socket.io emit callback调用探秘

socket.io

https://socket.io/

https://socket.io/docs/

What Socket.IO is

Socket.IO is a library that enables real-time, bidirectional and event-based communication between the browser and the server. It consists of:

• a Node.js server: Source | API
• a Javascript client library for the browser (which can be also run from Node.js): Source | API

emit callback 用法

https://stackoverflow.com/questions/20337832/is-socket-io-emit-callback-appropriate

Recently I have been messing around with socket.io and found this interesting thing, that I can have emit function callback like this.

I start emitting on client side like this:

client.emit('eventToEmit', dataToEmit, function(error, message){
    console.log(error);
    console.log(message);
});

Then I can fire a callback from server-side like this:

client.on('eventToEmit', function(data, callback){
    console.log(data);
    callback('error', 'message');
});

Everything works fine with no errors, but I am interested if doing something like this is appropriate since I have not seen anything similar in the documentation or any example so far.

见老外的疑惑，也是本篇的主题， 为什么服务器端能够直接调用客户端设置的回调函数。

跨进程可以调用函数，真是稀奇。 类似RPC。

官方文档的解释

https://socket.io/docs/#Sending-and-getting-data-acknowledgements

Sending and getting data (acknowledgements)

Sometimes, you might want to get a callback when the client confirmed the message reception.

To do this, simply pass a function as the last parameter of .send or .emit. What’s more, when you use .emit, the acknowledgement is done by you, which means you can also pass data along:

Server (app.js)

var io = require('socket.io')(80);

io.on('connection', function (socket) {
  socket.on('ferret', function (name, word, fn) {
    fn(name + ' says ' + word);
  });
});

Client (index.html)

<script>
  var socket = io(); // TIP: io() with no args does auto-discovery
  socket.on('connect', function () { // TIP: you can avoid listening on `connect` and listen on events directly too!
    socket.emit('ferret', 'tobi', 'woot', function (data) { // args are sent in order to acknowledgement function
      console.log(data); // data will be 'tobi says woot'
    });
  });
</script>

client-代码跟踪

https://github.com/socketio/socket.io-client

以客户端源码为研究对象。

在socket.js文件中，存在emit实现：

如下代码中， 17-20行代码中， 会将callback函数存储到本地的acks数组中， 并将基数记为 packet.id，

然后packet作为数据整体，传送的服务器端。

 Socket.prototype.emit = function (ev) {
   if (events.hasOwnProperty(ev)) {
     emit.apply(this, arguments);
     return this;
   }

   var args = toArray(arguments);
   var packet = {
     type: (this.flags.binary !== undefined ? this.flags.binary : hasBin(args)) ? parser.BINARY_EVENT : parser.EVENT,
     data: args
   };

   packet.options = {};
   packet.options.compress = !this.flags || false !== this.flags.compress;

   // event ack callback
   if ('function' === typeof args[args.length - 1]) {
     debug('emitting packet with ack id %d', this.ids);
     this.acks[this.ids] = args.pop();
     packet.id = this.ids++;
   }

   if (this.connected) {
     this.packet(packet);
   } else {
     this.sendBuffer.push(packet);
   }

   this.flags = {};

   return this;
 };

服务器端处理完数据后， 调用callback接口后，服务器端调用的接口为包装接口， 包装了数据为packet， 并将id打在packet上， 表示此packet为emit时候的packet对应。

服务器端数据到来后， 根据packet.id定位到 callback函数， 并将packet.data作为参数传递到callback中。

/**
 * Called upon a server acknowlegement.
 *
 * @param {Object} packet
 * @api private
 */

Socket.prototype.onack = function (packet) {
  var ack = this.acks[packet.id];
  if ('function' === typeof ack) {
    debug('calling ack %s with %j', packet.id, packet.data);
    ack.apply(this, packet.data);
    delete this.acks[packet.id];
  } else {
    debug('bad ack %s', packet.id);
  }
};

server-代码跟踪

https://github.com/socketio/socket.io

socket.js中 在onevent中 在数据的args数组之后， 添加了 acknowledge 回调函数

 /**
  * Called upon event packet.
  *
  * @param {Object} packet object
  * @api private
  */

 Socket.prototype.onevent = function(packet){
   var args = packet.data || [];
   debug('emitting event %j', args);

   if (null != packet.id) {
     debug('attaching ack callback to event');
     args.push(this.ack(packet.id));
   }

   this.dispatch(args);
 };

 /**
  * Produces an ack callback to emit with an event.
  *
  * @param {Number} id packet id
  * @api private
  */

 Socket.prototype.ack = function(id){
   var self = this;
   var sent = false;
   return function(){
     // prevent double callbacks
     if (sent) return;
     var args = Array.prototype.slice.call(arguments);
     debug('sending ack %j', args);

     self.packet({
       id: id,
       type: hasBin(args) ? parser.BINARY_ACK : parser.ACK,
       data: args
     });

     sent = true;
   };
 };

在 dispatch 负责调用 emitter 原生接口 on 绑定的 事件处理函数：

/**
 * `EventEmitter#emit` reference.
 */

var emit = Emitter.prototype.emit;

/**
  * Dispatch incoming event to socket listeners.
  *
  * @param {Array} event that will get emitted
  * @api private
  */

 Socket.prototype.dispatch = function(event){
   debug('dispatching an event %j', event);
   var self = this;
   function dispatchSocket(err) {
     process.nextTick(function(){
       if (err) {
         return self.error(err.data || err.message);
       }
       emit.apply(self, event);
     });
   }
   this.run(event, dispatchSocket);
 };

 /**
  * Sets up socket middleware.
  *
  * @param {Function} middleware function (event, next)
  * @return {Socket} self
  * @api public
  */

 Socket.prototype.use = function(fn){
   this.fns.push(fn);
   return this;
 };

 /**
  * Executes the middleware for an incoming event.
  *
  * @param {Array} event that will get emitted
  * @param {Function} last fn call in the middleware
  * @api private
  */
 Socket.prototype.run = function(event, fn){
   var fns = this.fns.slice(0);
   if (!fns.length) return fn(null);

   function run(i){
     fns[i](event, function(err){
       // upon error, short-circuit
       if (err) return fn(err);

       // if no middleware left, summon callback
       if (!fns[i + 1]) return fn(null);

       // go on to next
       run(i + 1);
     });
   }

   run(0);
 };

至此实现上彻底明了了。