今儿个主要学习一下几个基本组件
Padding Row Column Expended 四个组件

Padding 组件用法

Flutter提供的组件是没有padding属性的，所以在处理内边距的时候，就需要用到Padding组件来设置容器和子元素之间的间距 ，贴代码

class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.fromLTRB(0, 0, 10, 0),
      child: GridView.count(
        crossAxisCount: 2,
        childAspectRatio: 1.7,
        children: <Widget>[
          Padding(
            padding: EdgeInsets.fromLTRB(10, 10, 0, 0),
            child: Image.network('https://www.itying.com/images/flutter/1.png',
                fit: BoxFit.cover),
          ),
          Padding(
            padding: EdgeInsets.fromLTRB(10, 10, 0, 0),
            child: Image.network('https://www.itying.com/images/flutter/2.png',
                fit: BoxFit.cover),
          )
        ],
    ));
  }
}

fit的几个属性
BoxFit.fill:充满父容器。 可能会拉伸变形
BoxFit.contain:尽可能大，保持图片分辨率。不变形，填不满
BoxFit.cover:充满容器，不变形可能会被截断。
BoxFit.none:图片居中显示，不改变分大小，可能会被截断。
BoxFit.fitWidth:图片填满宽度，高度可能会被截断
BoxFit.fitHeight:图片填满高度，宽度可能会被截断
BoxFit.scaleDown:图片可以完整显示，但是可能不能填充满。

Row组件横向

void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
        home: Scaffold(
      appBar: AppBar(title: Text('FlutterDemo')),
      body: LayoutDemo(),
    ));
  }
}
class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      height: 800.0,
      width: 400.0,
      color: Colors.pink,
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,    
        crossAxisAlignment: CrossAxisAlignment.start,     //用的比较少
        children: <Widget>[
          IconContainer(Icons.search,color: Colors.blue),
          IconContainer(Icons.home,color: Colors.orange),
          IconContainer(Icons.select_all,color: Colors.red),
        ],
      ),
    );
  }
}
class IconContainer extends StatelessWidget{
  double size=32.0;
  Color color=Colors.red;
  IconData icon;
  IconContainer(this.icon,{this.color,this.size});
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Container(
      height: 100.0,
      width: 100.0,
      color: this.color,
      child: Center(
        child: Icon(this.icon,size: this.size,color: Colors.white)
      ),
    );
  }
}

MainAxisAlignment.start表示居于Container容器X轴开始位置也就是左侧
CrossAxisAlignment.start表示居于Container容器Y轴最上边
MainAxisAlignment.spaceEvenly表示控件均匀铺开

Column水平布局跟ROW用法一样

class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      height: 800.0,
      width: 400.0,
      color: Colors.pink,
      child: Column(    
        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
        crossAxisAlignment: CrossAxisAlignment.end,   
        children: <Widget>[
          IconContainer(Icons.search,color: Colors.blue),
          IconContainer(Icons.home,color: Colors.orange),
          IconContainer(Icons.select_all,color: Colors.red),
        ],
      ),
    );
  }
}

Flutter Expanded 类似 Web 中的 Flex 布局（权重的方式布局）

class LayoutDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        Row(
          children: <Widget>[
            Expanded(
              child:   Container(
                height: 180,
                color: Colors.black,
                child: Text('你好Flutter'),
              ),
            )
          ],
        ),
        SizedBox(height: 10),
        Row(
          children: <Widget>[
            Expanded(
              flex: 2,
              child: Container(
                height: 180,
                child: Image.network("https://www.itying.com/images/flutter/2.png",fit: BoxFit.cover),
              )
            ),
            SizedBox(width: 10),
            Expanded(
              flex: 1,
              child: Container(
                height: 180,
                child: ListView(
                  children: <Widget>[
                    Container(
                      height: 85,
                      child: Image.network("https://www.itying.com/images/flutter/3.png",fit: BoxFit.cover),
                    ),
                    SizedBox(height: 10),
                    Container(
                      height: 85,
                      child: Image.network("https://www.itying.com/images/flutter/4.png",fit: BoxFit.cover),
                    )
                  ],
                )
              )
            ),
          ],
        )
      ],
    );
  }
}

flex表示权重比例，同android 中的 weight。
内容比较简单，看代码 ，就不多说了。