不能将类型为 'RenderObject?' 的值分配给类型为"RenderBox"的变量

Question

我正在尝试在flutter中构建一个非常简单的绘画应用程序。我得到了">"RenderObject？"类型的值无法将其分配给"RenderBox"类型的变量"；错误

我附加了下面的代码，错误在第18行(即RenderBox object=context.findRenderObject((；(

import 'package:flutter/material.dart';
class drawPage extends StatefulWidget {
@override
_drawPageState createState() => new _drawPageState();
}
class _drawPageState extends State<drawPage> {
List<Offset> _points = <Offset>[];
@override
Widget build(BuildContext context) {
return new Scaffold(
body: new Container(
child: new GestureDetector(
onPanUpdate: (DragUpdateDetails details) {
setState(() {
RenderBox object = context.findRenderObject();
Offset _localPosition =
object.globalToLocal(details.globalPosition);
_points = new List.from(_points)..add(_localPosition);
});
},
onPanEnd: (DragEndDetails details) => _points.add(Offset.zero),
child: new CustomPaint(
painter: new Signature(points: _points),
size: Size.infinite,
),
),
),
floatingActionButton: new FloatingActionButton(
child: new Icon(Icons.clear),
onPressed: () => _points.clear(),
),
);
}
}
class Signature extends CustomPainter {
List<Offset> points;
Signature({this.points});
@override
void paint(Canvas canvas, Size size) {
Paint paint = new Paint()
..color = Colors.blue
..strokeCap = StrokeCap.round
..strokeWidth = 10.0;
for (int i = 0; i < points.length - 1; i++) {
if (points[i] != Offset.zero && points[i + 1] != Offset.zero) {
canvas.drawLine(points[i], points[i + 1], paint);
}
}
}
@override
bool shouldRepaint(Signature oldDelegate) => oldDelegate.points != points;
}

Answer 1

试试这个：

RenderBox object = context.findRenderObject() as RenderBox;

我们应该将泛型类型转换为确定性类型，请参阅的官方链接