Flutter入门09 -- Widget之滚动组件类

常见的滚动组件有：ListView、GridView、SingleChildScrollView、CustomScrollView、NestedScrollView、Scrollable、Scrollbar、NotificationListener、ScrollConfiguration、RefreshIndicator、PageView

ListView

ListView创建的方式通常有三种，分别为ListView()，ListView.builder()，ListView.separated()；

ListView创建方式之ListView()

构造方法如下：

ListView({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    EdgeInsetsGeometry padding,
    this.itemExtent,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    double cacheExtent,
    List children = const [],
    int semanticChildCount,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
    ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
})

scrollDirection：滚动方向，属于Axis类型；
itemExtent：item单元格的高度；
children：单元格数组，这里的单元格指的是ListTile；
shrinkWrap：其值为true时，会将item子组件的高度相加，然后赋值给ListView的高度，容易引发ListView高度溢出异常的问题；
案例代码如下：

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ListView(
      scrollDirection: Axis.vertical,
      itemExtent: 100,//设置Item的高度
      children: List.generate(100, (index) {
        return ListTile(
          leading: Icon(Icons.people),
          trailing: Icon(Icons.delete),
          title: Text("联系人${index+1}"),
          subtitle: Text("联系人电话号码:19991604555"),
        );
      }),
    );
  }
}

效果图如下：

image.png

通过ListView()创建，会一次性创建100个Item，这样性能比较差，其适用于Item个数确定，且数量较少的情况下才会采用；

ListView创建方式之ListView.builder()

其不会一次性创建所有Item，而是需要展示的Item才会去创建，性能较好，构造方法如下：

ListView.builder({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    EdgeInsetsGeometry padding,
    this.itemExtent,
    @required IndexedWidgetBuilder itemBuilder,
    int itemCount,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    double cacheExtent,
    int semanticChildCount,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
    ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
})

scrollDirection：滚动方向，属于Axis类型；
itemExtent：单元格高度；
itemCount：单元格数量；
itemBuilder：获取item单元格，其类型为Widget Function(BuildContext context, int index)，调用函数创建item；
案例代码如下：

class SFHomeBody extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return ListView.builder(
      itemExtent: 80,
      itemCount: 100,
      itemBuilder: (BuildContext context,int index) {
        return Container(
            color: Colors.red,
            child: Text("Hello World!!! ${index}",style: TextStyle(fontSize: 20)),
            padding: EdgeInsets.all(10),
        );
      }
    );
  }
}

效果图如下：

Snip20220214_22.png

ListView创建方式之ListView.separated()

创建带有分割线的ListView，构造函数如下：

ListView.separated({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    EdgeInsetsGeometry padding,
    @required IndexedWidgetBuilder itemBuilder,
    @required IndexedWidgetBuilder separatorBuilder,
    @required int itemCount,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    double cacheExtent,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
    ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
})

scrollDirection：滚动方向，属于Axis类型；
itemExtent：单元格高度；
itemCount：单元格数量；
itemBuilder：获取item单元格组件，其类型为Widget Function(BuildContext context, int index)，调用函数创建item；
separatorBuilder：获取分割线组件，类型同上；
不能设置单元格item的高度；
案例代码如下：

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ListView.separated(
        itemBuilder: (BuildContext ctx, int index){
          return Text("Hello World!!! ${index}",style: TextStyle(fontSize: 20),);
        },
        //分割线
        separatorBuilder: (BuildContext ctx,int index){
          return Divider(color: Colors.red,indent: 20,endIndent: 20,thickness: 5);
        },
        itemCount: 100
    );
  }
}

效果图如下：

image.png

GridView

GridView的创建方式有：GridView()，GridView.builder()

创建方式之GridView()

构造方法如下所示：

GridView({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    EdgeInsetsGeometry padding,
    @required this.gridDelegate,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    double cacheExtent,
    List children = const [],
    int semanticChildCount,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
    ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
})

children：子组件数组，item单元格数组；
scrollDirection：表示滚动方向，默认为垂直方向，属于Axis类型；
gridDelegate：GridView的代理，属于SliverGridDelegate类型，其本质是控制GridView如何排列内部子元素的一个委托；
- SliverGridDelegate是一个抽象类，其有两个实现子类，如下所示：
- SliverGridDelegateWithFixedCrossAxisCount：用于固定列数的场景；
- SliverGridDelegateWithMaxCrossAxisExtent：用于子元素有最大宽度限制的场景；
controller：GridView的滚动监听者；
primary：当内容不足以滚动时，是否支持滚动；
使用gridDelegate为SliverGridDelegateWithFixedCrossAxisCount来创建GridView，其中SliverGridDelegateWithFixedCrossAxisCount的构造方法如下：

const SliverGridDelegateWithFixedCrossAxisCount({
    @required this.crossAxisCount,
    this.mainAxisSpacing = 0.0,
    this.crossAxisSpacing = 0.0,
    this.childAspectRatio = 1.0,
})

交叉轴方向上item数量固定，其宽度根据屏幕的宽度与item的数量进行计算；
crossAxisCount：交叉轴方向(水平方向)item的个数；
childAspectRatio：item的宽高比；
crossAxisSpacing：交叉轴方向上 item之间的间距；
mainAxisSpacing：主轴方向上 item之间的间距；
案例代码如下：

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GridView(
      gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
        crossAxisCount: 3,
        childAspectRatio: 1.5,
        crossAxisSpacing: 8,
        mainAxisSpacing: 8
      ),
      children:
        List.generate(100, (index) {
           return Container(
             color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256), Random().nextInt(256)),
           );
        }),
    );
  }
}

效果图如下所示：

image.png

使用gridDelegate为SliverGridDelegateWithMaxCrossAxisExtent来创建GridView，其中SliverGridDelegateWithMaxCrossAxisExtent的构造方法如下：

const SliverGridDelegateWithMaxCrossAxisExtent({
    @required this.maxCrossAxisExtent,
    this.mainAxisSpacing = 0.0,
    this.crossAxisSpacing = 0.0,
    this.childAspectRatio = 1.0,
})

交叉轴方向上的设置item的最大宽度，item的个数不固定；
maxCrossAxisExtent：交叉轴方向上item的最大宽度；
childAspectRatio：item的宽高比；
crossAxisSpacing：交叉轴方向上 item之间的间距；
mainAxisSpacing：主轴方向上 item之间的间距；
案例代码如下：

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GridView(
      gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
        maxCrossAxisExtent: 220,
        crossAxisSpacing: 8,
        mainAxisSpacing: 8,
        childAspectRatio: 1.5
      ),
      children: List.generate(100, (index) {
        return Container(
          color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256),Random().nextInt(256)),
        );
      })
    );
  }
}

效果图如下：

image.png

创建方式之GridView.builder()

构造方法如下：

  GridView.builder({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    EdgeInsetsGeometry padding,
    @required this.gridDelegate,
    @required IndexedWidgetBuilder itemBuilder,
    int itemCount,
    bool addAutomaticKeepAlives = true,
    bool addRepaintBoundaries = true,
    bool addSemanticIndexes = true,
    double cacheExtent,
    int semanticChildCount,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
    ScrollViewKeyboardDismissBehavior keyboardDismissBehavior = ScrollViewKeyboardDismissBehavior.manual,
})

属性大部分相同；
gridDelegate：GridView的代理，属于SliverGridDelegate类型，其本质是控制GridView如何排列内部子元素的一个委托；
itemBuilder：创建item的构造函数；
itemCount：item的数量；
案例代码如下：

class SFHomeBody extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return GridView.builder(
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 3,
          mainAxisSpacing: 10,
          crossAxisSpacing: 10,
          childAspectRatio: 1.5
        ),
        itemBuilder: (BuildContext ctx,int index){
          return Container(color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256),Random().nextInt(256)));
        },
      itemCount: 100,
    );
  }
}

效果图如下：

image.png

CustomScrollView

CustomScrollView：自定义滚动组件，需要传入Slivers即Sliver数组，我们知道ListView与GridView都是继承自BoxScrollView，而BoxScrollView是一个抽象类，从源码来看ListView与GridView在创建的过程中都需要执行buildSlivers方法，其内部调用buildChildLayout方法，这是一个抽象方法，分别由ListView与GridView来实现，最终提供一个Sliver数组，其中ListView提供的Sliver为SliverFixedExtentList，GridView提供的Sliver为SliverGrid；
CustomScrollView的构造函数如下：

const CustomScrollView({
    Key key,
    Axis scrollDirection = Axis.vertical,
    bool reverse = false,
    ScrollController controller,
    bool primary,
    ScrollPhysics physics,
    bool shrinkWrap = false,
    Key center,
    double anchor = 0.0,
    double cacheExtent,
    this.slivers = const [],
    int semanticChildCount,
    DragStartBehavior dragStartBehavior = DragStartBehavior.start,
})

slivers：Sliver的数组，最重要的一个参数；
常见的Sliver组件如下：
- SliverSafeArea
- SliverAppBar
- SliverList
- SliverGrid
- SliverPadding
- SliverFixedExtentList
- SliverPrototypeExtentList
- SliverToBoxAdapter
- SliverPersistentHeader

单个Sliver -- SliverSafeArea

SliverSafeArea的构造函数如下：

const SliverSafeArea({
    Key key,
    this.left = true,
    this.top = true,
    this.right = true,
    this.bottom = true,
    this.minimum = EdgeInsets.zero,
    @required this.sliver,
})

案例代码如下：

import 'dart:math';
import 'package:flutter/material.dart';

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

class SFMyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: SFHomePage());
  }
}

class SFHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text("基础widget")),
        body: SFHomeContent());
  }
}

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return SliverDemo1();
  }
}

class SliverDemo1 extends StatelessWidget {
  const SliverDemo1({
    Key key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      slivers: [
        SliverSafeArea(
          sliver: SliverPadding(
            padding: EdgeInsets.all(8),
            sliver: SliverGrid(
              gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
                crossAxisCount: 2,
                crossAxisSpacing: 8,
                mainAxisSpacing: 8,
                childAspectRatio: 1.5
              ),
              delegate: SliverChildBuilderDelegate(
                  (BuildContext ctx,int index){
                    return Container(
                      color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256),Random().nextInt(256)),
                    );
                  },
                childCount: 100
              ),
            ),
          ),
        )
      ],
    );
  }
}

自定义CustomScrollView，需传入Slivers数组，这里传入的Sliver为SliverSafeArea；
SliverPadding：属于Sliver组件，可设置内边距；
SliverGrid：属于Sliver组件，是实现网格视图的核心组件，其构造函数如下：

const SliverGrid({
    Key key,
    @required SliverChildDelegate delegate,
    @required this.gridDelegate,
 })

gridDelegate：GridView的代理，属于SliverGridDelegate类型，其本质是控制GridView如何排列内部子元素的一个委托；
delegate：是提供item组件，类型为SliverChildDelegate，
SliverChildDelegate是抽象类，其作用是用来创建滚动组件的item，其有两个子类分别为SliverChildListDelegate与SliverChildBuilderDelegate
- SliverChildListDelegate：性能较差，item一次性创建所有；
- SliverChildBuilderDelegate：性能较好，创建需要展示的item，其构造函数如下：

const SliverChildBuilderDelegate(
    this.builder, {
    this.findChildIndexCallback,
    this.childCount,
    this.addAutomaticKeepAlives = true,
    this.addRepaintBoundaries = true,
    this.addSemanticIndexes = true,
    this.semanticIndexCallback = _kDefaultSemanticIndexCallback,
    this.semanticIndexOffset = 0,
})

builder：是一个item的构造函数；
childCount：item的数量；
SafeArea与SliverSafeArea的区别：
- SafeArea：安全区域，让目标组件在安全区域内显示；
- SliverSafeArea：给Sliver设置安全区域，且在滚动时可以在非安全区域内滚动，而SafeArea不可以；
效果图如下：

image.png

多个Sliver (SliverAppBar + SliverGrid + SliverList)

import 'dart:math';
import 'package:flutter/material.dart';

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

class SFMyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: SFHomePage());
  }
}

class SFHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        // appBar: AppBar(title: Text("基础widget")),
        body: SFHomeContent());
  }
}

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      slivers: [
        SliverAppBar(
          pinned: true,
          expandedHeight: 200,
          flexibleSpace: FlexibleSpaceBar(
            title: Text("Hello World!!",style: TextStyle(fontSize: 25),),
          ),
        ),
        SliverGrid(
          gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
              crossAxisCount: 2,
              crossAxisSpacing: 8,
              mainAxisSpacing: 8,
              childAspectRatio: 2.5
          ),
            delegate: SliverChildBuilderDelegate(
              (BuildContext ctx,int index){
                  return Container(
                    color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256),Random().nextInt(256)),
                  );
               },
             childCount: 10
            ),
        ),
        SliverList(
          delegate: SliverChildBuilderDelegate(
              (BuildContext ctx,int index){
                return ListTile(
                  leading: Icon(Icons.people),
                  title: Text("联系人$index"),
                );
              },
            childCount: 20
          ),
        )
      ],
    );
  }
}

slivers数组中传入了SliverAppBar，SliverGrid与SliverList三种类型的Sliver，效果如下：

image.png

滚动组件的监听

滚动组件的监听通常有两种方式，分别为controller与NotificationListener

controller监听

可以设置默认值offset；
监听滚动，也可以监听滚动的位置；
案例代码：

import 'package:flutter/material.dart';

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

class SFMyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: SFHomePage());
  }
}

class SFHomePage extends StatefulWidget {
  @override
  _SFHomePageState createState() => _SFHomePageState();
}

class _SFHomePageState extends State {
  ScrollController controller = ScrollController(initialScrollOffset: 300);
  bool isShowFloatButton = false;

  @override
  void initState() {
    super.initState();
    controller.addListener(() {
      print("监听到滚动: ${controller.offset}");
      setState(() {
        isShowFloatButton = controller.offset >= 1000;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("基础widget")),
      body: SFHomeContent(controller),
      floatingActionButton: isShowFloatButton ? FloatingActionButton(
        child: Icon(Icons.arrow_upward),
        onPressed: (){
          controller.animateTo(0, duration: Duration(seconds: 1), curve: Curves.easeIn);
        },
      ) : null,
    );
  }
}

class SFHomeContent extends StatelessWidget {
  final ScrollController controller;
  SFHomeContent(this.controller);

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      controller: controller,
      itemBuilder: (BuildContext ctx, int index) {
        return ListTile(
          leading: Icon(Icons.people),
          title: Text("联系人$index"),
        );
      },
      itemCount: 100,
    );
  }
}

右下角悬浮按钮，当前滚动偏移量>=1000时显示；

NotificationListener监听

案例代码：

import 'package:flutter/material.dart';

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

class SFMyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: SFHomePage());
  }
}

class SFHomePage extends StatefulWidget {
  @override
  _SFHomePageState createState() => _SFHomePageState();
}

class _SFHomePageState extends State {
  bool isShowFloatButton = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("基础widget")),
      body: SFHomeContent(),
      floatingActionButton: isShowFloatButton
          ? FloatingActionButton(
              child: Icon(Icons.arrow_upward),
              onPressed: () {

              },
            )
          : null,
    );
  }
}

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return NotificationListener(
      onNotification: (ScrollNotification notification){
        if(notification is ScrollStartNotification){
          print("开始滚动");
        }else if (notification is ScrollUpdateNotification){
          print("正在滚动 -- 总区域:${notification.metrics.maxScrollExtent} 当前位置: ${notification.metrics.pixels}");
        }else if (notification is ScrollEndNotification){
          print("结束滚动");

        }

        return true;
      },
      child: ListView.builder(
        itemBuilder: (BuildContext ctx, int index) {
          return ListTile(
            leading: Icon(Icons.people),
            title: Text("联系人$index"),
          );
        },
        itemCount: 100,
      ),
    );
  }
}

Flutter入门09 -- Widget之滚动组件类

ListView

ListView创建方式之ListView()

ListView创建方式之ListView.builder()

ListView创建方式之ListView.separated()

GridView

创建方式之GridView()

创建方式之GridView.builder()

CustomScrollView

单个Sliver -- SliverSafeArea

多个Sliver (SliverAppBar + SliverGrid + SliverList)

滚动组件的监听

controller监听

NotificationListener监听

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