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:
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
.sendor.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>
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。
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 };
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);
}
};
致此实现上彻底明了了。
原文地址：https://www.cnblogs.com/lightsong/p/10226940.html