flutter-todos/lib/widgets/loading_widget.dart

import 'dart:math';

import 'package:flutter/material.dart';

class LoadingWidget extends StatefulWidget {
  final Widget child;

  const LoadingWidget({Key key, @required this.child}) : super(key: key);

  @override
  _LoadingWidgetState createState() => _LoadingWidgetState();
}

class _LoadingWidgetState extends State<LoadingWidget>
    with SingleTickerProviderStateMixin {
  AnimationController _controller;
  Animation animation;

  @override
  void initState() {
    _controller =
        AnimationController(vsync: this, duration: Duration(seconds: 2));
    animation = Tween(begin: 0.0, end: 1.0).animate(
        CurvedAnimation(parent: _controller, curve: Curves.ease));
    _controller.addStatusListener((status) {
      if (status == AnimationStatus.completed) {
        _controller.reset();
        _controller.forward();
      }
    });
    _controller.forward();
    super.initState();
  }

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

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
      animation: animation,
      builder: (ctx, child) {
        return Transform.scale(
          scale: ((animation.value - 0.5) * 2).abs(),
          child: child,
        );
      },
      child: widget.child,
    );
  }
}