【译】定制Flutter滚动效果

时间 2019-11-07

标签定制 flutter 滚动效果繁體版

原文原文链接

原文地址：medium.com/flutter-com…git

[译者注：ScrollPhysics 很是强大好用，能够定制各类滑动效果，经过设置阻尼系数等等实现]github

在这篇文章中咱们将定制ScrollPhysics来改变ListView的滚动行为

KISS （保持简单....别想多了，哈哈)

在一个多页面集合或者幻灯片集合循环访问，是一个常常出现的场景。spring

实现这个效果的代码很是简单，咱们只须要使用PageView的默认属性就能够了。bash

import 'dart:math';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final List<int> pages = List.generate(4, (index) => index);
@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: PageView.builder(
          itemCount: pages.length,
          itemBuilder: (context, index) {
            return Container(
              color: randomColor,
              margin: const EdgeInsets.all(20.0),
            );
          },
        ),
      ),
    );
  }
Color get randomColor =>
      Color((Random().nextDouble() * 0xFFFFFF).toInt() << 0).withOpacity(1.0);
}
复制代码

源码引用至GitHubapp

很是酷炫. 可是.less

有时候，咱们想要给用户一些提示；或者咱们滚动的列表中的元素不是真的全页面。这种状况下，若是当前的页面只填充视图的一部分，让咱们能看到下个元素（或者上个元素）那就太好了。dom

不用担忧，在Flutter中使用PageController就能作到。ide

代码依然很是简单。咱们只须要把想要的视图的百分比设置到viewportFraction属性就能够了。

class MyHomePage extends StatelessWidget {
  final List<int> pages = List.generate(4, (index) => index);
  final _pageController = PageController(viewportFraction: 0.8);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: PageView.builder(
          controller: _pageController,
          itemCount: pages.length,
          itemBuilder: (context, index) {
            return Container(
              color: randomColor,
              margin: const EdgeInsets.all(20.0),
            );
          },
        ),
      ),
    );
  }
复制代码

很是酷炫. 可是.函数

若是这不是咱们想要的效果呢？我想要元素想一整个列表同样，而不是居中；可是又想要一次滚动一个元素。ui

为了实现这个效果，咱们须要深刻了解一下个咱们还没用过的属性：ScrollPhysics

Row vs PageView

PageView更多的是为用户滑动的一组页面设计的，有点像播放幻灯片。咱们的状况有些不一样，由于咱们想要一个列表的效果，但同时又想一次滚动一个元素。放弃PageView，而使用ListView更加符合需求。

@override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: ListView.builder(
          scrollDirection: Axis.horizontal,
          itemCount: pages.length,
          itemBuilder: (context, index) => Container(
            height: double.infinity,
            width: 300,
            color: randomColor,
            margin: const EdgeInsets.all(20.0),
          ),
        ),
      ),
    );
  }
复制代码

简单。可是若是你向右滑动，就会发现不能一次滑动一个元素。咱们如今是在处理List里面的元素了，再也不是页面。因此咱们须要本身创建页面的概念，咱们可使用ListView的physics属性作到这种效果。

ScrollPhysics

已经有不一样的ScrollPhysics之类能够用来控制滑动效果；其中有一个看起来很是有趣，PageScrollPhysics。PageView内部使用的就是PageScrollPhysics，不兴的是，在ListView中使用无效。咱们能够本身设计一个出来，先看看PageScrollPhysics的实现。

class PageScrollPhysics extends ScrollPhysics {
  /// Creates physics for a [PageView].
  const PageScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);

  @override
  PageScrollPhysics applyTo(ScrollPhysics ancestor) {
    return PageScrollPhysics(parent: buildParent(ancestor));
  }

  double _getPage(ScrollPosition position) {
    if (position is _PagePosition)
      return position.page;
    return position.pixels / position.viewportDimension;
  }

  double _getPixels(ScrollPosition position, double page) {
    if (position is _PagePosition)
      return position.getPixelsFromPage(page);
    return page * position.viewportDimension;
  }

  double _getTargetPixels(ScrollPosition position, Tolerance tolerance, double velocity) {
    double page = _getPage(position);
    if (velocity < -tolerance.velocity)
      page -= 0.5;
    else if (velocity > tolerance.velocity)
      page += 0.5;
    return _getPixels(position, page.roundToDouble());
  }

  @override
  Simulation createBallisticSimulation(ScrollMetrics position, double velocity) {
    // If we're out of range and not headed back in range, defer to the parent // ballistics, which should put us back in range at a page boundary. if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) || (velocity >= 0.0 && position.pixels >= position.maxScrollExtent)) return super.createBallisticSimulation(position, velocity); final Tolerance tolerance = this.tolerance; final double target = _getTargetPixels(position, tolerance, velocity); if (target != position.pixels) return ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance); return null; } @override bool get allowImplicitScrolling => false; } 复制代码

源码引用至GitHub

方法createBallisticSimulation是这个类的入口，将滚动条中的位置和速度做为输入参数。首先这是在检查用户是向右滚动仍是向左滚动，接着计算滚动条中的新位置，也就是将当前加或减视图的范围，由于页面视图中的滚动是一个接一个的。

咱们要作的很是相似，可是咱们没有使用视图(viewport)，而是使用自定义的大小，由于每一个视图有多个元素。

这个自定义的大小咱们能够本身计算，它是滚动条的总大小除以列表元素个数再减1.为何要减1呢？列表中有1个元素不能滑动，2个元素就能滑1个元素...因此N个元素就能滑N-1

CustomScrollPhysics

class CustomScrollPhysics extends ScrollPhysics {
  final double itemDimension;

  CustomScrollPhysics({this.itemDimension, ScrollPhysics parent})
      : super(parent: parent);

  @override
  CustomScrollPhysics applyTo(ScrollPhysics ancestor) {
    return CustomScrollPhysics(
        itemDimension: itemDimension, parent: buildParent(ancestor));
  }

  double _getPage(ScrollPosition position) {
    return position.pixels / itemDimension;
  }

  double _getPixels(double page) {
    return page * itemDimension;
  }

  double _getTargetPixels(
      ScrollPosition position, Tolerance tolerance, double velocity) {
    double page = _getPage(position);
    if (velocity < -tolerance.velocity) {
      page -= 0.5;
    } else if (velocity > tolerance.velocity) {
      page += 0.5;
    }
    return _getPixels(page.roundToDouble());
  }
@override
  Simulation createBallisticSimulation(
      ScrollMetrics position, double velocity) {
    // If we're out of range and not headed back in range, defer to the parent // ballistics, which should put us back in range at a page boundary. if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) || (velocity >= 0.0 && position.pixels >= position.maxScrollExtent)) return super.createBallisticSimulation(position, velocity); final Tolerance tolerance = this.tolerance; final double target = _getTargetPixels(position, tolerance, velocity); if (target != position.pixels) return ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance); return null; } @override bool get allowImplicitScrolling => false; } 复制代码

源码引用至GitHub

咱们重写getPixels()让它返回基于页码的位置，重写getPage()返回基于位置的页码，最后再把itemDimension传入构造函数。

使用CustomScrollPhysics

幸运的是ScrollController能够获取滚动条的长度；可是呢，须要等到widget被建立出来以后才能够拿到。咱们须要把咱们的Page改成StatefulWidget，去监听dimensions的有效性通知，而后初始化CustomScrollPhysics

final _controller = ScrollController();

final List<int> pages = List.generate(4, (index) => index);

ScrollPhysics _physics;

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

  _controller.addListener(() {
    if (_controller.position.haveDimensions && _physics == null) {
      setState(() {
        var dimension =
            _controller.position.maxScrollExtent / (pages.length - 1);
        _physics = CustomScrollPhysics(itemDimension: dimension);
      });
    }
  });
}
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到此，咱们可让List一次滑动一个元素了。

总结

这是一个自定义ScrollPhysics来定制滑动效果的简单例子；在示例中咱们让ListView一次滑动一个元素。完整的代码以下：

import 'dart:math';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final _controller = ScrollController();

  final List<int> pages = List.generate(4, (index) => index);

  ScrollPhysics _physics;

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

    _controller.addListener(() {
      if (_controller.position.haveDimensions && _physics == null) {
        setState(() {
          var dimension =
              _controller.position.maxScrollExtent / (pages.length - 1);
          _physics = CustomScrollPhysics(itemDimension: dimension);
        });
      }
    });
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: ListView.builder(
          scrollDirection: Axis.horizontal,
          controller: _controller,
          physics: _physics,
          itemCount: pages.length,
          itemBuilder: (context, index) => Container(
            height: double.infinity,
            width: 300,
            color: randomColor,
            margin: const EdgeInsets.all(20.0),
          ),
        ),
      ),
    );
  }

  Color get randomColor =>
      Color((Random().nextDouble() * 0xFFFFFF).toInt() << 0).withOpacity(1.0);
}

class CustomScrollPhysics extends ScrollPhysics {
  final double itemDimension;

  CustomScrollPhysics({this.itemDimension, ScrollPhysics parent})
      : super(parent: parent);

  @override
  CustomScrollPhysics applyTo(ScrollPhysics ancestor) {
    return CustomScrollPhysics(
        itemDimension: itemDimension, parent: buildParent(ancestor));
  }

  double _getPage(ScrollPosition position) {
    return position.pixels / itemDimension;
  }
  
  double _getPixels(double page) {
    return page * itemDimension;
  }

  double _getTargetPixels(
      ScrollPosition position, Tolerance tolerance, double velocity) {
    double page = _getPage(position);
    if (velocity < -tolerance.velocity) {
      page -= 0.5;
    } else if (velocity > tolerance.velocity) {
      page += 0.5;
    }
    return _getPixels(page.roundToDouble());
  }

  @override
  Simulation createBallisticSimulation(
      ScrollMetrics position, double velocity) {
    // If we're out of range and not headed back in range, defer to the parent // ballistics, which should put us back in range at a page boundary. if ((velocity <= 0.0 && position.pixels <= position.minScrollExtent) || (velocity >= 0.0 && position.pixels >= position.maxScrollExtent)) return super.createBallisticSimulation(position, velocity); final Tolerance tolerance = this.tolerance; final double target = _getTargetPixels(position, tolerance, velocity); if (target != position.pixels) return ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance); return null; } @override bool get allowImplicitScrolling => false; } 复制代码

源码引用至GitHub

就写这么多，欢迎提问互动。感谢您的阅读！