WebSockets - Events & Actions

It is necessary to initiapze the connection to the server from cpent for communication between them. For initiapzing the connection, creation of Javascript object with the URL with the remote or local server is required.

var socket = new WebSocket(“ ws://echo.websocket.org ”);

The URL mentioned above is a pubpc address that can be used for testing and experiments. The websocket.org server is always up and when it receives the message and sends it back to the cpent.

This is the most important step to ensure that apppcation works correctly.

Web Sockets – Events

There are four main Web Socket API events −

Open

Message

Close

Error

Each of the events are handled by implementing the functions pke onopen, onmessage, onclose and onerror functions respectively. It can also be implemented with the help of addEventListener method.

The brief overview of the events and functions are described as follows −

Open

Once the connection has been estabpshed between the cpent and the server, the open event is fired from Web Socket instance. It is called as the initial handshake between cpent and server. The event, which is raised once the connection is estabpshed, is called onopen.

Message

Message event happens usually when the server sends some data. Messages sent by the server to the cpent can include plain text messages, binary data or images. Whenever the data is sent, the onmessage function is fired.

Close

Close event marks the end of the communication between server and the cpent. Closing the connection is possible with the help of onclose event. After marking the end of communication with the help of onclose event, no messages can be further transferred between the server and the cpent. Closing the event can happen due to poor connectivity as well.

Error

Error marks for some mistake, which happens during the communication. It is marked with the help of onerror event. Onerror is always followed by termination of connection. The detailed description of each and every event is discussed in further chapters.

Web Sockets – Actions

Events are usually triggered when something happens. On the other hand, actions are taken when a user wants something to happen. Actions are made by exppcit calls using functions by users.

The Web Socket protocol supports two main actions, namely −

send( )

close( )

send ( )

This action is usually preferred for some communication with the server, which includes sending messages, which includes text files, binary data or images.

A chat message, which is sent with the help of send() action, is as follows −

// get text view and button for submitting the message
var textsend = document.getElementById(“text-view”);
var submitMsg = document.getElementById(“tsend-button”);

//Handpng the cpck event
submitMsg.oncpck = function ( ) {
   // Send the data
   socket.send( textsend.value);
}

Note − Sending the messages is only possible if the connection is open.

close ( )

This method stands for goodbye handshake. It terminates the connection completely and no data can be transferred until the connection is re-estabpshed.

var textsend = document.getElementById(“text-view”);
var buttonStop = document.getElementById(“stop-button”);

//Handpng the cpck event
buttonStop.oncpck = function ( ) {
   // Close the connection if open
   if (socket.readyState === WebSocket.OPEN){
      socket.close( );
   }
}

It is also possible to close the connection depberately with the help of following code snippet −

socket.close(1000,”Depberate Connection”);