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NowJS User Manual

Installing NowJS is very easy. In order to get started, you'll first need to have node.js and npm.

Installing

At your command line, simply enter npm install now.

Setup on the server

NowJS needs an instance of a node.js http server in order to communicate. If your application is already using an http server, NowJS can use the existing instance. Otherwise, you will need to create one. Here's an example server:

var yourHttpServer = require('http').createServer(function(req, response){ /* Serve your static files */ });
yourHttpServer.listen(8080);

Using this http server, get NowJS and use it to get a reference to the 'everyone' object. var nowjs = require('now'); var everyone = nowjs.initialize(yourHttpServer);

Setup on the client

On pages that you would like to use NowJS on, simply include this script tag in your HTML head: <script src="/nowjs/now.js"></script>

NowJS only works on pages that are served through the same http server instance that was passed into the initialize function above.

Using NowJS

The secret to NowJS lies in two magic objects called now and everyone.now. In the Setup on the server section, we created the everyone object. Including the script tag in Setup on the client automatically added the now object to the document's global namespace.

###The shared namespaces Each client has a now object, which is a namespace that is shared between the server and that particular client. Any variables or functions that are changed/added in this shared namespace get synced automatically to the server.

The server has the everyone.now object, which is a namespace that is shared between the server and all clients. When the server adds/changes variables and functions in everyone, those changes are reflected in each client's now namespace. Variables added or changed in the everyone object will be reflected in all currently connected clients as well as all future clients.

###Remote function calls Syncing variables is useful but the true power of NowJS lies in remote function calls. This means that the client can call functions that execute on the server, or vice-versa.

Functions that are placed in a shared namespace can be called by either the server or the client. Functions are executed on the machine on which the function was created. When calling a remote function, pass in the arguments as usual; closures and callbacks work fine.

Note that when making a remote function call, return values are ignored. Any type of value you need to return should be in a callback. Quick example:

now.sayHi = function() {
  return "hi"
}

Should be changed to

now.sayHi = function(callback) {
  callback("hi")
}

This change is because return is a synchronous operation while communication over the network with Node is asynchronous. Callbacks allow greater flexibility without blocking the main thread.

When a remote machine invokes a function, the now namespace that is shared between it and the remote machine is in scope. This namespace can be accessed in the function body through the this.now object (i.e. this client's now). If the function is being executed on the server, the everyone.now namespace also remains available.

###Special behavior of everyone.now When you call a function inside the everyone.now namespace, NowJS will attempt to call the corresponding function in each connected client's now namespace. If the corresponding function exists, a remote function call will be made to that client. If not, a call will not be made.

Setting variables inside the everyone.now namespace will set the same value uniformly across all clients' now namespaces. It is possible to also get/read values from everyone.now, but since clients may change the value of the variable in their own now namespace, the returned value is indeterminate/meaningless.

###Client connected/disconnected events on the server NowJS allows you to specify a callback to be fired when a client connects or disconnects on the server. To set a listener for the events, do the following:

everyone.connected(function(){});
everyone.disconnected(function(){});

The callbacks are run in the context of the connecting/disconnecting client's now namespace. This makes it easy to access information about that client for setup or setdown procedures.

###Client ready event on the client NowJS allows you to specify a callback to be fired when the client has successfully connected to the NowJS server. To set a listener for the events, do the following:

now.ready(function(){});

##The NowJS module object

Calling require('now') on the server returns a reference to the NowJS module object. You should store this in a variable for easy access like this:

var nowjs = require('now');

In previous versions of NowJS, before 0.5.0, it was common practice to immediately chain an initialize call after requiring the module, rather than storing a reference to the module object. Such code would look like this:

var everyone = require('now').initialize(httpServer);    // pre 0.5.0

As of NowJS 0.5.0, it is strongly encouraged to break this up into two different calls, so that a reference to the NowJS module object can be stored, like this:

var nowjs = require('now');
var everyone = nowjs.initialize(httpServer);

The module object exposes several methods that can be used:

###.initialize(httpServer) The initialize function takes a Node.js http server such as the one available in the http module or a module like Express. Returns a reference to the everyone object.

###.getGroup(groupName) This method takes an arbitrary string groupName and returns an ClientGroup object whose name is groupName. If a group with that name was already created by a previous call to getGroup, that group will be returned. Otherwise, a new group will be created and returned.

##Groups in NowJS While the everyone object is used to perform actions on all connected clients, it is sometimes useful to be able to address a smaller subset of clients. For this reason, NowJS 0.5.0 and above exposes the groups API.

A group is created or retrieved by passing a string to the getGroup method of the NowJS module object. Calling this method returns a group object. For example, to create a group called "foo," one would do this:

var fooGroup = nowjs.getGroup("foo");`

Users can be added to a group by passing their this.user.clientId string to the addUser method of the group object. A similar call to removeUser will remove the user from the group. Continuing the example above, one could do this:

everyone.now.addToFooGroup = function(){
  var fooGroup = nowjs.getGroup("foo");
  fooGroup.addUser(this.user.clientId);
}

everyone.now.removeFromGroup = function(){
  var fooGroup = nowjs.getGroup("foo");
  fooGroup.removeUser(this.user.clientId);
}

The groups behave similarly to the everyone object explained earlier. Each group has a now namespace that can be used to perform actions on all members of that group. For example,

everyone.now.sendToFooGroup = function(){
  var fooGroup = nowjs.getGroup("foo");
  fooGroup.now.receiveMessage("Hello, members of foo group");
}

In the above function, receiveMessage would be called on only users who had previously added to the group named "foo."

While the everyone object and group objects expose similar functionality, there are subtle yet crucial differences in how they work. For that reason, the everyone object cannot be retrived like a regular group using getGroup. It is also highly discouraged to use groups to set variables for only a subset of users, like this: fooGroup.now.x = 3. A discussion of this topic can be found in the Best Practices document.

###The ClientGroup object ####.addUser(clientId) Takes a user's socket.io sessionId string, which is available using this.user.clientId, and adds that user to the group. Throws an error if clientId is not a valid sessionId.

####.removeUser(clientId) Takes a user's socket.io client.sessionId string, which is available using this.user.clientId and removes that user to the group. Throws an error if clientId is not a valid sessionId.

####.now A now namespace similar to everyone.now. Actions to this namespace affect all users that are members of the group. For example, actions on 'fooGroup.now' from the above example affects all users in the group 'foo'.

####.on Every group receives two events connect and disconnect. Connect is triggered when a user is added to the group. Disconnect is triggered when a user is removed from the group. These event handlers are called with one parameter, the this.user.clientId of the user that has connected or disconnected. Additionally you can also access that users this.now object (and also this.user.clientId).

 everyone.on('connect', function(clientId){
   //this.now.setup();
 });

 everyone.on('disconnect', function(clientId){
   //this.now.destruct();
 });

Further Reading

Now that you've read the User Manual guide, try the NowJS Quick Start and Best Practices

Have more questions? You can reach us in #nowjs on freenode