Stack个人感觉就是iOS中的 x,y,left,right …
Stack与Positioned配合使用，可以用于从left top bottom right等方面指定子widget的位置

Stack({
    Key key,
    this.alignment = AlignmentDirectional.topStart,  //对齐方式,9个位置.
    this.textDirection,                              //对齐方向
    this.fit = StackFit.loose,                       //此参数用于决定 non-positioned子Widget 如何去适应Stack的大小
    this.overflow = Overflow.clip,                   //决定如何显示超出 Stack显示空间的 子widget
    List<Widget> children = const <Widget>[],        // 
  }) : super(key: key, children: children);

Stack(
  children: <Widget>[
    //因为默认对齐方式是 topStart 所以右上角开始
    Image.network(
      "https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1586249335016&di=3e3b2783d09385b45eb65293f8101352&imgtype=0&src=http%3A%2F%2Fb-ssl.duitang.com%2Fuploads%2Fitem%2F201805%2F12%2F20180512104335_ka4UX.thumb.700_0.jpeg",
      width: 200,
      height: 200,
    ),
    Text(
        "嗨,大长腿",
      style: TextStyle(
        fontSize: 30,
        color: Colors.white
      ),
    ),
  ],
),

1. alignment、textDirection 略过其他文章里有

2. fit: non-positioned子Widget 如何去适应Stack的大小

fit: StackFit.expand, //子Widget 扩伸到Stack的大小

StackFit.loose   // 使用 子Widget 自身的大小

StackFit.passthrough   //Stack的父Widget 的约束无修改的传递给 Stack的子Widget

3. overflow 决定如何显示超出 Stack显示空间的widget

Positioned

 child: Stack(
    alignment: AlignmentDirectional.centerStart,
    children: <Widget>[
      Positioned(
        left: 10,
        top: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.orange),
        ),
      ),
      Positioned(
        right: 10,
        top: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
      Positioned(
        right: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
      Positioned(
        bottom: 10,
        left: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
      Positioned(
        bottom: 10,
        right: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
      Positioned(
        right: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
      Positioned(
        //当Stack 设置 alignment: AlignmentDirectional.centerStart 时 不设置上下间距默认是居中,其他同理
        left: 10,
        child: Text(
          "Positioned",style: TextStyle(backgroundColor: Colors.blue),
        ),
      ),
    ],
  ),

Positioned 和iOS的UIView布局感觉很多都一样.就不在记录了