flutter-获取父widget的大小

方法就是 在原组件的外面加一层LayoutBuilder
注释的第一句话白话翻译: 依据父widget的大小创建一棵widget树

/// Builds a widget tree that can depend on the parent widget's size.
///
/// Similar to the [Builder] widget except that the framework calls the [builder]
/// function at layout time and provides the parent widget's constraints. This
/// is useful when the parent constrains the child's size and doesn't depend on
/// the child's intrinsic size. The [LayoutBuilder]'s final size will match its
/// child's size.
///
/// {@youtube 560 315 https://www.youtube.com/watch?v=IYDVcriKjsw}
///
/// If the child should be smaller than the parent, consider wrapping the child
/// in an [Align] widget. If the child might want to be bigger, consider
/// wrapping it in a [SingleChildScrollView].
///
/// See also:
///
///  * [SliverLayoutBuilder], the sliver counterpart of this widget.
///  * [Builder], which calls a `builder` function at build time.
///  * [StatefulBuilder], which passes its `builder` function a `setState` callback.
///  * [CustomSingleChildLayout], which positions its child during layout.
///  * The [catalog of layout widgets](https://flutter.dev/widgets/layout/).
class LayoutBuilder extends ConstrainedLayoutBuilder {
...}

 @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return LayoutBuilder(
        builder: (context, constrains){
         var viewWidth = constrains.maxWidth;
          var viewHeight =  constrains.maxHeight ;
          return getContentView();
        });

  }