CustomScrollView
CustomScrollView是可以使用Sliver来自定义滚动模型（效果）的组件。它可以包含多种滚动模型。包括header，footer，以及类似Android Coordinglayout布局的滑动效果，CustomScrollView可以实现把多个彼此独立的可滑动widget组合起来
Sliver的概念
Flutter中提出一个Sliver（中文为“薄片”的意思）概念，如果一个可滚动组件支持Sliver模型，那么该滚动可以将子组件分成好多个“薄片”（Sliver），只有当Sliver出现在视口中时才会去构建它，这种模型也称为“基于Sliver的延迟构建模型”。可滚动组件中有很多都支持基于Sliver的延迟构建模型，如ListView、GridView，但是也有不支持该模型的，如SingleChildScrollView。
这个跟安卓的思维是一致的，类似与分页加载，复用加载，数据量比较大的时候对提高加载效率非常有帮助。SingleChildScrollView只包含一个子widget，本身也只适用于子widget数量有限的场景。
1、添加Header和Footer
import 'dart:ui';
import 'package:flutter/material.dart';
class CsWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return Scaffold(
appBar: PreferredSize(
preferredSize:
Size.fromHeight(MediaQueryData.fromWindow(window).padding.top),
child: SafeArea(
top: true,
child: Offstage(),
),
),
body: CustomScrollViewWidget(),
);
}
}
class CustomScrollViewWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return CustomScrollView(
scrollDirection: Axis.vertical,
slivers: <Widget>[
SliverAppBar(
pinned: false,
expandedHeight: 250.0,
backgroundColor: Colors.blue,
flexibleSpace: FlexibleSpaceBar(
background: Image.network("http://b-ssl.duitang.com/uploads/blog/201312/04/20131204184148_hhXUT.jpeg",fit: BoxFit.cover,),
),
),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
color: Colors.red,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 20),
itemExtent: 50),
SliverAppBar(
pinned: false,
expandedHeight: 250.0,
backgroundColor: Colors.blue,
flexibleSpace: FlexibleSpaceBar(
background: Image.network("http://b-ssl.duitang.com/uploads/blog/201312/04/20131204184148_hhXUT.jpeg",fit: BoxFit.cover,),
),
),
],
);
}
}
CustomScrollView里添加的子widget都必须是Sliver，比如添加Image或者Text都会报错
分析一下SliverAppBar
SliverAppBar
const SliverAppBar({
Key key,
this.leading,//左侧的图标或文字，多为返回箭头
this.automaticallyImplyLeading = true,//没有leading为true的时候，默认返回箭头，没有leading且为false，则显示title
this.title,//标题
this.actions,//标题右侧的操作
this.flexibleSpace,//可以理解为SliverAppBar的背景内容区
this.bottom,//SliverAppBar的底部区
this.elevation,//阴影
this.forceElevated = false,//是否显示阴影
this.backgroundColor,//背景颜色
this.brightness,//状态栏主题，默认Brightness.dark，可选参数light
this.iconTheme,//SliverAppBar图标主题
this.actionsIconTheme,//action图标主题
this.textTheme,//文字主题
this.primary = true,//是否显示在状态栏的下面,false就会占领状态栏的高度
this.centerTitle,//标题是否居中显示
this.titleSpacing = NavigationToolbar.kMiddleSpacing,//标题横向间距
this.expandedHeight,//合并的高度，默认是状态栏的高度加AppBar的高度
this.floating = false,//滑动时是否悬浮
this.pinned = false,//标题栏是否固定
this.snap = false,//配合floating使用
})其中比较常用的leading，title，actions，backgroundColor，flexibleSpace，pinned,expandedHeight
class CustomScrollViewWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return CustomScrollView(
scrollDirection: Axis.vertical,
slivers: <Widget>[
SliverAppBar(
leading: IconButton(icon: Icon(Icons.arrow_back), onPressed: () {}),
title: Text("标题"),
actions: <Widget>[
IconButton(icon: Icon(Icons.share), onPressed: () {})
],
primary: true,
pinned: true,
centerTitle: true,
expandedHeight: 250.0,
backgroundColor: Colors.blue,
flexibleSpace: FlexibleSpaceBar(
background: Image.network(
"http://b-ssl.duitang.com/uploads/blog/201312/04/20131204184148_hhXUT.jpeg",
fit: BoxFit.cover,
),
),
),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
height: 30,
color: index % 2 == 0 ? Colors.red : Colors.purpleAccent,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 20),
itemExtent: 50),
],
);
}
}
SliverAppBar+SliverGrid+SliverFixedExtentList实现滑动列表
class CustomScrollViewWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return CustomScrollView(
scrollDirection: Axis.vertical,
slivers: <Widget>[
SliverAppBar(
leading: IconButton(icon: Icon(Icons.arrow_back), onPressed: () {}),
title: Text("标题"),
actions: <Widget>[
IconButton(icon: Icon(Icons.share), onPressed: () {})
],
primary: true,
pinned: true,
centerTitle: true,
expandedHeight: 250.0,
backgroundColor: Colors.blue,
flexibleSpace: FlexibleSpaceBar(
background: Image.network(
"http://b-ssl.duitang.com/uploads/blog/201312/04/20131204184148_hhXUT.jpeg",
fit: BoxFit.cover,
),
),
),
SliverPadding(
padding: EdgeInsets.all(20),
sliver: SliverGrid(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
color: index % 2 == 0 ? Colors.greenAccent : Colors.yellow,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 8),
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 3,
crossAxisSpacing: 20,
mainAxisSpacing: 20,
childAspectRatio: 1)),
),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
height: 30,
color: index % 2 == 0 ? Colors.red : Colors.purpleAccent,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 20),
itemExtent: 50),
],
);
}
}
这样就能实现多维列表+单列表的组合布局。但是距离android的多type布局还有一定距离，因为CustomScrollView子widget必须是Sliver，但是换个思路，gradview和listview的子widget没有这个限制，是否以此作为多type实现的方式呢？
我实验了一下是可以的，由此可以断言，CustomScrollView可以完全实现Android多type布局的效果
class CustomScrollViewWidget extends StatelessWidget {
@override
Widget build(BuildContext context) {
// TODO: implement build
return CustomScrollView(
scrollDirection: Axis.vertical,
slivers: <Widget>[
SliverAppBar(
leading: IconButton(icon: Icon(Icons.arrow_back), onPressed: () {}),
title: Text("标题"),
actions: <Widget>[
IconButton(icon: Icon(Icons.share), onPressed: () {})
],
primary: true,
pinned: true,
centerTitle: true,
expandedHeight: 250.0,
backgroundColor: Colors.blue,
flexibleSpace: FlexibleSpaceBar(
background: Image.network(
"http://b-ssl.duitang.com/uploads/blog/201312/04/20131204184148_hhXUT.jpeg",
fit: BoxFit.cover,
),
),
),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
color: Colors.deepPurpleAccent,
alignment: Alignment.center,
child: Text("这只是中间type"),
);
}, childCount: 1),
itemExtent: 60),
SliverPadding(
padding: EdgeInsets.all(20),
sliver: SliverGrid(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
color: index % 2 == 0 ? Colors.greenAccent : Colors.yellow,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 8),
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 3,
crossAxisSpacing: 20,
mainAxisSpacing: 20,
childAspectRatio: 1)),
),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
height: 30,
color: index % 2 == 0 ? Colors.red : Colors.purpleAccent,
alignment: Alignment.center,
child: Text("$index"),
);
}, childCount: 20),
itemExtent: 50),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
color: Colors.deepPurpleAccent,
alignment: Alignment.center,
child: Text("这只是一个footer"),
);
}, childCount: 1),
itemExtent: 60),
SliverFixedExtentList(
delegate:
SliverChildBuilderDelegate((BuildContext context, int index) {
return Container(
child: Image.network(
"http://b-ssl.duitang.com/uploads/item/201208/08/20120808125749_UxUu8.jpeg",
height: 60,
fit: BoxFit.fitWidth,
),
);
}, childCount: 1),
itemExtent: 60),
],
);
}
}
今天的学习就到此为止
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