samples/animations/lib/src/basics/10_animated_switcher.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';

Color generateColor() => Color(0xFFFFFFFF & Random().nextInt(0xFFFFFFFF));

Widget generateContainer(int keyCount) => Container(
      key: ValueKey<int>(keyCount),
      height: Random().nextDouble() * 200,
      width: Random().nextDouble() * 200,
      decoration: BoxDecoration(
        color: generateColor(),
        borderRadius: BorderRadius.circular(Random().nextDouble() * 100),
        border: Border.all(
          color: generateColor(),
          width: Random().nextDouble() * 5,
        ),
      ),
    );

class AnimatedSwitcherDemo extends StatefulWidget {
  static String routeName = '/basics/10_animated_switcher';

  _AnimatedSwitcherDemoState createState() => _AnimatedSwitcherDemoState();
}

class _AnimatedSwitcherDemoState extends State<AnimatedSwitcherDemo> {
  Widget container;
  int keyCount;

  void initState() {
    super.initState();
    keyCount = 0;
    container = generateContainer(keyCount);
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        actions: [
          MaterialButton(
            onPressed: () => setState(
              () => container = generateContainer(++keyCount),
            ),
            child: Text(
              "Change Widget",
              style: TextStyle(
                  color: Theme.of(context).buttonTheme.colorScheme.onPrimary),
            ),
          ),
        ],
      ),
      body: Center(
        // AnimatedSwitcher Widget is used to switch between different widgets
        // with a given transition. You can change the transitions by using
        // transitionBuilder property.
        child: AnimatedSwitcher(
          duration: Duration(seconds: 1),
          child: container,
          transitionBuilder: (child, animation) => ScaleTransition(
            child: child,
            scale: animation,
          ),
        ),
      ),
    );
  }
}