Flutter 自定义涂鸦画板，基于 WebSocket 实现...

随着 flutter 的兴起，越来越多的公司开始使用 flutter ，最近一老同事问我关于如何使用 flutter 实现一个你画我猜的小游戏，现把这个分享给大家~

已实现的功能

• 画板自由涂鸦
• 选择画笔颜色
• 选择画笔大小
• 撤销到上一步
• 反撤销
• 清空画布
• 橡皮擦
• 基于 WebSocket 实时发送到服务器
• WebSocket 服务端转发给其它连接
• 接受 WebSocket 的消息内容绘制

使用到的技术

• 基础组件的使用（Scaffold、AppBar、IconButton、Container、Column、Stack、Padding、Icon 等）
• 自定义 CustomPainter ，在 Canvas 上使用 Paint 绘制
• 手势识别 GestureDetector 事件的使用
• Flutter 基于 Provider 插件的状态管理实现
• 简单的实现 WebSocket 通讯（真实项目考虑的问题要更多，比如心跳，重连，网络波动处理等）

最终效果

三屏实时同步

实战开始

• 打开 pubspec.yaml 引用状态管理和 WebSocket 库

dev_dependencies:
  flutter_test:
    sdk: flutter

  provider: ^4.0.1
  web_socket_channel: ^1.1.0

• 创建 draw_entity.dart 实体类

import 'package:flutter/widgets.dart';

//基础实体（ pengzhenkun - 2020.04.30 ）
class DrawEntity {
  Offset offset;
  String color;
  double strokeWidth;

  DrawEntity(this.offset, {this.color = "default", this.strokeWidth = 5.0});
}

• 创建 signature_painter.dart 自定义画板

import 'package:flutter/material.dart';
import 'package:fluttercontrol/page/drawguess/draw_entity.dart';
import 'package:fluttercontrol/page/drawguess/draw_provider.dart';

//自定义 Canvas 画板（ pengzhenkun - 2020.04.30 ）
class SignaturePainter extends CustomPainter {
  List<DrawEntity> pointsList;
  Paint pt;

  SignaturePainter(this.pointsList) {
    pt = Paint() //设置笔的属性
      ..color = pintColor["default"]
      ..strokeCap = StrokeCap.round
      ..isAntiAlias = true
      ..strokeWidth = 3.0
      ..style = PaintingStyle.stroke
      ..strokeJoin = StrokeJoin.bevel;
  }

  void paint(Canvas canvas, Size size) {
    for (int i = 0; i < pointsList.length - 1; i++) {
      //画线
      if (pointsList[i] != null && pointsList[i + 1] != null) {
        pt
          ..color = pintColor[pointsList[i].color]
          ..strokeWidth = pointsList[i].strokeWidth;

        canvas.drawLine(pointsList[i].offset, pointsList[i + 1].offset, pt);
      }
    }
  }

//是否重绘
  bool shouldRepaint(SignaturePainter other) => other.pointsList != pointsList;
}

• 创建 draw_provider.dart 状态管理
• 记录 撤销的数据、存储要画的数据、预处理的数据、默认颜色、默认字体大小、Socket连接（为了好理解，Socket连接也写在了此类）

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:fluttercontrol/page/drawguess/draw_entity.dart';
import 'package:web_socket_channel/web_socket_channel.dart';
import 'package:web_socket_channel/io.dart';

//可选的画板颜色
Map<String, Color> pintColor = {
  'default': Color(0xFFB275F5),
  'black': Colors.black,
  'brown': Colors.brown,
  'gray': Colors.grey,
  'blueGrey': Colors.blueGrey,
  'blue': Colors.blue,
  'cyan': Colors.cyan,
  'deepPurple': Colors.deepPurple,
  'orange': Colors.orange,
  'green': Colors.green,
  'indigo': Colors.indigo,
  'pink': Colors.pink,
  'teal': Colors.teal,
  'red': Colors.red,
  'purple': Colors.purple,
  'blueAccent': Colors.blueAccent,
  'white': Colors.white,
};

//数据管理 WebSocket，基础数据，通讯，连接维护等（ pengzhenkun - 2020.04.30 ）
class DrawProvider with ChangeNotifier {
  final String _URL = 'ws://10.10.3.55:8080/mini';

  List<List<DrawEntity>> undoPoints = List<List<DrawEntity>>(); // 撤销的数据
  List<List<DrawEntity>> points = List<List<DrawEntity>>(); // 存储要画的数据
  List<DrawEntity> pointsList = List<DrawEntity>(); //预处理的数据，避免绘制时处理卡顿
  String pentColor = "default";//默认颜色
  double pentSize = 5;//默认字体大小

  //Socket 连接
  WebSocketChannel _channel;

  //开始连接
  connect() {
    _socketConnect();
  }

  _socketConnect() {
    _channel = IOWebSocketChannel.connect(_URL);
    _channel.stream.listen(
      (message) {
        //监听到的消息
        print("收到消息：$message");
        message = jsonDecode(message);
        if (message["type"] == "sendDraw") {
          //正在连续绘制
          if (points.length == 0) {
            points.add(List<DrawEntity>());
            points.add(List<DrawEntity>());
          }
          pentColor = message["pentColor"];
          pentSize = message["pentSize"];
          //添加绘制
          //添加绘制
          points[points.length - 2].add(DrawEntity(
              Offset(message["dx"], message["dy"]),
              color: pentColor,
              strokeWidth: pentSize));
          //通知更新
          setState();
        } else if (message["type"] == "sendDrawNull") {
          //手抬起，添加占位
          //添加绘制标识
          points.add(List<DrawEntity>());
          //通知更新
          setState();
        } else if (message["type"] == "clear") {
          //清空画板
          points.clear();
          //通知更新
          setState();
        } else if (message["type"] == "sendDrawUndo") {
          //撤销，缓存到撤销容器
          undoPoints.add(points[points.length - 3]); //添加到撤销的数据里
          points.removeAt(points.length - 3); //移除数据
          //通知更新
          setState();
        } else if (message["type"] == "reverseUndoDate") {
          //反撤销数据
          List<DrawEntity> ss = undoPoints.removeLast();
          points.insert(points.length - 2, ss);
          //通知更新
          setState();
        }
      },
      onDone: () {
        print("连接断开 onDone");
        //尝试重新连接
        _socketConnect();
      },
      onError: (err) {
        print("连接异常 onError");
      },
      cancelOnError: true,
    );
  }

  //清除数据
  clear() {
    //清除数据
    points.clear();
    //通知更新
    setState();
    _channel.sink
        .add(jsonEncode({'uuid': 'xxxx', 'type': 'clear', 'msg': 'clear'}));
  }

  //绘制数据
  sendDraw(Offset localPosition) {
    if (points.length == 0) {
      points.add(List<DrawEntity>());
      points.add(List<DrawEntity>());
    }
    //添加绘制
    points[points.length - 2].add(
        DrawEntity(localPosition, color: pentColor, strokeWidth: pentSize));
//    points.add(localPosition);
    //通知更新
    setState();

    //发送绘制消息给服务端
    _channel.sink.add(jsonEncode({
      'uuid': 'xxxx',
      'type': 'sendDraw',
      'pentColor': pentColor,
      'pentSize': pentSize,
      "dx": localPosition.dx,
      "dy": localPosition.dy
    }));
  }

  //绘制Null数据隔断标识
  sendDrawNull() {
    //添加绘制标识
    points.add(List<DrawEntity>());
    //通知更新
    setState();
    //发送绘制消息给服务端
    _channel.sink.add(jsonEncode({'uuid': 'xxxx', 'type': 'sendDrawNull'}));
  }

  //撤销一条数据
  undoDate() {
    //撤销，缓存到撤销容器
    undoPoints.add(points[points.length - 3]); //添加到撤销的数据里
    points.removeAt(points.length - 3); //移除数据
    setState();
    //发送绘制消息给服务端
    _channel.sink.add(jsonEncode({'uuid': 'xxxx', 'type': 'sendDrawUndo'}));
  }

  //反撤销一条数据
  reverseUndoDate() {
    List<DrawEntity> ss = undoPoints.removeLast();
    points.insert(points.length - 2, ss);

    setState();
    //发送绘制消息给服务端
    _channel.sink.add(jsonEncode({'uuid': 'xxxx', 'type': 'reverseUndoDate'}));
  }

  @override
  void dispose() {
    _channel.sink?.close();
    super.dispose();
  }

  _update() {
    pointsList = List<DrawEntity>();
    for (int i = 0; i < points.length - 1; i++) {
      pointsList.addAll(points[i]);
      pointsList.add(null);
    }
  }

  setState() {
    _update();
    notifyListeners();
  }
}

• 有了以上的实现后，创建 draw_page.dart 搭建我们的主页面

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'package:fluttercontrol/page/drawguess/draw_provider.dart';
import 'package:fluttercontrol/page/drawguess/widget/signature_painter.dart';
import 'package:provider/provider.dart';

//绘制布局页面 （ pengzhenkun - 2020.04.30 ）
class DrawPage extends StatefulWidget {
  @override
  _DrawPageState createState() => _DrawPageState();
}

class _DrawPageState extends State<DrawPage> {
  DrawProvider _provider = DrawProvider();

  @override
  void initState() {
    super.initState();
    _provider.connect();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("WebSocket Draw"),
          actions: <Widget>[
            IconButton(
              icon: Icon(Icons.call_missed_outgoing),
              onPressed: () {
                //撤销一步
                _provider.undoDate();
              },
            ),
            IconButton(
              icon: Icon(Icons.call_missed),
              onPressed: () {
                //反撤销
                _provider.reverseUndoDate();
              },
            ),
          ],
        ),
        body: ChangeNotifierProvider.value(
          value: _provider,
          child: Consumer<DrawProvider>(
            builder: (context, drawProvider, _) {
              return Container(
                color: Color(0x18262B33),
                child: Column(
                  children: <Widget>[
                    Expanded(
                      child: Stack(
                        children: [
                          Container(
                            color: Colors.white,
                          ),
                          Text(drawProvider.points.length.toString()),
                            GestureDetector(
                            //手势探测器，一个特殊的widget，想要给一个widge添加手势，直接用这货包裹起来
                            onPanUpdate: (DragUpdateDetails details) {
                              //按下
                              RenderBox referenceBox =
                                  context.findRenderObject();
                              Offset localPosition = referenceBox
                                  .globalToLocal(details.globalPosition);
                              drawProvider.sendDraw(localPosition);
                            },
                            onPanEnd: (DragEndDetails details) {
                              drawProvider.sendDrawNull();
                            }, //抬起来
                          ),
                          CustomPaint(
                              painter:
                                  SignaturePainter(drawProvider.pointsList)),
                        ],
                      ),
                    ),
                    Padding(
                      padding: EdgeInsets.only(left: 10, right: 80, bottom: 20),
                      child: Wrap(
                        spacing: 5,
                        runSpacing: 5,
                        crossAxisAlignment: WrapCrossAlignment.center,
                        children: <Widget>[
                          buildInkWell(drawProvider, 5),
                          buildInkWell(drawProvider, 8),
                          buildInkWell(drawProvider, 10),
                          buildInkWell(drawProvider, 15),
                          buildInkWell(drawProvider, 17),
                          buildInkWell(drawProvider, 20),
                        ],
                      ),
                    ),
                    Padding(
                      padding: EdgeInsets.only(left: 10, right: 80, bottom: 20),
                      child: Wrap(
                        spacing: 5,
                        runSpacing: 5,
                        children: pintColor.keys.map((key) {
                          Color value = pintColor[key];
                          return InkWell(
                            onTap: () {
//                          setColor(context, key);
                              drawProvider.pentColor = key;
                              drawProvider.notifyListeners();
                            },
                            child: Container(
                              width: 32,
                              height: 32,
                              color: value,
                              child: drawProvider.pentColor == key
                                  ? Icon(
                                      Icons.done,
                                      color: Colors.white,
                                    )
                                  : null,
                            ),
                          );
                        }).toList(),
                      ),
                    )
                  ],
                ),
              );
            },
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: _provider.clear,
          tooltip: '',
          child: Icon(Icons.clear),
        ));
  }

  InkWell buildInkWell(DrawProvider drawProvider, double size) {
    return InkWell(
      onTap: () {
        drawProvider.pentSize = size;
        drawProvider.notifyListeners();
      },
      child: Container(
        width: 40,
        height: 40,
        child: Center(
          child: Container(
            decoration: new BoxDecoration(
              color: pintColor[drawProvider.pentColor],
              //设置四周圆角 角度
              borderRadius: BorderRadius.all(Radius.circular(size / 2)),
              //设置四周边框
              border: drawProvider.pentSize == size
                  ? Border.all(width: 1, color: Colors.black)
                  : null,
            ),
            width: size,
            height: size,
          ),
        ),
      ),
    );
  }

  @override
  void dispose() {
    _provider.dispose();
    super.dispose();
  }
}

• WebSocket 服务端使用 Dart 编写，没有太多逻辑，只做了数据的转发.
• 核心代码如下

//处理消息
  void handMsg(dynamic msg, sct) {
    print('收到客户端消息：${msg}' + webSockets.length.toString());

    msg = jsonDecode(msg);
    if (msg["type"] == "sendDraw" ||//正在连续绘制
        msg["type"] == "clear" ||//清空画板
        msg["type"] == "sendDrawNull" ||//手抬起，添加占位
        msg["type"] == "sendDrawUndo" ||//撤销，缓存到撤销容器
        msg["type"] == "reverseUndoDate")//反撤销数据
      //给其它所有客户端回复当前客户端发了什么
      for (WebSocket webSocket in webSockets) {
        //判断是否有关闭代码，如果没有证明客户端当前未关闭，给它回复
        if (webSocket.closeCode == null && webSocket != sct) {
          //回复客户端一条消息
          webSocket.add(jsonEncode(msg));
        }
      }
  }

大功告成

附源码：

• Flutter：https://gitee.com/pengzhenkun/flutter_draw
• DartServer：https://gitee.com/pengzhenkun/dart_server