Recipe 24.1. Connecting to a Socket Server

Problem

You want to establish a connection with a socket server.

Solution

Use either the Socket.connect( ) or XMLSocket.connect( ) method and listen for the connect event to be notified when a connection is made.

Discussion

To connect to a socket server, there are two critical pieces of information that you need to know before attempting to make a connection. The first is the domain name or IP address of the server to which the connection will be made, and the second is the port over which the connection should take place.

Whether you are using a Socket or an XMLSocket instance, the connection process is exactly the same; both classes define a connect( ) method that takes two parameters:

host

A string value specifying the host to connect to, either with a domain name such as www.example.com, or with an IP address such as 192.168.1.101. To connect to the web server the Flash movie is being served from, pass null instead of a string hostname.

port

An int value specifying the port number that should be used to connect to the host. The port value must be at least 1024, unless the server has a policy file specifically allowing ports less than 1024.

Because of the asynchronous nature of socket programming in Flash, the connect( ) method does not wait for a connection to happen before continuing to the next line of ActionScript code. If you try to interact with a socket before a connection has been fully established, you'll encounter unexpected results and your code won't work correctly.

The proper way to connect to a socket is to first add an event listener for the connect event before attempting to call connect( ). The connect event is dispatched by both Socket and XMLSocket objects when a successful connection has been made, letting you know that the socket is ready to be interacted with.

The code for connecting a Socket instance to a socket server running on localhost over port 2900 looks like this:

package {   import flash.display.Sprite;   import flash.events.*;   import flash.net.Socket;   public class SocketExample extends Sprite {        private var socket:Socket;        public function SocketExample(  ) {       socket = new Socket(  );              // Add an event listener to be notified when the connection        // is made       socket.addEventListener( Event.CONNECT, onConnect );              // Connect to the server       socket.connect( "localhost", 2900 );     }          private function onConnect( event:Event ):void {       trace( "The socket is now connected..." );       }        } }

If you want to connect via XMLSocket instead, the code is almost exactly the same. First, you create the event listener for the connect event, and then you invoke the connect( ) method. The only difference is that all Socket references should be replaced with XMLSocket:

package {   import flash.display.Sprite;   import flash.events.*;   import flash.net.XMLSocket;   public class SocketExample extends Sprite {        private var socket:XMLSocket;        public function SocketExample(  ) {       socket = new XMLSocket(  );              // Add an event listener to be notified when the connection is made       socket.addEventListener( Event.CONNECT, onConnect );              // Connect to the server       socket.connect( "localhost", 2900 );     }          private function onConnect( event:Event ):void {       trace( "The xml socket is now connected..." );       }        } }

If the connection fails, one of two things can happen: either the connection fails right away and a runtime error is generated, or an ioError or securityError event is raised to indicate that the connection could not be completed successfully. For more information about handling the error events, see Recipe 24.6.

Remember, when connecting to a host with a socket connection, the following Flash Player security sandbox rules apply:

1. The .swf and host must be in the exact same domain in order for a successful connection to be made.

2. A .swf delivered over a network cannot connect to a local server.

3. A local untrusted .swf cannot access any network resources.

4. To allow cross-domain access or connections to a port lower than 1024, a cross-domain policy file can be used.

Violating the security sandbox by attempting to connect to an untrusted domain or on a low port raises a securityError event. These issues can both be worked around by using a cross-domain policy file, discussed in Recipe 3.12. To use a cross-domain policy file with either a Socket or XMLSocket object, you need to load the policy file by using flash.system.Security.loadPolicyFile( ), as shown here:

Security.loadPolicyFile("http://www.rightactionscript.com/crossdomain.xml");

When assembling the cross-domain policy file, you should specify not only the allowed domains, but also the allowed ports. If you do not specify allowed ports, Flash Player assumes that port 80 (standard HTTP port) is the only allowed port. You can specify a comma-delimited list using the port attribute of the <allow-access-from> tag. The following policy file allows all domains to connect to ports 80 and 110 (standard HTTP and POP mail ports):

<?xml version="1.0"?> <!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd"> <cross-domain-policy>    <allow-access-from domain="*" to-ports="80,110" /> </cross-domain-policy>

See Also

Recipes 3.12, 24.5, and 24.6