CustomPainter在AlertDialog上不起作用

Question

我正在尝试使用CustomPainter类从用户那里获得签名。 当我尝试在新页面上使用它时，它效果很好。问题是，它在AlertDialog上不起作用。

这是我的CustomPainter的代码。

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

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

class _SignatureState extends State<Signature> {

  List<Offset> _points = <Offset>[];

  Future<ui.Image> get rendered{
    ui.PictureRecorder recorder = ui.PictureRecorder();
    Canvas canvas = Canvas(recorder);
    SignaturePainter painter = SignaturePainter(points: _points);
    var size = context.size;
    painter.paint(canvas, size);
    return recorder.endRecording().toImage(size.width.floor(), size.height.floor());

  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: new Container(
          child: GestureDetector(
            onPanUpdate: (DragUpdateDetails details) {
              setState(() {
                RenderBox _object = context.findRenderObject();
                Offset _locationPoints = _object.localToGlobal(details.globalPosition);
                _points = new List.from(_points)..add(_locationPoints);
              });
            },
            onPanEnd: (DragEndDetails details) {
              setState(() {
                _points.add(null);
              });
            },
            child: CustomPaint(painter: SignaturePainter(points: _points),
                size: Size.infinite),
          ),
        )
    );
  }

  void clearPoints(){
    setState(() {
      _points.clear();
    });
  }
}

class SignaturePainter extends CustomPainter {

  List<Offset> points = <Offset>[];

  SignaturePainter({this.points});
  @override
  void paint(Canvas canvas, Size size) {
    var paint = Paint()
      ..color = Colors.black
      ..strokeCap = StrokeCap.square
      ..strokeWidth = 5.0;

    for(int i=0; i < points.length - 1; i++) {
      if(points[i] != null && points[i+1] != null) {
        canvas.drawLine(points[i], points[i+1], paint);
      }
    }
  }

  @override
  bool shouldRepaint(SignaturePainter oldDelegate) {
    return oldDelegate.points != points;
  }

}

这是调用AlertDialog的支架主体。

body: new Center(
        child: new Container(
          width: screenWidth(context, ratio:0.589),
          height: screenHeight(context, ratio: 0.9),
          child: new AlertDialog(
            contentPadding: EdgeInsets.only(left: screenWidth(context, ratio: 0.065), right: screenWidth(context, ratio: 0.065)),
            content: new Column(
              children: <Widget>[
                new Padding(padding: EdgeInsets.only(top: screenHeight(context, ratio: 0.028)),),
                new Container(
                  color: Colors.lightGreenAccent,
                  width: screenWidth(context, ratio: 0.4414),
                  height: screenHeight(context, ratio: 0.105),
                  child: Text("General condition part.1 gonna be here"),
                ),
                new Padding(padding: EdgeInsets.only(top: screenHeight(context, ratio: 0.0156)),),
                new Container(
                  color: Colors.lightGreenAccent,
                  width: screenWidth(context, ratio: 0.4414),
                  height: screenHeight(context, ratio: 0.105),
                  child: Text("General condition part.2 gonna be here"),
                ),
                new Padding(padding: EdgeInsets.only(top: screenHeight(context, ratio: 0.0156)),),
                new Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    new Text("Acknowledged and agreed on: ${DateTime.now().month}, ${DateTime.now().day}, ${DateTime.now().year}"),
                    new Padding(padding: EdgeInsets.only(top: screenHeight(context, ratio: 0.02))),
                    new Row(
                      children: <Widget>[
                        new Text("Name"),
                        new Padding(padding: EdgeInsets.only(left: screenWidth(context, ratio: 0.08)), child: Text(globals.name),),
                        new Padding(padding: EdgeInsets.only(left: screenWidth(context, ratio: 0.05)), child: Text("Date"),),
                        new Padding(padding: EdgeInsets.only(left: screenWidth(context, ratio: 0.08)), child: Text("${DateTime.now().month}, ${DateTime.now().day}, ${DateTime.now().year}"),),

                      ],
                    ),
                    new Container(
                      color: Colors.amberAccent,
                      height: screenHeight(context, ratio: 0.2),
                      child: new Signature(key: signatureKey),
                    )
                  ],
                ),
                new MaterialButton(
                  minWidth: screenWidth(context, ratio: 0.37),
                  height: screenHeight(context, ratio: 0.061),
                  color: Color.fromRGBO(66, 230, 123, 100),
                  shape: OutlineInputBorder(),
                  child: Text("Submit", style: TextStyle(color: Colors.white),),
                  onPressed: (){
                    setState(() {
                      signFlag = false;
                    });

                )
              ],
            ),
          ),
        ),
      )

我希望我可以在画布区域上绘制线条，但是当我尝试在该区域上绘制某些东西时，什么也不会发生。 我不能在对话框上使用CustomPainter吗？