Signature Class需要修改以响应VerticalDrag，我将其重命名为Signature1

现在签名区域键盘不应该滚动，您可以按照其行为检查下面的完整代码。您会发现Signature区域不再用滚动SingleChildScrollView。

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'dart:async';
import 'dart:ui' as ui;

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  var color = Colors.black;
  var strokeWidth = 3.0;
  final _sign = GlobalKey<Signature1State>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body:
           SingleChildScrollView(
             child: Column(
               children: <Widget>[
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 SizedBox(height: 15),
                 _showCategory(),
                 _showSignaturePad()
               ],
             ),
           )

      ,
    );
  }

  Widget _showCategory() {
    return TextField(
        onTap: () {
          FocusScope.of(context).requestFocus(FocusNode());
        },
        style: TextStyle(fontSize: 12.0, height: 1.0),
        decoration: InputDecoration(hintText: "TextView"));
  }

  Widget _showSignaturePad() {
    return Container(
      width: double.infinity,
      height: 200,
      child: Padding(
        padding: const EdgeInsets.all(8.0),
        child: Container(
          height: 200,
          //color: Colors.red,
          child: Signature1(
            color: color,
            key: _sign,
            strokeWidth: strokeWidth,
          ),
        ),
      ),
      color: Colors.grey.shade300,
    );
  }
}
class Signature1 extends StatefulWidget {
  final Color color;
  final double strokeWidth;
  final CustomPainter backgroundPainter;
  final Function onSign;

  Signature1({
    this.color = Colors.black,
    this.strokeWidth = 5.0,
    this.backgroundPainter,
    this.onSign,
    Key key,
  }) : super(key: key);

  Signature1State createState() => Signature1State();

  static Signature1State of(BuildContext context) {
    return context.findAncestorStateOfType<Signature1State>();
  }
}

class _SignaturePainter extends CustomPainter {
  Size _lastSize;
  final double strokeWidth;
  final List<Offset> points;
  final Color strokeColor;
  Paint _linePaint;

  _SignaturePainter({@required this.points, @required this.strokeColor, @required this.strokeWidth}) {
    _linePaint = Paint()
      ..color = strokeColor
      ..strokeWidth = strokeWidth
      ..strokeCap = StrokeCap.round;
  }

  @override
  void paint(Canvas canvas, Size size) {
    _lastSize = size;
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) canvas.drawLine(points[i], points[i + 1], _linePaint);
    }
  }

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

class Signature1State extends State<Signature1> {
  List<Offset> _points = <Offset>[];
  _SignaturePainter _painter;
  Size _lastSize;

  Signature1State();

  void _onDragStart(DragStartDetails details){
    RenderBox referenceBox = context.findRenderObject();
    Offset localPostion = referenceBox.globalToLocal(details.globalPosition);
    setState(() {
      _points = List.from(_points)
        ..add(localPostion)
        ..add(localPostion);
    });
  }
  void _onDragUpdate (DragUpdateDetails details) {
    RenderBox referenceBox = context.findRenderObject();
    Offset localPosition = referenceBox.globalToLocal(details.globalPosition);

    setState(() {
      _points = List.from(_points)..add(localPosition);
      if (widget.onSign != null) {
        widget.onSign();
      }
    });
  }
  void _onDragEnd (DragEndDetails details) => _points.add(null);

  @override
  Widget build(BuildContext context) {
    WidgetsBinding.instance.addPostFrameCallback((_) => afterFirstLayout(context));
    _painter = _SignaturePainter(points: _points, strokeColor: widget.color, strokeWidth: widget.strokeWidth);
    return ClipRect(
      child: CustomPaint(
        painter: widget.backgroundPainter,
        foregroundPainter: _painter,
        child: GestureDetector(

          onVerticalDragStart: _onDragStart,
          onVerticalDragUpdate: _onDragUpdate,
          onVerticalDragEnd: _onDragEnd,

          onPanStart: _onDragStart,
          onPanUpdate: _onDragUpdate,
          onPanEnd: _onDragEnd
        ),
      ),
    );
  }

  Future<ui.Image> getData() {
    var recorder = ui.PictureRecorder();
    var origin = Offset(0.0, 0.0);
    var paintBounds = Rect.fromPoints(_lastSize.topLeft(origin), _lastSize.bottomRight(origin));
    var canvas = Canvas(recorder, paintBounds);
    if(widget.backgroundPainter != null) {
      widget.backgroundPainter.paint(canvas, _lastSize);
    }
    _painter.paint(canvas, _lastSize);
    var picture = recorder.endRecording();
    return picture.toImage(_lastSize.width.round(), _lastSize.height.round());
  }

  void clear() {
    setState(() {
      _points = [];
    });
  }

  bool get hasPoints => _points.length > 0;

  List<Offset> get points => _points;

  afterFirstLayout(BuildContext context) {
    _lastSize = context.size;
  }
}