Flutter FractionallySizedBox组件

FractionallySizedBox组件常用来设置子元素占用父元素的宽度，先看下它的构造函数：

const FractionallySizedBox({
    Key key,
    this.alignment = Alignment.center,
    this.widthFactor,
    this.heightFactor,
    Widget child,
})

具体解析：
1.widthFactor:FractionallySizedBox组件的宽度因子
    widthFactor != null时,子组件的宽度等于FractionallySizedBox组件的约束的最大宽度乘以widthFractor
    widthFractor == null ```时,FractionallySizedBox组件的宽度约束会原封不动的传递给子组件
2.heightFractor: FractionallySizedBox组件的高度因子
    heightFractor != null时,子组件的高度等于FractionallySizedBox组件的约束的最大高度乘以heightFractor
    heightFractor == null ```时,FractionallySizedBox组件的高度约束会原封不动的传递给子组件
3.alignment:FractionallySizedBox组件中的子组件的对齐方式
4.child:FractionallySizedBox组件中的子组