Expanded 这是个用来让子项具有伸缩能力的widget

Expanded继承自Flexible，但是它们两个的区别并不大，看它们的构造方法：

class Expanded extends Flexible {
  /// Creates a widget that expands a child of a [Row], [Column], or [Flex]
  /// expand to fill the available space in the main axis.
  const Expanded({
    Key key,
    int flex: 1,
    @required Widget child,
  }) : super(key: key, flex: flex, fit: FlexFit.tight, child: child);
}

class Flexible extends ParentDataWidget<Flex> {
  /// Creates a widget that controls how a child of a [Row], [Column], or [Flex]
  /// flexes.
  const Flexible({
    Key key,
    this.flex: 1,
    this.fit: FlexFit.loose,
    @required Widget child,
  }) : super(key: key, child: child);
    ……
}

可见它们两个的默认灵活系数是一样的，但是fit参数不同，Expanded是默认要占满分配的空间的，而Flexible则默认不需要