1 ConstrainedBox、SizedBox、UnconstrainedBox介绍

1)、ConstrainedBox用于对子组件添加额外的约束。例如，如果你想让子组件的最小高度是80像素

      ConstrainedBox({
        Key key,
        @required this.constraints,
        Widget child,
      })

我们可以看到这里有个constraints

  final BoxConstraints constraints;

    class BoxConstraints extends Constraints {
      /// Creates box constraints with the given constraints.
      const BoxConstraints({
        this.minWidth = 0.0,
        this.maxWidth = double.infinity,
        this.minHeight = 0.0,
        this.maxHeight = double.infinity,
      }) : assert (minWidth != null),
           assert (maxWidth != null),
           assert (minHeight != null),
           assert (maxHeight != null);

我们可以看到BoxConstraints继承Constraints，然后一些属性设置。

有多重ConstrainedBox限制时，对于minWidth和minHeight来说，是取父子中相应数值较大的

2)、SizedBox：用于给子元素指定固定的宽高

3)、UnconstrainedBox：不会对子组件产生任何限制，它允许其子组件按照其本身大小绘制

一般用来去掉父约束

 
2 测试代码

测试1、

      @override
      Widget build(BuildContext context) {
          return MaterialApp(
              title: 'open url',
              home: Scaffold(
                appBar: AppBar(
                  // Here we take the value from the MyHomePage object that was created by
                  // the App.build method, and use it to set our appbar title.
                  title: Text('hello flutter'),
                ),
                body: Center(
                  child: ConstrainedBox(
                      constraints: BoxConstraints(
                          minWidth: double.infinity,
                          minHeight: 100
                      ),
                    child: Container(
                        width: 50,
                        height: 50,
                        color: Colors.green,
                    ),
                  ),
                ),
              ),
          );
      }
    }

测试2、

      @override
      Widget build(BuildContext context) {
          return MaterialApp(
              title: 'open url',
              home: Scaffold(
                appBar: AppBar(
                  title: Text('hello flutter'),
                ),
                body: Center(
                  child: ConstrainedBox(
                      constraints: BoxConstraints(
                          minWidth: 200,
                          minHeight: 100
                      ),
                    child: ConstrainedBox(
                      constraints: BoxConstraints(
                          minWidth: 100,
                          minHeight: 200
                      ),
                      child:  Container(
                        width: 50,
                        height: 50,
                        color: Colors.green,

更多请见：http://www.mark-to-win.com/tutorial/51853.html