flutter开发Row嵌套Row，Column嵌套Column，Row嵌套Column widget占空间大小

如果Row里面嵌套Row，或者Column里面再嵌套Column，那么只有对最外面的Row或Column会占用尽可能大的空间，里面Row或Column所占用的空间为实际大小，下面以Column为例说明：

Container(
  color: Colors.green,
  child: Padding(
    padding: const EdgeInsets.all(16.0),
    child: Column(
      crossAxisAlignment: CrossAxisAlignment.start,
      mainAxisSize: MainAxisSize.max, //有效，外层Colum高度为整个屏幕
      children: [
        Container(
          color: Colors.red,
          child: Column(
            mainAxisSize: MainAxisSize.max,//无效，内层Colum高度为实际高度  
            children: [
              Text("hello world "),
              Text("I am Jack "),
            ],
          ),
        )
      ],
    ),
  ),
);

如果是Row嵌套Cloumn，Cloumn会占用Row最大的高度，Row嵌套Cloumn不会影响到Cloumn空间占比，不方便我们widget排放，如果需要使Cloumn占用最小空间，进行设置 mainAxisSize: MainAxisSize.min,

Widget getUserInfo(PetCardViewModel model) {
    return Container(
      height: 150,
      margin: EdgeInsets.only(top: 16,bottom: 16),
      padding: EdgeInsets.symmetric(horizontal: 16),
      child: Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: [
          Row(
            children: [
              CircleAvatar(

                radius: 20,
                backgroundImage: NetworkImage(model.userImgUrl),
                backgroundColor: Colors.red,
              ),
              Padding(padding: EdgeInsets.only(left: 8)),
            Container(
              color: Colors.orange,
              child:     Column(
                mainAxisSize: MainAxisSize.min,
                children: [
                  Text(
                    model.userName,
                    style: TextStyle(
                        fontSize: 14,
                        fontWeight: FontWeight.bold,
                        color: Colors.black),
                  ),
                  Padding(padding: EdgeInsets.only(top: 6)),
                  Text(
                    model.description,
                    style: TextStyle(fontSize: 12, color: Colors.black38),
                  )
                ],
              ),
            )
            ],
          ),
          Text(
            model.publishTime,
            style: TextStyle(fontSize: 12, color: Colors.black38),
          ),
        ],
      ),
    );
  }

设置最小占用空间

Row嵌套Column，会默认占用Row最大高度