多人共享、同时操作的电子白板，让不同的参入者以不同的颜色来画画；可以保存当前room的内容，以让后来者可以直接加载所有内容。
在github上找到一个用html5 canvas实现的一个电子白板的例子：
https://github.com/kblcuk/canvas-whiteboard
它是基于socket.io来实现多人白板的共享、操作。本文在它的基础上加上了房间，这样只有同一房间的人才会共享。
1 加入房间
客户端：
var roomName = location.search.split(‘?‘)[1];
// Init socket.io
App.socket = io.connect(‘http://localhost‘);
App.socket.emit(‘create‘, roomName);
服务端：
io.sockets.on(‘connection‘, function(socket) {
socket.on(‘create‘, function(room) {
socket.room = room;
socket.join(room);
//get existed image as soon as join room
io.sockets.in(socket.room).emit(‘setup‘, colors[i++], imageData[socket.room]);
});
……
2 后进房间的人，要可以看到前面所有的内容
io.sockets.in(socket.room).emit(‘setup‘, colors[i++], imageData[socket.room]);
3 只在房间内广播
服务端：
socket.emit(‘draw‘, data);
4 发送、接收
发送：
App.socket.emit(‘do-the-draw‘, data); // Broadcast draw.
注： 客户端发送时，不需要加上broadcast.to(socket.room)
接收：
// Broadcast all draw clicks.
socket.on(‘do-the-draw‘, function(data) {
socket.broadcast.to(socket.room).emit(‘draw‘, data);
imageData[socket.room] = data.imageData;
});
5. 客户端
客户端代码<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>White Board</title>
<style>
.whiteboard-canvas {
margin: 0 auto;
border: 1px solid yellowgreen
}
#clear {
display: block;
}
</style>
</head>
<body>
<script src="https://code.jquery.com/jquery-1.9.1.js"></script>
<script src="/socket.io/socket.io.js"></script>
<script>
// Keep everything we need outside global scope
var App = {};
var roomName = location.search.split(‘?‘)[1];
// Init socket.io
App.socket = io.connect(‘http://localhost‘);
App.socket.emit(‘create‘, roomName);
// Flag to indicate whether we‘re currently drawing or not
App._startedDrawing = false;
/**
* Do the draw. Acts differently
* depending on event type passed.
*/
App.draw = function (data) {
var originalColor = App.ctx.strokeStyle;
App.ctx.strokeStyle = data.color;
if (data.type == "mousedown") { // Start drawing
App.ctx.beginPath();
App.ctx.moveTo(data.x, data.y)
} else if (data.type == "mouseup") { // Stop drawing
App.ctx.stroke();
App.ctx.closePath();
App._startedDrawing = false;
App.socket.emit(‘save-data‘, App.whiteboard[0].toDataURL());
} else { // Continue
App.ctx.lineTo(data.x, data.y);
App.ctx.stroke();
}
App.ctx.strokeStyle = originalColor;
};
/**
* We want to draw whenever we receive
* ‘draw‘ event from other sockets as well.
*/
App.socket.on(‘draw‘, App.draw);
/**
* Clears the canvas.
*/
App.clear = function () {
App.ctx.clearRect(0, 0, App.whiteboard[0].width, App.whiteboard[0].height);
};
App.socket.on(‘clear‘, App.clear);
/**
* Fetch color that this user should
* use for drawing.
*/
App.socket.on(‘setup‘, function (color, dataUrl) {
App.ctx.strokeStyle = color;
if (dataUrl) {
// load image from data url
var imageObj = new Image();
imageObj.onload = function () {
App.ctx.drawImage(this, 0, 0);
};
imageObj.src = dataUrl;
}
});
/**
* Bind mouse events to our canvas once DOM has loaded.
* At this point we are also ready to emit
* ‘doTheDraw‘ events.
*/
$(function () {
App.whiteboard = $(‘#whiteboard‘);
App.ctx = App.whiteboard[0].getContext("2d");
// Connect mouse events
App.whiteboard.on(‘mousedown mouseup mousemove‘, null, function (e) {
if (!App._startedDrawing && e.type != "mousedown") return;
App._startedDrawing = true;
var offset = $(this).offset();
var data = {
x: (e.pageX - offset.left),
y: (e.pageY - offset.top),
type: e.handleObj.type,
color: App.ctx.strokeStyle,
imageData: App.whiteboard[0].toDataURL()
};
App.draw(data); // Draw yourself.
App.socket.emit(‘do-the-draw‘, data); // Broadcast draw.
});
// Clear button
$(‘#clear‘).on(‘click‘, function () {
App.clear(); // Clear our screen
App.socket.emit(‘clear‘); // Broadcast clear.
});
});
</script>
<canvas id="whiteboard" width="700px" height="500px" class="whiteboard-canvas"></canvas>
<button id="clear">Clear this up</button>
</body>
</html>
6. 服务端
服务端代码/**
* Our app starts here.
*/
// Setup
var express = require(‘express‘);
var app = express();
var server = app.listen(3210, function() {
console.log(‘Listening on port %d‘, server.address().port);
});
var io = require(‘socket.io‘)(server);
// Return canvas.html for whatever request we get.
app.get(‘/‘, function(req, res) {
res.sendFile(__dirname + ‘/canvas.html‘);
});
// We will store image data here, so users that
// connect after startup receive the current
// state of the image.
var imageData = {};
// Color palette by Skyblue2u: http://www.colourlovers.com/palette/580974/Adrift_in_Dreams
// Nice one!
var colors = ["#CFF09E", "#A8DBA8", "#79BD9A", "#3B8686", "#0B486B"];
var i = 0;
// Setup emitters for each connection
io.sockets.on(‘connection‘, function(socket) {
// Each new user gets a different color
if (i == 5) i = 0;
socket.on(‘create‘, function(room) {
socket.room = room;
socket.join(room);
//get existed image as soon as join room
io.sockets.in(socket.room).emit(‘setup‘, colors[i++], imageData[socket.room]);
});
// Broadcast all draw clicks.
socket.on(‘do-the-draw‘, function(data) {
socket.broadcast.to(socket.room).emit(‘draw‘, data);
imageData[socket.room] = data.imageData;
});
// ...and clear clicks as well
socket.on(‘clear‘, function() {
socket.broadcast.to(socket.room).emit(‘clear‘);
imageData[socket.room] = null;
});
// Users modified image, let‘s save it
socket.on(‘save-data‘, function(data) {
imageData[socket.room] = data;
});
});
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