. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:math';

import 'package:flutter/material.dart';

class AnimatedPositionedDemo extends StatefulWidget {
  static String routeName = '/basics/09_animated_positioned';

  _AnimatedPositionedDemoState createState() => _AnimatedPositionedDemoState();
}

class _AnimatedPositionedDemoState extends State<AnimatedPositionedDemo> {
  double topPosition;
  double leftPosition;

  double generateTopPosition(double top) => Random().nextDouble() * top;

  double generateLeftPosition(double left) => Random().nextDouble() * left;

  void initState() {
    super.initState();
    topPosition = generateTopPosition(30);
    leftPosition = generateLeftPosition(30);
  }

  void changePosition(double top, double left) {
    setState(() {
      topPosition = generateTopPosition(top);
      leftPosition = generateLeftPosition(left);
    });
  }

  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    final appBar = AppBar();
    final topPadding = MediaQuery.of(context).padding.top;
    // AnimatedPositioned animates changes to a widget's position within a Stack
    return SafeArea(
      child: Scaffold(
        appBar: appBar,
        body: Container(
          height: size.height,
          width: size.width,
          child: Stack(
            children: [
              AnimatedPositioned(
                top: topPosition,
                left: leftPosition,
                duration: Duration(seconds: 1),
                child: InkWell(
                  onTap: () => changePosition(
                      size.height -
                          (appBar.preferredSize.height + topPadding + 50),
                      size.width - 150),
                  child: Container(
                    alignment: Alignment.center,
                    width: 150,
                    height: 50,
                    child: Text(
                      "Click Me",
                      style: TextStyle(
                        color:
                            Theme.of(context).buttonTheme.colorScheme.onPrimary,
                      ),
                    ),
                    color: Theme.of(context).primaryColor,
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}