Flutter入门05 -- 滚动组件

作者: zyc_在路上 | 来源:发表于2022-01-18 11:28 被阅读0次

滚动组件有ListView，GridView，Sliver

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

第一种方式：ListView()
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 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 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"),
        );
      }),
    );
  }
}

ListTile组件就是ListView的Item；
itemExtent：设置Item的高度；
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；
第二种方式：ListView.builder()
ListView.builder()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；
案例代码如下：

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ListView.builder(
        itemCount: 100,
        itemExtent: 50,
        itemBuilder: (BuildContext ctx, int index){
          return Text("Hello World!!! ${index}",style: TextStyle(fontSize: 20),);
    }
    );
  }
}

第三种方式： ListView.separated()
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
    );
  }
}

itemBuilder：创建Item；
separatorBuilder：创建分割线；

GridView

GridView的创建方式有：GridView()，GridView.builder()
案例代码一：GridView()+ SliverGridDelegateWithFixedCrossAxisCount

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)),
           );
        }),
    );
  }
}

SliverGridDelegateWithFixedCrossAxisCount：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：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；
案例代码二：GridView()+ SliverGridDelegateWithMaxCrossAxisExtent

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)),
        );
      })
    );
  }
}

SliverGridDelegateWithMaxCrossAxisExtent：交叉轴方向上的设置item宽度，个数不固定；
- maxCrossAxisExtent：item的最大宽度；
案例代码三：GridView.builder() + SliverGridDelegateWithFixedCrossAxisCount

class SFHomeContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return GridView.builder(
        gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
          crossAxisCount: 3,
          crossAxisSpacing: 8,
          mainAxisSpacing: 8
        ),
        itemBuilder: (BuildContext ctx,int index){
          return Container(
            color: Color.fromARGB(255, Random().nextInt(256), Random().nextInt(256),Random().nextInt(256)),
          );
        }
    );
  }
}

Slivers -- 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在察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或者是SliverList，GridView提供的Sliver为SliverGrid
案例代码：单个Sliver

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SliverGrid；
参数gridDelegate：是提供布局信息；
参数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；
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；
SliverPadding：是Sliver自己的设置内边距的组件；
案例代码：多个Sliver

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<SFHomePage> {
  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<SFHomePage> {
  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05 -- 滚动组件

ListView

ListView创建方式

GridView

Slivers -- CustomScrollView

滚动组件的监听

controller监听

NotificationListener监听

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