flutter - 在构建期间获取 Widget 的位置和大小

Question

如何在 build() 期间获取小部件的大小和位置？方法？在下面的代码中，我想在 CustomPaint 中绘制矩形居中的小部件，占视口(viewport)剩余区域的一定百分比，即 Text 下方的所有区域小部件，标记为 (A) .我可以从 MediaQuery 获得宽度，但是我如何获得 (A) 之后的剩余高度?我必须知道 Text 的位置小部件。
This answer不起作用，因为 Flutter 在尝试检索 RenderBox 时抛出异常在构建()期间。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('App')),
      body: Column(children: [
        Text('Test'), // (A) Get position of this
        CustomPaint(
          painter: DrawArea(MediaQuery.of(context).size),
          size: Size(1000, 800),
        ),
      ]),
    );
  }
}

class DrawArea extends CustomPainter {
  final Size viewportSize;

  DrawArea(this.viewportSize);

  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint()
      ..color = Colors.black
      ..strokeWidth = 1
      ..style = PaintingStyle.stroke;

    double yOffset = (this.viewportSize.width - this.viewportSize.width * .95) / 2;
    canvas.drawRect(Rect.fromLTWH(yOffset, 0, this.viewportSize.width * .95, 800), paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

Answer 1

paint 的第二个参数方法是CustomPaint的大小小部件。您只需要确保您的小部件尺寸合适。
制作 CustomPaint占用 Column 中的剩余空间，您需要将其包装在 Expanded 中小部件:

      body: Column(children: [
        Text('Test'),
        Expanded(
          child: CustomPaint(
            painter: DrawArea(),
            size: Size.infinite, // make it as large as possible
          ),
        ),
      ]),

然后你可以使用传递的size自定义画家中的参数:

class DrawArea extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint()
      ..color = Colors.black
      ..strokeWidth = 1
      ..style = PaintingStyle.stroke;

    double xOffset = size.width * 0.025
    double yOffset = size.height * 0.025
    canvas.drawRect(Rect.fromLTWH(xOffset, yOffset, size.width * .95, size.height * .95), paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}