nowjs和nodejs实现服务器端与客户端实时数据传输的例子

参考：[url]http://www.bennadel.com/blog/2171-Realtime-Messaging-And-Synchronization-With-NowJS-And-Node-js.htm[/url]

先说例子实现的功能。网页上有一图片，图片可以拖动。用浏览器打开多个同一网页，当图片移动时，其它页面的图片会同步移动。例子也展示了用jQuery实现图片的拖动。
测试环境window7,nodejs v0.6.5 分别用ie,firefox,chrome打开[url]http://127.0.0.1:8080/client.html[/url]，所有该网页上的图片会同步移动。贴上代码。
server.js端：
需要用sock.io和nowjs第三包，推荐用npm方式安装。nowjs包在window的安装可参考：
[url]http://blog.nowjs.com/running-nowjs-natively-on-windows[/url]

// Include the necessary modules.
var sys = require("util");
var http = require("http");
var url = require("url");
var path = require("path");
var fileSystem = require("fs");


// ---------------------------------------------------------- //
// ---------------------------------------------------------- //


// Create an instance of the HTTP server.
var server = http.createServer(
    function (request, response) {

// Get the requested "script_name". This is the part of the
// path after the server_name.
        var scriptName = request.url;

// Convert the script name (expand-path) to a physical file
// on the local file system.
        var requestdFilePath = path.join(process.cwd(), scriptName);

// Read in the requested file. Remember, since all File I/O
// (input and output) is asynchronous in Node.js, we need to
// ask for the file to be read and then provide a callback
// for when that file data is available.
//
// NOTE: You can check to see if the file exists *before* you
// try to read it; but for our demo purposes, I don't see an
// immediate benefit since the readFile() method provides an
// error object.
        fileSystem.readFile(
            requestdFilePath,
            "binary",
            function (error, fileBinary) {

// Check to see if there was a problem reading the
// file. If so, we'll **assume** it is a 404 error.
                if (error) {

// Send the file not found header.
                    response.writeHead(404);

// Close the response.
                    response.end();

// Return out of this guard statement.
                    return;

                }

// If we made it this far then the file was read in
// without a problem. Set a 200 status response.
                response.writeHead(200);

// Serve up the file binary data. When doing this, we
// have to set the encoding as binary (it defaults to
// UTF-8).
                response.write(fileBinary, "binary");

// End the response.
                response.end();

            }
        );

    }
);

// Point the server to listen to the given port for incoming
// requests.
server.listen(8080);


// ---------------------------------------------------------- //
// ---------------------------------------------------------- //


// Create a local memory space for further now-configuration.
(function () {

// Now that we have our HTTP server initialized, let's configure
// our NowJS connector.
    var nowjs = require("now");


// After we have set up our HTTP server to serve up "Static"
// files, we pass it off to the NowJS connector to have it
// augment the server object. This will prepare it to serve up
// the NowJS client module (including the appropriate port
// number and server name) and basically wire everything together
// for us.
//
// Everyone contains an object called "now" (ie. everyone.now) -
// this allows variables and functions to be shared between the
// server and the client.
    var everyone = nowjs.initialize(server);


// Create primary key to keep track of all the clients that
// connect. Each one will be assigned a unique ID.
    var primaryKey = 0;


// When a client has connected, assign it a UUID. In the
// context of this callback, "this" refers to the specific client
// that is communicating with the server.
//
// NOTE: This "uuid" value is NOT synced to the client; however,
// when the client connects to the server, this UUID will be
// available in the calling context.
    everyone.connected(
        function () {
            this.now.uuid = ++primaryKey;
        }
    );


// Add a broadcast function to *every* client that they can call
// when they want to sync the position of the draggable target.
// In the context of this callback, "this" refers to the
// specific client that is communicating with the server.
    everyone.now.syncPosition = function (position) {//syncPosition()在这里定义，在客户端调用

// Now that we have the new position, we want to broadcast
// this back to every client except the one that sent it in
// the first place! As such, we want to perform a server-side
// filtering of the clients. To do this, we will use a filter
// method which filters on the UUID we assigned at connection
// time.
        everyone.now.filterUpdateBroadcast(this.now.uuid, position);

    };


// We want the "update" messages to go to every client except
// the one that announced it (as it is taking care of that on
// its own site). As such, we need a way to filter our update
// broadcasts. By defining this filter method on the server, it
// allows us to cut down on some server-client communication.
    everyone.now.filterUpdateBroadcast = function (masterUUID, position) {

// Make sure this client is NOT the same client as the one
// that sent the original position broadcast.
        if (this.now.uuid == masterUUID) {

// Return out of guard statement - we don't want to
// send an update message back to the sender.
            return;

        }

// If we've made it this far, then this client is a slave
// client, not a master client.
        this.now.updatePosition(position);//updatePosition()为客户端定义的方法，在这里可调用，用this修饰now。

    };

})();


// ---------------------------------------------------------- //
// ---------------------------------------------------------- //


// Write debugging information to the console to indicate that
// the server has been configured and is up and running.
sys.puts("Server is running on 8080");

我把重要的东西摘录下来：
[quote]Once the core HTTP server is configured and the NowJS module is initialized, we are given access to the "everyone" object. This everyone object then provides us with access to the server-side "now" scope. This "now" scope is shared between the server and every one of the clients. Anything added to or removed from the server-side "now" scope is also added to or removed from every client currently (or eventually) connected to the server.

This is true for both variables and functions! Notice that my server-side Node.js code defines two methods: syncPosition() and filterUpdateBroadcast(). By defining them in the "everyone.now" scope, I am making them available to both the server and to every single one of the connected clients.

But what about that, "everyone.now.updatePosition()", function? Where did that come from? Ah-ha! Here's the real, "there is no spoon" mind-screw - that function is defined on the client (which we'll see in a minute). And, since it's defined in the client's "now" scope, the server-side Javascript can then invoke it as if there were no separation between the server and client contexts.[/quote]

client.html

<!DOCTYPE html>
<html>
<head>
    <title>NowJS And Node.js Realtime Communication</title>

    <style type="text/css">

        html,
        body {
            height: 100%;
            overflow: hidden;
            width: 100%;
        }

        img {
            left: 9px;
            position: absolute;
            top: 70px;
        }

    </style>

    <!-- We have this file stored explicitly. -->
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"/>
    </script>

    <!--
    The NowJS HTTP augmentation will take care of routing
    this - we don't actually have this physical file stored
    at this file path.
    -->
    <script type = "text/javascript" src = "/nowjs/now.js" ></script>
</head>
<body>

<h1>
    NowJS And Node.js Realtime Communication
</h1>

<!--
This will be draggable. When this image drags, we are
going to sync the position of it across browsers.
-->
<img
        id="myhead"
        src="./myhead.gif"
        width="100"
        height="100"
        alt="It's my head, do you like it?"
        />


<!-- Configure the client-side script. -->
<script type="text/javascript">

    // Get a reference to the target draggable.
    var myhead = $("#myhead");

    // Get a reference to the body - this is the element on which
    // we'll be tracking mouse movement once the draggable
    // tracking has been turned on.
    var body = $("body");


    // On mouse-down, turn on draggability.
    myhead.mousedown(
            function (event) {
// Prevent the default behavior.
                event.preventDefault();

// Get the current position of the mouse within the
// bounds of the target.
                var localOffset = {
                    x:(event.pageX - myhead.position().left),
                    y:(event.pageY - myhead.position().top)
                };

// Start tracking the mouse movement on the body.
// We're tracking on the body so that the mouse can
// move faster than the tracking.
                body.mousemove(
                        function (event) {
// Create a new position object.
                            var newPosition = {
                                left:(event.pageX - localOffset.x),
                                top:(event.pageY - localOffset.y)
                            };

// Update the target position locally.
                            myhead.css(newPosition);

// Announce the updated position so that we
// can sync accross all clients with NowJS.
                            now.syncPosition(newPosition);//syncPosition()是在服务器端定义的方法，可在客户端调用。
                        }
                );
            }
    );


    // On mouse-up, turn off draggability.
    myhead.mouseup(
            function (event) {
// Unbind the mousemove - no need to track movement
// once the mouse has been lifted.
                body.unbind("mousemove");
            }
    );


    // I allow the remove server to make a request to update the
    // position of the target.
    //
    // NOTE: By defining this function in the NOW scope, it gives
    // the server access to it as well.
    now.updatePosition = function (newPosition){
//updatePosition()这个方法在客户端定义，可在服务器端调用
// Check to see if this client is in master mode; if so,
// we won't update the position as this client is
// actively updating its own position.
        myhead.css(newPosition);

    };

</script>
</body>
</html>

[quote]When the user moves the image on the client, the client broadcasts the new position using the "now.syncPosition()" function. This function, which was defined on the server, then pushes the updated position down to all the other clients using the "now.updatePosition()" function, which was defined on the client.

Even after coding this myself, it's still somewhat confusing; so, let's look at a quick rundown of the various functions to see where they were defined and where they were invoked:

[b]syncPosition()[/b]
[list]
[*]Defined: Server-side
[*]Invoked: Client-side
[/list]
[b]filterUpdateBroadcast()[/b]
[list]
[*]Defined: Server-side
[*]Invoked: Server-side
[/list]
[b]updatePosition()[/b]
[list]
[*]Defined: Client-side
[*]Invoked: Server-side
[/list][/quote]