Flutter pageview切换指示器的实现代码

// Copyright 2017, the Flutter project authors. Please see the AUTHORS file// for details. 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';void main() { runApp(new MyApp());}class MyApp extends StatelessWidget { @override Widget build(BuildContext context) {  return new MaterialApp(   title: 'Flutter Demo',   home: new MyHomePage(),   debugShowCheckedModeBanner: false,  ); }}/// An indicator showing the currently selected page of a PageControllerclass DotsIndicator extends AnimatedWidget { DotsIndicator({  this.controller,  this.itemCount,  this.onPageSelected,  this.color: Colors.white, }) : super(listenable: controller); /// The PageController that this DotsIndicator is representing. final PageController controller; /// The number of items managed by the PageController final int itemCount; /// Called when a dot is tapped final ValueChanged<int> onPageSelected; /// The color of the dots. /// /// Defaults to `Colors.white`. final Color color; // The base size of the dots static const double _kDotSize = 8.0; // The increase in the size of the selected dot static const double _kMaxZoom = 2.0; // The distance between the center of each dot static const double _kDotSpacing = 25.0; Widget _buildDot(int index) {  double selectedness = Curves.easeOut.transform(   max(    0.0,    1.0 - ((controller.page ?? controller.initialPage) - index).abs(),   ),  );  double zoom = 1.0 + (_kMaxZoom - 1.0) * selectedness;  return new Container(   width: _kDotSpacing,   child: new Center(    child: new Material(     color: color,     type: MaterialType.circle,     child: new Container(      width: _kDotSize * zoom,      height: _kDotSize * zoom,      child: new InkWell(       onTap: () => onPageSelected(index),      ),     ),    ),   ),  ); } Widget build(BuildContext context) {  return new Row(   mainAxisAlignment: MainAxisAlignment.center,   children: new List<Widget>.generate(itemCount, _buildDot),  ); }}class MyHomePage extends StatefulWidget { @override State createState() => new MyHomePageState();}class MyHomePageState extends State<MyHomePage> { final _controller = new PageController(); static const _kDuration = const Duration(milliseconds: 300); static const _kCurve = Curves.ease; final _kArrowColor = Colors.black.withOpacity(0.8); final List<Widget> _pages = <Widget>[  new ConstrainedBox(   constraints: const BoxConstraints.expand(),   child: new FlutterLogo(colors: Colors.blue),  ),  new ConstrainedBox(   constraints: const BoxConstraints.expand(),   child: new FlutterLogo(style: FlutterLogoStyle.stacked, colors: Colors.red),  ),  new ConstrainedBox(   constraints: const BoxConstraints.expand(),   child: new FlutterLogo(style: FlutterLogoStyle.horizontal, colors: Colors.green),  ), ]; @override Widget build(BuildContext context) {  return new Scaffold(   body: new IconTheme(    data: new IconThemeData(color: _kArrowColor),    child: new Stack(     children: <Widget>[      new PageView.builder(       physics: new AlwaysScrollableScrollPhysics(),       controller: _controller,       itemBuilder: (BuildContext context, int index) {        return _pages[index % _pages.length];       },      ),      new Positioned(       bottom: 0.0,       left: 0.0,       right: 0.0,       child: new Container(        color: Colors.grey[800].withOpacity(0.5),        padding: const EdgeInsets.all(20.0),        child: new Center(         child: new DotsIndicator(          controller: _controller,          itemCount: _pages.length,          onPageSelected: (int page) {           _controller.animateToPage(            page,            duration: _kDuration,            curve: _kCurve,           );          },         ),        ),       ),      ),     ],    ),   ),  ); }}