samples/isolate_example/lib/page_one.dart

// Copyright 2019-present the Flutter authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import 'package:flutter/material.dart';

class PerformancePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          SmoothAnimationWidget(),
        ],
      ),
    );
  }
}

class SmoothAnimationWidget extends StatefulWidget {
  @override
  SmoothAnimationWidgetState createState() => SmoothAnimationWidgetState();
}

class SmoothAnimationWidgetState extends State<SmoothAnimationWidget>
    with TickerProviderStateMixin {
  AnimationController _controller;
  var borderRadius;

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

    _controller = AnimationController(
      duration: const Duration(seconds: 1),
      vsync: this,
    )..addStatusListener(
        (status) {
          if (status == AnimationStatus.completed) {
            _controller.reverse();
          } else if (status == AnimationStatus.dismissed) {
            _controller.forward();
          }
        },
      );

    borderRadius = BorderRadiusTween(
      begin: BorderRadius.circular(100.0),
      end: BorderRadius.circular(0.0),
    ).animate(
      CurvedAnimation(
        parent: _controller,
        curve: Curves.linear,
      ),
    );

    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: borderRadius,
      builder: (context, child) {
        return Center(
          child: Container(
            child: new FlutterLogo(
              size: 200,
            ),
            alignment: Alignment.bottomCenter,
            width: 350,
            height: 200,
            decoration: BoxDecoration(
              gradient: LinearGradient(
                begin: Alignment.topLeft,
                colors: [Colors.blueAccent, Colors.redAccent],
              ),
              borderRadius: borderRadius.value,
            ),
          ),
        );
      },
    );
  }

  @override
  void dispose() {
    _controller.dispose();
    super.dispose();
  }
}