Isolate and same demonstration how you can kill your UI )

main.dart

import 'package:flutter/material.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/foundation.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: PerformancePage(),
        ),
      ),
    );
  }
}


// Computes the nth number in the Fibonacci sequence.
int fib(int n) {
  var a = n - 1;
  var b = n - 2;

  if (n == 1) {
    return 0;
  } else if (n == 0) {
    return 1;
  } else {
    return (fib(a) + fib(b));
  }
}

class PerformancePage extends StatefulWidget {
  @override
  _PerformancePageState createState() => _PerformancePageState();
}

class _PerformancePageState extends State<PerformancePage> {
  Future<void> computeFuture = Future.value();

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          SmoothAnimationWidget(),
          Container(
            alignment: Alignment.bottomCenter,
            padding: EdgeInsets.only(top: 150),
            child: Column(
              children: [
                FutureBuilder(
                  future: computeFuture,
                  builder: (context, snapshot) {
                    return RaisedButton(
                      child: const Text('Compute on Main'),
                      elevation: 8.0,
                      onPressed:
                          snapshot.connectionState == ConnectionState.done
                              ? () => handleComputeOnMain(context)
                              : null,
                    );
                  },
                ),
                FutureBuilder(
                  future: computeFuture,
                  builder: (context, snapshot) {
                    return RaisedButton(
                        child: const Text('Compute on Secondary'),
                        elevation: 8.0,
                        onPressed:
                            snapshot.connectionState == ConnectionState.done
                                ? () => handleComputeOnSecondary(context)
                                : null);
                  },
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }

  void handleComputeOnMain(BuildContext context) {
    var future = computeOnMainIsolate()
      ..then((_) {
        var snackBar = SnackBar(
          content: Text('Main Isolate Done!'),
        );
        Scaffold.of(context).showSnackBar(snackBar);
      });

    setState(() {
      computeFuture = future;
    });
  }

  void handleComputeOnSecondary(BuildContext context) {
    var future = computeOnSecondaryIsolate()
      ..then((_) {
        var snackBar = SnackBar(
          content: Text('Secondary Isolate Done!'),
        );
        Scaffold.of(context).showSnackBar(snackBar);
      });

    setState(() {
      computeFuture = future;
    });
  }

  Future<void> computeOnMainIsolate() async {
    // A delay is added here to give Flutter the chance to redraw the UI at
    // least once before the computation (which, since it's run on the main
    // isolate, will lock up the app) begins executing.
    await Future<void>.delayed(Duration(milliseconds: 100));
    fib(45);
  }

  Future<void> computeOnSecondaryIsolate() async {
    // Compute the Fibonacci series on a secondary isolate.
    await compute(fib, 45);
  }
}

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

class SmoothAnimationWidgetState extends State<SmoothAnimationWidget>
    with TickerProviderStateMixin {
  AnimationController _animationController;
  Animation<BorderRadius> _borderAnimation;

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

    _animationController =
        AnimationController(duration: const Duration(seconds: 1), vsync: this);

    _borderAnimation = BorderRadiusTween(
            begin: BorderRadius.circular(100.0),
            end: BorderRadius.circular(0.0))
        .animate(_animationController);

    _animationController.repeat(reverse: true);
  }

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

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

solution.dart

import 'package:flutter/material.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/foundation.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: PerformancePage(),
        ),
      ),
    );
  }
}


// Computes the nth number in the Fibonacci sequence.
int fib(int n) {
  var a = n - 1;
  var b = n - 2;

  if (n == 1) {
    return 0;
  } else if (n == 0) {
    return 1;
  } else {
    return (fib(a) + fib(b));
  }
}

class PerformancePage extends StatefulWidget {
  @override
  _PerformancePageState createState() => _PerformancePageState();
}

class _PerformancePageState extends State<PerformancePage> {
  Future<void> computeFuture = Future.value();

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        children: [
          SmoothAnimationWidget(),
          Container(
            alignment: Alignment.bottomCenter,
            padding: EdgeInsets.only(top: 150),
            child: Column(
              children: [
                FutureBuilder(
                  future: computeFuture,
                  builder: (context, snapshot) {
                    return RaisedButton(
                      child: const Text('Compute on Main'),
                      elevation: 8.0,
                      onPressed:
                          snapshot.connectionState == ConnectionState.done
                              ? () => handleComputeOnMain(context)
                              : null,
                    );
                  },
                ),
                FutureBuilder(
                  future: computeFuture,
                  builder: (context, snapshot) {
                    return RaisedButton(
                        child: const Text('Compute on Secondary'),
                        elevation: 8.0,
                        onPressed:
                            snapshot.connectionState == ConnectionState.done
                                ? () => handleComputeOnSecondary(context)
                                : null);
                  },
                ),
              ],
            ),
          ),
        ],
      ),
    );
  }

  void handleComputeOnMain(BuildContext context) {
    var future = computeOnMainIsolate()
      ..then((_) {
        var snackBar = SnackBar(
          content: Text('Main Isolate Done!'),
        );
        Scaffold.of(context).showSnackBar(snackBar);
      });

    setState(() {
      computeFuture = future;
    });
  }

  void handleComputeOnSecondary(BuildContext context) {
    var future = computeOnSecondaryIsolate()
      ..then((_) {
        var snackBar = SnackBar(
          content: Text('Secondary Isolate Done!'),
        );
        Scaffold.of(context).showSnackBar(snackBar);
      });

    setState(() {
      computeFuture = future;
    });
  }

  Future<void> computeOnMainIsolate() async {
    // A delay is added here to give Flutter the chance to redraw the UI at
    // least once before the computation (which, since it's run on the main
    // isolate, will lock up the app) begins executing.
    await Future<void>.delayed(Duration(milliseconds: 100));
    fib(45);
  }

  Future<void> computeOnSecondaryIsolate() async {
    // Compute the Fibonacci series on a secondary isolate.
    await compute(fib, 45);
  }
}

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

class SmoothAnimationWidgetState extends State<SmoothAnimationWidget>
    with TickerProviderStateMixin {
  AnimationController _animationController;
  Animation<BorderRadius> _borderAnimation;

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

    _animationController =
        AnimationController(duration: const Duration(seconds: 1), vsync: this);

    _borderAnimation = BorderRadiusTween(
            begin: BorderRadius.circular(100.0),
            end: BorderRadius.circular(0.0))
        .animate(_animationController);

    _animationController.repeat(reverse: true);
  }

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

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