Flutter之ConstrainedBox、SizedBox、UnconstrainedBox（尺寸限制类容器）

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、

  @overrideWidget 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、

  @overrideWidget 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,),),),),),);}
}

测试3、

@overrideWidget 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: UnconstrainedBox(child: ConstrainedBox(constraints: BoxConstraints(minWidth: 50,minHeight: 50),child: Container(width: 10,height: 15,color: Colors.red,),),),),),),);}
}