flutter - 自定义 Painter 类在 Stack Flutter 中不可见

Question

由于某种原因，Stack 小部件没有显示带有 CustomPaint 的 Container。

但是，如果从 Stack 中删除，它可以正常工作。我在这里错过了什么？

class _DemoNavBar extends State<DemoNavBar> {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
        home:
        Stack(
          children: <Widget>[
            Container(child: CustomPaint(painter: CurvePainter()))
      ],
    )
    );
  }
}

class CurvePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint();
    paint.color = Colors.green[800];
    paint.style = PaintingStyle.fill;

    var path = Path();

    path.moveTo(0, size.height - 100); 

    path.lineTo(size.width * 0.5, size.height - 100); 
    path.quadraticBezierTo(size.width * 0.7, size.height, size.width * 0.9,
        size.height - 100); 
    path.lineTo(size.width, size.height - 100); 
    path.lineTo(size.width, size.height); 
    path.lineTo(0, size.height);
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }

谢谢!

Answer 1

检查 CustomPaint 的来源

  /// The size that this [CustomPaint] should aim for, given the layout
  /// constraints, if there is no child.
  ///
  /// Defaults to [Size.zero].
  ///
  /// If there's a child, this is ignored, and the size of the child is used
  /// instead.

所以，给它一个大小。其他解决方案包括 1) 为 CustomPaint 的父 Container 提供宽度和高度，以及 2) 为 CustomPaint 提供一个子项，该子项将忽略 size在下面的解决方案中提供。

---

我检查了这段代码工作正常。size: MediaQuery.of(context).size 使用完整的屏幕尺寸。

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

class SO extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: DemoNavBar(),
    );
  }
}

class DemoNavBar extends StatefulWidget {
  @override
  _DemoNavBar createState() => _DemoNavBar();
}

class _DemoNavBar extends State<DemoNavBar> {
  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        CustomPaint(
          size: MediaQuery.of(context).size,
          painter: CurvePainter(),
        )
      ],
    );
  }
}

class CurvePainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint();
    paint.color = Colors.green[800];
    paint.style = PaintingStyle.fill;

    var path = Path();

    path.moveTo(0, size.height - 100);

    path.lineTo(size.width * 0.5, size.height - 100);
    path.quadraticBezierTo(size.width * 0.7, size.height, size.width * 0.9, size.height - 100);
    path.lineTo(size.width, size.height - 100);
    path.lineTo(size.width, size.height);
    path.lineTo(0, size.height);
    canvas.drawPath(path, paint);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}

---

现在，原因是因为 Container 没有父或子来提供它需要完整屏幕大小的大小，并且在没有 Stack 的情况下也可以正常工作。当使用堆栈时，大小会变为零，这会提供给自定义画家。

等价的代码可以写成

Stack(
  children: <Widget>[
    Container(
      width: MediaQuery.of(context).size.width,
      height: MediaQuery.of(context).size.height,
      child: CustomPaint(
        painter: CurvePainter(),
      ),
    )
  ],
);

最终结果是