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.

見老外的疑惑，也是本篇的主題， 為什麼伺服器端能夠直接呼叫客戶端設定的回撥函式。

跨程序可以呼叫函式，真是稀奇。 類似RPC。

官方文件的解釋

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>

程式碼跟蹤

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

以客戶端原始碼為研究物件。

在socket.js檔案中，存在emit實現：

如下程式碼中， 17-20行程式碼中， 會將callback函式儲存到本地的acks陣列中， 並將基數記為 packet.id，

然後packet作為資料整體，傳送的伺服器端。

 1 Socket.prototype.emit = function (ev) {
 2   if (events.hasOwnProperty(ev)) {
 3     emit.apply(this, arguments);
 4     return this;
 5   }
 6 
 7   var args = toArray(arguments);
 8   var packet = {
 9     type: (this.flags.binary !== undefined ? this.flags.binary : hasBin(args)) ? parser.BINARY_EVENT : parser.EVENT,
10     data: args
11   };
12 
13   packet.options = {};
14   packet.options.compress = !this.flags || false !== this.flags.compress;
15 
16   // event ack callback
17   if ('function' === typeof args[args.length - 1]) {
18     debug('emitting packet with ack id %d', this.ids);
19     this.acks[this.ids] = args.pop();
20     packet.id = this.ids++;
21   }
22 
23   if (this.connected) {
24     this.packet(packet);
25   } else {
26     this.sendBuffer.push(packet);
27   }
28 
29   this.flags = {};
30 
31   return this;
32 };

伺服器端處理完資料後， 呼叫callback介面後，伺服器端呼叫的介面為包裝介面， 包裝了資料為packet， 並將id打在packet上， 表示此packet為emit時候的packet對應。

伺服器端資料到來後， 根據packet.id定位到 callback函式， 並將packet.data作為引數傳遞到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);
  }
};

致此實現上徹底明瞭了。