Flutter动画和运动控件之AnimatedWidget 动画小组件

https://api.flutter.dev/flutter/widgets/AnimatedWidget-class.html

实际上这是一个抽象类，它的实现类有：

• AnimatedBuilder, which is useful for complex animation use cases and a notable exception to the naming scheme of AnimatedWidget subclasses.
• AlignTransition, which is an animated version of Align.
• DecoratedBoxTransition, which is an animated version of DecoratedBox.
• DefaultTextStyleTransition, which is an animated version of DefaultTextStyle.
• PositionedTransition, which is an animated version of Positioned.
• RelativePositionedTransition, which is an animated version of Positioned.
• RotationTransition, which animates the rotation of a widget.
• ScaleTransition, which animates the scale of a widget.
• SizeTransition, which animates its own size.
• SlideTransition, which animates the position of a widget relative to its normal position.
• FadeTransition, which is an animated version of Opacity.
• AnimatedModalBarrier, which is an animated version of ModalBarrier.

示例：

/// Flutter code sample for AnimatedWidget

// This code defines a widget called `Spinner` that spins a green square
// continually. It is built with an [AnimatedWidget].

import 'dart:math' as math;

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

/// This is the main application widget.
class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class SpinningContainer extends AnimatedWidget {
  const SpinningContainer({
    Key? key,
    required AnimationController controller,
  }) : super(key: key, listenable: controller);

  Animation<double> get _progress => listenable as Animation<double>;

  @override
  Widget build(BuildContext context) {
    return Transform.rotate(
      angle: _progress.value * 2.0 * math.pi,
      child: Container(width: 200.0, height: 200.0, color: Colors.green),
    );
  }
}

/// This is the stateful widget that the main application instantiates.
class MyStatefulWidget extends StatefulWidget {
  const MyStatefulWidget({Key? key}) : super(key: key);

  @override
  State<MyStatefulWidget> createState() => _MyStatefulWidgetState();
}

/// This is the private State class that goes with MyStatefulWidget.
/// AnimationControllers can be created with `vsync: this` because of TickerProviderStateMixin.
class _MyStatefulWidgetState extends State<MyStatefulWidget>
    with TickerProviderStateMixin {
  late final AnimationController _controller = AnimationController(
    duration: const Duration(seconds: 10),
    vsync: this,
  )..repeat();

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

  @override
  Widget build(BuildContext context) {
    return SpinningContainer(controller: _controller);
  }
}