1 FractionallySizedBox
一个widget，它把它的子项放在可用空间的一小部分
2 构造函数
FractionallySizedBox({
Key key,
this.alignment = Alignment.center,
this.widthFactor,
this.heightFactor,
Widget child,
})3 常用属性
3.1 alignment：对齐方式
alignment:Alignment.topLeft,3.1.1 顶部左边
alignment:Alignment.topLeft,3.1.2 顶部中间
alignment:Alignment.topCenter,3.1.3 顶部右边
alignment:Alignment.topRight,3.1.4 中部左边
alignment:Alignment.centerLeft,3.1.5 中部中间
alignment:Alignment.center,3.1.6 中部右边
alignment:Alignment.centerRight,3.1.7 底部左边
alignment:Alignment.bottomLeft,3.1.8 底部中间
alignment:Alignment.bottomCenter,3.1.9 底部右边
alignment:Alignment.bottomRight,3.2 widthFactor：FractionallySizedBox组件的宽度
widthFactor: 0.5,3.3 heightFactor：FractionallySizedBox组件的高度
heightFactor: 0.5,3.4 child：子widget
child: Text('你好 Flutter'),正文完