Stack

 Stack({
    Key key,
    this.alignment = AlignmentDirectional.topStart,
    this.textDirection,
    this.fit = StackFit.loose,
    this.overflow = Overflow.clip,
    List<Widget> children = const <Widget>[],
  })

AlignmentDirectional

  static const AlignmentDirectional topStart = AlignmentDirectional(-1.0, -1.0);

  static const AlignmentDirectional topCenter = AlignmentDirectional(0.0, -1.0);

  static const AlignmentDirectional topEnd = AlignmentDirectional(1.0, -1.0);

  static const AlignmentDirectional centerStart = AlignmentDirectional(-1.0, 0.0);

  static const AlignmentDirectional center = AlignmentDirectional(0.0, 0.0);

  static const AlignmentDirectional centerEnd = AlignmentDirectional(1.0, 0.0);

  static const AlignmentDirectional bottomStart = AlignmentDirectional(-1.0, 1.0);

  static const AlignmentDirectional bottomCenter = AlignmentDirectional(0.0, 1.0);

  static const AlignmentDirectional bottomEnd = AlignmentDirectional(1.0, 1.0);

StackFit

loose, 指的是子Widget 多大就多大
expand, 使子Widget的大小和父组件一样大
passthrough, 从其父堆栈传递到堆栈的约束未经修改地传递到未定位的子级。

Overflow

Widget 超出Stack时候如何显示

visible 超出部分还会显示的
clip 子Widget超出Stack会被截断，