column和row这两者的混合使用会经常导致[RenderFlex children have non-zero flex but incoming height constraints are unbounded]问题。
起因：
「column里面套column」、「row里面套row」，都会导致上面的问题。原因是column和row都会给自己的孩子「在主方向上设置一个类似null或者0的弹性因子，并且主方向上无限制」
分析：
column源码注释：
/// Layout for a [Column] proceeds in six steps:
///
/// 1. Layout each child a null or zero flex factor (e.g., those that are not
/// [Expanded]) with unbounded vertical constraints and the incoming
/// horizontal constraints. If the [crossAxisAlignment] is
/// [CrossAxisAlignment.stretch], instead use tight horizontal constraints
/// that match the incoming max width.
/// 2. Divide the remaining vertical space among the children with non-zero
/// flex factors (e.g., those that are [Expanded]) according to their flex
/// factor. For example, a child with a flex factor of 2.0 will receive twice
/// the amount of vertical space as a child with a flex factor of 1.0.
/// 3. Layout each of the remaining children with the same horizontal
/// constraints as in step 1, but instead of using unbounded vertical
/// constraints, use vertical constraints based on the amount of space
/// allocated in step 2. Children with [Flexible.fit] properties that are
/// [FlexFit.tight] are given tight constraints (i.e., forced to fill the
/// allocated space), and children with [Flexible.fit] properties that are
/// [FlexFit.loose] are given loose constraints (i.e., not forced to fill the
/// allocated space).
/// 4. The width of the [Column] is the maximum width of the children (which
/// will always satisfy the incoming horizontal constraints).
/// 5. The height of the [Column] is determined by the [mainAxisSize] property.
/// If the [mainAxisSize] property is [MainAxisSize.max], then the height of
/// the [Column] is the max height of the incoming constraints. If the
/// [mainAxisSize] property is [MainAxisSize.min], then the height of the
/// [Column] is the sum of heights of the children (subject to the incoming
/// constraints).
/// 6. Determine the position for each child according to the
/// [mainAxisAlignment] and the [crossAxisAlignment]. For example, if the
/// [mainAxisAlignment] is [MainAxisAlignment.spaceBetween], any vertical
/// space that has not been allocated to children is divided evenly and
/// placed between the children.
第一步：对每个孩子设置一个竖直方向上null或者0的弹性因子（Expanded除外）且竖直方向上没有限制
第二步：将剩下的空间（孩子中有一些能确定自己大小的组件，如Text，能明确自己的大小，会成为第一批获得空间的组件，所以第二步中，描述为剩下的空间），给孩子中的Expanded们根据他们设置的flex分配。（执行完第二步之后，所有能确定自己大小的组件（如Text、固定高度的Container），以及使用Expanded的组件，都能明确自己的竖直方向的约束）
第三四五六步对本次的问题没有影响，不讨论。
可以看到，除了固定高度的组件以及Expanded组件，其他组件没有都是没办法限制高度的。因为Column本身的高度是有限制的，如果它的孩子高度没有限制，那是渲染不出来的。
异常例子
比如说:
Column(children:[
Text("a widget that can determine height and width"),
Column(children:[
Expanded(child:Container(color:Colors.blue),),
]),
]);
原因：因为第一层column会给它的所有孩子设置0的弹性，所以第二层的column是不知道自己的约束的，最后来到Expanded，这时候Expanded向自己的父节点要求撑满，轮到第二层的column懵逼了。它心里逼逼赖赖：我的父节点没给我约束，我的子节点又要求撑满，左右为难！直接抛出错误！
解决方法1：
如果第二层column在没有获得父组件给的约束时，自己的孩子又都是能确定大小的话，是不会报错的：
Column(children: [
Text("a widget that can determine height and width"),
Column(
children: [
Text("one"),
Text("Two"),
Container(
height: 50,
child: Column(children: [
Expanded(
child: Container(color: Colors.blue),
),
]),
),
],
),
]);
解决方法2：
如果第二层column外面裹上一层Expanded，那就可以在第二步里面获得剩余的空间，即获得约束，这样在第二层Column的孩子要撑满的时候，就可以给他提供确切的大小。
Column(children:[
Text("a widget that can determine height and width"),
Expanded(child:
Column(children:[
Expanded(child:Container(
color:Colors.blue),),
Text("Two"),
]),
),
]);
参考：stackoverflow.com/questions/5…
同样的情况适用于row。
