Flutter，如何获取小部件的大小？

Question

我遇到这个例子来获取小部件的 Size/Position： https://medium.com/@diegoveloper/flutter-widget-size-and-position-b0a9ffed9407

我的代码有什么问题？

我收到错误：

The following NoSuchMethodError was thrown while handling a gesture:
I/flutter ( 8566): The method 'findRenderObject' was called on null.
I/flutter ( 8566): Receiver: null
I/flutter ( 8566): Tried calling: findRenderObject()

void main() {
      runApp(new MyApp());
    }

    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(home: MyWidgetSizeTMP());
      }
    }

    class MyWidgetSizeTMP extends StatefulWidget{
      @override
      MyWidgetSizeTMPState createState() {
        return new MyWidgetSizeTMPState();
      }
    }

    class MyWidgetSizeTMPState extends State<MyWidgetSizeTMP> {

      //creating Key for red panel
      GlobalKey _keyRed = GlobalKey();

      _getSizes() {
        final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
        final sizeRed = renderBoxRed.size;
        print("SIZE of Red: $sizeRed");
      }

      _getPositions() {
        final RenderBox renderBoxRed = _keyRed.currentContext.findRenderObject();
        final positionRed = renderBoxRed.localToGlobal(Offset.zero);
        print("POSITION of Red: $positionRed ");
      }

      _afterLayout(_) {
        _getSizes();
        _getPositions();
      }

      @override
      void initState() {
        // TODO: implement initState
        super.initState();
        WidgetsBinding.instance.addPostFrameCallback(_afterLayout);
      }

      @override
      Widget build(BuildContext context) {
        return Scaffold(
          appBar: AppBar(
          ),
          body: Column(
            children: <Widget>[
              Flexible(
                flex: 2,
                child: Container(
                  color: Colors.red,
                ),
              ),
              Flexible(
                flex: 1,
                child: Container(
                  color: Colors.purple,
                ),
              ),
              Flexible(
                flex: 3,
                child: Container(
                  color: Colors.green,
                ),
              ),
              Spacer(),
              Padding(
                padding: const EdgeInsets.only(bottom: 8.0),
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.spaceAround,
                  crossAxisAlignment: CrossAxisAlignment.center,
                  children: <Widget>[
                    MaterialButton(
                      elevation: 5.0,
                      padding: EdgeInsets.all(15.0),
                      color: Colors.grey,
                      child: Text("Get Sizes"),
                      onPressed: _getSizes,
                    ),
                    MaterialButton(
                      elevation: 5.0,
                      color: Colors.grey,
                      padding: EdgeInsets.all(15.0),
                      child: Text("Get Positions"),
                      onPressed: _getPositions,
                    )
                  ],
                ),
              )
            ],
          ),
        );
      }
    }

Answer 1

这是因为 _keyRed 没有附加到代码中的上下文。

修复它 -

 @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _keyRed,  //Add this.
      appBar: AppBar(

Answer 2

您可以在 LayoutBuilder 中包装：

return LayoutBuilder(
    builder: (BuildContext context, BoxConstraints constraints) {
//      height: constraints.maxHeight,
//      width: constraints.maxWidth
        return YourWidget();
    }
);

Answer 3

在您提到的 link 中，在创建 GlobalKey 之后，它被分配给 red 容器

但是在您的代码中您没有将它分配给容器，因此可以从 _getSizes() 和 _getPositions(), _keyRed 在使用之前必须附加到代码中的上下文。

//creating Key for red panel
GlobalKey _keyRed = GlobalKey();
    ...
    //set key
        Flexible(
                 flex: 2,
                 child: Container(
                   key: _keyRed, // assigned as a key of the red container.
                   color: Colors.red,
                 ),
         ),

Answer 4

没有直接的方法来计算小部件的大小，所以要找到我们必须借助小部件的上下文。

调用 context.size returns 我们 Size 对象，它包含小部件的高度和宽度。 context.size 计算小部件的渲染框和 returns 大小。

结帐https://medium.com/flutterworld/flutter-how-to-get-the-height-of-the-widget-be4892abb1a2

Flutter，如何获取小部件的大小？

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