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 PageController
class 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,
           );
          },
         ),
        ),
       ),
      ),
     ],
    ),
   ),
  );
 }
}