global position

观察,

GestureDetector(
            child: CustomPaint(painter: StudyPaint(points: _points)),
            onPanEnd: (DragEndDetails details)=>_points.add(null),
            onPanUpdate: (DragUpdateDetails details){
              var a = details.globalPosition;
              RenderBox referenceBox = context.findRenderObject();
              Offset localPosition = referenceBox.globalToLocal(details.globalPosition);
              print('a: $a, localposition: $localPosition');
              setState(() {
                _points = List.from(_points)..add(localPosition);
              });
            },
          ),

　　

 减了3个 h

import 'package:flutter/material.dart';

main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return HomePageState();
  }
}

class HomePageState extends State<HomePage> {
  List<Offset> _points = <Offset>[];

  @override
  Widget build(BuildContext context) {
//    print(_points);
  var h = MediaQuery.of(context).padding.top;
  var b = MediaQuery.of(context).padding.bottom;
//  print('h: $h, b: $b');

    return Scaffold(
      appBar: AppBar(title: Text('Painter'),),
      body: Container(
           MediaQuery.of(context).size.width, height: MediaQuery.of(context).size.height-h,
          child: Stack(
            children: <Widget>[
              GestureDetector(
//                child:
                onPanEnd: (DragEndDetails details)=>_points.add(null),
                onPanUpdate: (DragUpdateDetails details){
                  var a = details.globalPosition;
                  var x = Offset(a.dx, a.dy-h-h-h);
                  RenderBox referenceBox = context.findRenderObject();
                  Offset localPosition = referenceBox.globalToLocal(details.globalPosition);
//              print('a: $a, x: $x, localposition: $localPosition');
                  setState(() {
                    _points = List.from(_points)..add(x);
                  });
                },
              ),
              CustomPaint(painter: StudyPaint(points: _points)),
            ],
          )
      ),
    );
  }
}

class StudyPaint extends CustomPainter {
  StudyPaint({this.points});
  List<Offset> points;

  @override
  void paint(Canvas canvas, Size size) {
    print(points);
    Paint paint = Paint()
        ..color = Colors.blue
        ..strokeCap = StrokeCap.round
        ..isAntiAlias = true
      ..strokeJoin = StrokeJoin.bevel
        ..strokeWidth = 12.0;

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

  }

  @override
  bool shouldRepaint(StudyPaint other) => other.points!=points;
}

　　

import 'package:flutter/material.dart';

main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return HomePageState();
  }
}

class HomePageState extends State<HomePage> {
  List<Offset> _points = <Offset>[];

  @override
  Widget build(BuildContext context) {
//    print(_points);
  var h = MediaQuery.of(context).padding.top;
  print('h: $h');
    return Scaffold(
      appBar: AppBar(title: Text('Painter'),),
      body: Center(
        child: Container(
           double.infinity, height: double.infinity,
          child: GestureDetector(
            child: CustomPaint(painter: StudyPaint(points: _points)),
            onPanEnd: (DragEndDetails details)=>_points.add(null),
            onPanUpdate: (DragUpdateDetails details){
              var a = details.globalPosition;
              RenderBox referenceBox = context.findRenderObject();
              Offset localPosition = referenceBox.globalToLocal(details.globalPosition);
              print('a: $a, localposition: $localPosition');
              setState(() {
                _points = List.from(_points)..add(localPosition);
              });
            },
          ),
        ),
      ),
    );
  }
}

class StudyPaint extends CustomPainter {
  StudyPaint({this.points});
  List<Offset> points;

  @override
  void paint(Canvas canvas, Size size) {
    Paint paint = Paint()
        ..color = Colors.blue
        ..strokeCap = StrokeCap.round
        ..isAntiAlias = true
        ..strokeWidth = 12.0
        ..strokeJoin = StrokeJoin.bevel;

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

  }

  @override
  bool shouldRepaint(StudyPaint other) => other.points!=points;
}