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engine.io分析3--socket.io的基石
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上一篇讲了第一次请求（握手）的执行流程，这篇说说握手后，engine.io是怎么传输数据的。
engine.io对http的request事件进行了绑定。在回调函数中，根据有没有socket id来判断是不是握手请求。
if (req._query.sid) { debug(‘setting new request for existing client‘); self.clients[req._query.sid].transport.onRequest(req); } else { self.handshake(req._query.transport, req); }
上一篇走的是self.handshake(req._query.transport, req);，这篇走self.clients[req._query.sid].transport.onRequest(req);。
上一篇的握手请求是get方法。维持长连接的请求也是get方法。广播时，engine.io收到数据，会向长连接中写入数据，返回长连接，然后关闭。如果长连接超时了或者返回错误，客户端也会再次发出。如果在规定的时间(pingTimeout,默认60秒)内，服务器端没有收到客户端发出的长连接请求，则认定这个socket无效，从clients中清除该socket.
function Socket (id, server, transport, req) {。。。。 this.setTransport(transport); this.onOpen();}
Socket.prototype.setPingTimeout = function () { var self = this; clearTimeout(self.pingTimeoutTimer); self.pingTimeoutTimer = setTimeout(function () { self.onClose(‘ping timeout‘); }, self.server.pingInterval + self.server.pingTimeout);};
Socket.prototype.onOpen = function () { this.readyState = ‘open‘; // sends an `open` packet this.transport.sid = this.id; this.sendPacket(‘open‘, JSON.stringify({ sid: this.id , upgrades: this.getAvailableUpgrades() , pingInterval: this.server.pingInterval , pingTimeout: this.server.pingTimeout })); this.emit(‘open‘); this.setPingTimeout();};
可以看到，每次握手成功，初始化一个socket的时候，就会设置一个超时定时器。超时的话执行onClose(),里面就是各种清空，初始化，然后触发close事件，取消其他事件绑定等。
又回到onRequest()，针对请求方法的不同，执行不同的策略，
Polling.prototype.onRequest = function (req) { var res = req.res; if (‘GET‘ == req.method) { this.onPollRequest(req, res); } else if (‘POST‘ == req.method) { this.onDataRequest(req, res); } else { res.writeHead(500); res.end(); }};
如果是get方法
Polling.prototype.onPollRequest = function (req, res) { debug(‘setting request‘); this.req = req; this.res = res; var self = this; function onClose () { self.onError(‘poll connection closed prematurely‘); } function cleanup () { req.removeListener(‘close‘, onClose); self.req = self.res = null; } req.cleanup = cleanup; req.on(‘close‘, onClose); this.writable = true; this.emit(‘drain‘);};
onPollRequest（）里面其实就做了close事件绑定，触发drain事件。drain事件是在socket.js里面 this.setTransport(transport);中绑定的。
this.transport.on(‘drain‘, this.flush.bind(this));可以看到，最终还是用了flush().
如果是post方法
Polling.prototype.onDataRequest = function (req, res) { if (this.dataReq) { // assert: this.dataRes, ‘.dataReq and .dataRes should be (un)set together‘ this.onError(‘data request overlap from client‘); res.writeHead(500); return; } var isBinary = ‘application/octet-stream‘ == req.headers[‘content-type‘]; this.dataReq = req; this.dataRes = res; var chunks = isBinary ? new Buffer(0) : ‘‘; var self = this; function cleanup () { chunks = isBinary ? new Buffer(0) : ‘‘; req.removeListener(‘data‘, onData); req.removeListener(‘end‘, onEnd); req.removeListener(‘close‘, onClose); self.dataReq = self.dataRes = null; } function onClose () { cleanup(); self.onError(‘data request connection closed prematurely‘); } function onData (data) { var contentLength; if (typeof data =http://www.mamicode.com/= ‘string‘) { chunks += data; contentLength = Buffer.byteLength(chunks); } else { chunks = Buffer.concat([chunks, data]); contentLength = chunks.length; } if (contentLength > self.maxHttpBufferSize) { chunks = ‘‘; req.connection.destroy(); } } function onEnd () { self.onData(chunks); var headers = { // text/html is required instead of text/plain to avoid an // unwanted download dialog on certain user-agents (GH-43) ‘Content-Type‘: ‘text/html‘, ‘Content-Length‘: 2 }; // prevent XSS warnings on IE // https://github.com/LearnBoost/socket.io/pull/1333 var ua = req.headers[‘user-agent‘]; if (ua && (~ua.indexOf(‘;MSIE‘) || ~ua.indexOf(‘Trident/‘))) { headers[‘X-XSS-Protection‘] = ‘0‘; } res.writeHead(200, self.headers(req, headers)); res.end(‘ok‘); cleanup(); } req.abort = cleanup; req.on(‘close‘, onClose); req.on(‘data‘, onData); req.on(‘end‘, onEnd); if (!isBinary) req.setEncoding(‘utf8‘);};
里面其实就是对http request里面事件的绑定。当有数据传入时，onData(),判断是不是超出设定的buffer最大长度，如果没有超出，则将数据写入chunks。
当收到数据后，onEnd().然后是self.onData(chunks);
Polling.prototype.onData = http://www.mamicode.com/function (data) { debug(‘received "%s"‘, data); var self = this; var callback = function(packet) { if (‘close‘ == packet.type) { debug(‘got xhr close packet‘); self.onClose(); return false; } self.onPacket(packet); }; parser.decodePayload(data, callback);};
parser.decodePayload(data, callback);用来处理字符编码的，可以简单的看成是对数据处理后执行回调函数。
self.onPacket(packet);在父类transport.js里面。
Transport.prototype.onPacket = function (packet) { this.emit(‘packet‘, packet);};
触发transport上的packet事件，这个又是在socket.js的setTransport（）里绑定的。前面的onPollRequest（）里的drain事件也是。
Socket.prototype.setTransport = function (transport) { this.transport = transport; this.transport.once(‘error‘, this.onError.bind(this)); this.transport.on(‘packet‘, this.onPacket.bind(this)); this.transport.on(‘drain‘, this.flush.bind(this)); this.transport.once(‘close‘, this.onClose.bind(this, ‘transport close‘)); //this function will manage packet events (also message callbacks) this.setupSendCallback();};
然后是onPacket()回调
Socket.prototype.onPacket = function (packet) { if (‘open‘ == this.readyState) { // export packet event debug(‘packet‘); this.emit(‘packet‘, packet); // Reset ping timeout on any packet, incoming data is a good sign of // other side‘s liveness this.setPingTimeout(); switch (packet.type) { case ‘ping‘: debug(‘got ping‘); this.sendPacket(‘pong‘); this.emit(‘heartbeat‘); break; case ‘error‘: this.onClose(‘parse error‘); break; case ‘message‘: this.emit(‘data‘, packet.data); this.emit(‘message‘, packet.data); break; } } else { debug(‘packet received with closed socket‘); }};
重新设置超时定时器.由于每次长连接关闭的时候总会再发出一个post请求，engine.io由此确定客户端还在，这时，packet.type是ping,然后返回响应的心跳。客户端收到才会重新发出新的get长连接请求。
如果post请求中带有数据，packet.type会是message.触发data,message事件。绑定，回调在服务端自定义。这样服务器端就收到了客户端的数据。
最后说下socket.send,socket.write.这也是向客户端返回数据的方法。很简单。
Socket.prototype.send =Socket.prototype.write = function(data, callback){ this.sendPacket(‘message‘, data, callback); return this;};
sendPacket()在握手时也是用的这个返回握手响应，这个方法最后也用了flush().
engine.io分析3--socket.io的基石