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AJAX and PHP: Building Responsive Web Applications: Chapter 5: AJAX chat and JSON

, 27 Jun 2006 CPOL
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In this chapter, we'll implement our own online chat application. We'll use this occasion to learn about JSON (JavaScript Object Notation), which represents an alternative to XML for representing data exchanged between the web browser and the web server.

Title AJAX and PHP: Building Responsive Web Applications
Author(s) Cristian Darie, Bogdan Brinzarea, Filip Cherecheş-Toşa, Mihai Bucica
Publisher Packt Publishing Ltd.
Published March 2006
ISBN 1904811825
Price USD 31.49
Pages 275

Introduction

Online chat solutions have been very popular long before AJAX was born. There are numerous reasons for this popularity, and you're probably familiar with them if you've ever used an Internet Relay Chat (IRC) client, or an Instant Messenger (IM) program, or a Java chat applet.

AJAX has pushed online chat solutions forward by making it easy to implement features that are causing trouble or are harder to implement with other technologies. First of all, an AJAX chat application inherits all the typical AJAX benefits, such as integration with existing browser features, and (if written well) cross-platform compatibility.

An additional advantage is that an AJAX chat application avoids the connectivity problems that are common with other technologies, because many firewalls block the communication ports they use. On the other hand, AJAX uses, exclusively, HTTP for communicating with the server.

Probably the most impressive AJAX chat application available today is Meebo. We are pretty sure that some of you have heard about it, and if you haven't, we recommend you have a look at it. The first and the most important feature in Meebo is that it allows you to log in into your favorite IM system by using only a web interface. At the time of writing, Meebo lets you connect to AIM or ICQ, Yahoo! Messenger, Jabber, or GTalk, and MSN. You can access all these services from a single web page with a user friendly interface, with no pop-up windows or Java applets.

Meebo isn't the only web application that offers chat functionality. Even if AJAX is very young, a quick Google search on "AJAX Chat" will reveal several other applications.

It's time to get to work. In the rest of the chapter, we'll implement our own online chat application. We'll use this occasion to learn about JSON (JavaScript Object Notation), which represents an alternative to XML for representing data exchanged between a web browser and the web server.

Introducing JSON

JSON is a data format that you can use instead of XML for exchanging information between a JavaScript client and a PHP server script. Interestingly enough, JSON's popularity increased together with the AJAX phenomenon, although the AJAX acronym implies using XML.

Because XML is a more popular and more widely supported format, we've chosen to use XML for all examples in this book, except this one. However, if you like, it's fairly easy to update the other code samples to use JSON instead of XML. (As you know, this AJAX Chat chapter also has a version that uses XML, and you can compare them to see the differences.)

Perhaps the best short description of JSON is the one proposed by its official website: "JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write. It is easy for machines to parse and generate".

If you're new to JSON, a fair question you could ask would be: why another data exchange format? JSON, just as XML, is a text-based format that is easy to write and to understand for both humans and computers. The key word in the definition above is "lightweight". JSON data structures occupy less bandwidth than their XML versions.

To make an idea of how JSON compares to XML, let's take the same data structure and see how we would represent it using both standards. In the chat application you'll write, the server composes messages for the client that would look like this, if written in XML:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<response>
  <clear>false</clear>
  <messages>
    <message>
      <id>1</id>
      <color>#000000</color>
      <time>2006-01-17 09:07:31</time>
      <name>Guest550</name>
      <text>Hello there! What's up?</text>
    </message>
    <message>
      <id>2</id>
      <color>#000000</color>
      <time>2006-01-17 09:21:34</time>
      <name>Guest499</name>
      <text>This is a test message</text>
    </message>
  </messages>
</response>

The same message, written in JSON, this time, looks like this:

[
  {"clear":"false"},
  "messages":
    [
      {"message":
        {"id":"1",
         "color":"#000000",
         "time":"2006-01-17 09:07:31",
         "name":"Guest550",
         "text":"Hello there! What's up?"}
      },
      {"message":
        {"id":"2",
         "color":"#000000",
         "time":"2006-01-17 09:21:34",
         "name":"Guest499",
         "text":"This is a test message"}
      }      
    ]
  }
]

As you can see, they aren't that different. If we disregard the extra formatting spaces that we added for better readability, the XML message occupies 396 bytes while the JSON message has only 274 bytes.

JSON is said to be a subset of JavaScript because it contains two basic structures: the object, and the array. An object is an unordered collection of name/value pairs, defined in this form:

{ name1:value1, name2:value2, ... }

The array is an ordered list of values defined in this form:

[ value1, value2, ... ]

The type of the value can be an object, a string, a number, an array, true, false, or null. A string is a collection of Unicode characters surrounded by double quotes. For escaping, we use the backslash.

For more detailed information, please refer to www.json.org, which covers the theory wonderfully.

Installing JSON

It's obvious that when you plan to use JSON, you need to be able to parse and generate JSON structures in JavaScript and PHP. JSON libraries are available for most of today's programming languages: ActionScript, C, C++, C#, Delphi, E, Erlang, Java, JavaScript, Lisp, Lua, ML and Ruby, Objective CAML, OpenLazslo, Perl, PHP, Python, Rebol, Ruby, and Squeak.

For JavaScript, we'll use the library listed here. The direct link to the small JSON library is this. The entire installation process consists in copying this file to your application's folder, and referencing it from the files that need its functionality.

The JSON solution we're using for PHP is the library developed by Michal Migurski that can be downloaded from here. Installing the library implies simply downloading the PHP class file, and copying it into the application's folder. Next, we need to reference it from our application by using the require_once directive like this:

require_once('JSON.php');

In order to effectively start using it, we need to instantiate the class:

$json= new Services_JSON();

Then you're ready to use it. The encode and decode methods allow us to encode a PHP object into JSON format and to decode a JSON string into a PHP object.

Using JSON with JavaScript and PHP

A typical way to build a JSON structure in JavaScript:

var myMessages = 
  {'messages' : [ 
    {'username': 'Guest0740', 
     'message': 'This is a JSON message', 
     'color': '#eeeeee'},
    {'username': 'Guest0740', 
     'message': 'This is another JSON message', 
     'color': '#eeeeee'} ]};
alert(myMessages.toJSONString());

The first command defines an object called myMessages, which contains a single member called messages, which at its turn contains an array containing two objects that are made of three members (username, message, and color). In order to access the "This is another JSON message" text, you could use the following syntax:

myMessages[0].messages[1].message

The following two code snippets show typical ways to read a JSON structure in JavaScript:

myMessagesJSON = myMessages.toJSONString();
myMessagesFromJSON = myMessagesJSON.parseJSON();

Or

myMessagesFromJSON = eval ('(' + myMessagesJSON + ') ');

The eval function is very fast but it can allow any JavaScript code to be executed, so it's generally much safer to use the parseJSON method.

In PHP, you encode and decode JSON messages using the encode and decode methods. Let's take the previous example and do something similar in PHP. A typical way to build a JSON structure in PHP:

require_once('JSON.php');
$json = new Services_JSON();
$myMessages = array ('messages' => 
  array( 
    array ('username' => 'Guest0740', 
           'message' => 'This is a JSON message', 
           'color' => '#eeeeee'), 
    array('username' => 'Guest0740', 
          'message' => 'This is another JSON message', 
          'color' => '#eeeeee')));
echo $json->encode($myMessages);

A typical way to read a JSON structure in PHP is:

require_once('JSON.php');
$json = new Services_JSON();
$myMessagesFromJSON = $json->decode($myMessagesJSON);

Implementing AJAX Chat

Now, it's time to implement the AJAX chat application. We'll keep the application simple, modular, and extensible. We won't implement a login module, support for chat rooms, the online users list, etc. By keeping it simple, we try to focus on what the goal of this chapter is: posting and retrieving messages without causing any page reloads. We'll also let the user pick a color for her or his messages, because this involves an AJAX mechanism that will be another good exercise.

The chat application can be tested online, and it looks like in Figure 5.1.

Figure 5.1: AJAX Chat

A novelty in this chapter is that you will have two XMLHttpRequest objects. The first one will handle updating the chat window, and the second will handle the color picker (when you click on the image, the coordinates are sent to the server, and the server replies with the color code).

The messages for the AJAX chat are saved in a queue (a FIFO structure), whose functionality was covered in Chapter 4, so that messages are not lost even if the server is slow, and they always get to the server in the same order as you sent them. Unlike with other patterns you can find on Internet these days, we also ensure we don't load the server with any more requests until the current one is finished.

In order to have this example working, you need the GD library. The installation instructions in Appendix A include support for the GD library.

Time for Action—AJAX Chat
  1. Connect to the AJAX database, and create a table named chat with the following code:
    CREATE TABLE chat 
    (
      chat_id int(11) NOT NULL auto_increment,
      posted_on datetime NOT NULL,
      user_name varchar(255) NOT NULL,
      message text NOT NULL,
      color char(7) default '#000000',
      PRIMARY KEY (chat_id)
    );
  2. In your ajax folder, create a new folder named chat.
  3. Copy the palette.png file from the code download to the chat folder.
  4. We will create the application starting with the server functionality. In the chat folder, create a file named config.php, and add the database configuration code to it (change these values to match your configuration):
    <?php
    // defines database connection data
    define('DB_HOST', 'localhost');
    define('DB_USER', 'ajaxuser');
    define('DB_PASSWORD', 'practical');
    define('DB_DATABASE', 'ajax');
    ?>
  5. Now, add the standard error handling file, error_handler.php:
    <?php
    // set the user error handler method to be error_handler
    set_error_handler('error_handler', E_ALL);
    // error handler function
    function error_handler($errNo, $errStr, $errFile, $errLine)
    {
      // clear any output that has already been generated
      if(ob_get_length()) ob_clean();
      // output the error message 
      $error_message = 'ERRNO: ' . $errNo . chr(10) .
                       'TEXT: ' . $errStr . chr(10) .
                       'LOCATION: ' . $errFile . 
                       ', line ' . $errLine;
      echo $error_message;
      // prevent processing any more PHP scripts
      exit;
    }
    ?>
  6. Create another file named chat.php and add this code to it:
    <?php
    // reference the file containing the Chat class
    require_once('chat.class.php');
    // reference the file containing the JSON class
    require_once('JSON.php');
    // create an instance of the JSON class
    $json=new Services_JSON();
    // retrieve the operation to be performed
    $mode = $_POST['mode'];
    // default the last id to 0
    $id = 0;
    // create a new Chat instance
    $chat = new Chat();
    // if the operation is SendAndRetrieve
    if($mode == 'SendAndRetrieveNew')
    {
      // retrieve the action parameters used to add a new message
      $name = $_POST['name']; 
      $message = $_POST['message'];
      $color = $_POST['color'];
      $id = $_POST['id'];
      // check if we have valid values 
      if ($name != '' && $message != '' && $color != '') 
      {
        // post the message to the database     
        $chat->postMessage($name, $message, $color); 
    
    
      }
    }
    // if the operation is DeleteAndRetrieve
    elseif($mode == 'DeleteAndRetrieveNew')
    {
      // delete all existing messages
      $chat->deleteMessages();         
    }
    // if the operation is Retrieve
    elseif($mode == 'RetrieveNew')
    {
      // get the id of the last message retrieved by the client
      $id = $_POST['id'];    
    }
    // Clear the output
    if(ob_get_length()) ob_clean();
    // Headers are sent to prevent browsers from caching
    header('Expires: Mon, 26 Jul 1997 05:00:00 GMT'); 
    header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . 'GMT'); 
    header('Cache-Control: no-cache, must-revalidate'); 
    header('Pragma: no-cache');
    header('Content-Type: text/javascript');
    // retrieve new messages from the server and send them in JSON format
    echo $json->encode($chat->retrieveNewMessages($id));
    ?>
  7. Create another file named chat.class.php, and add this code to it:
    <?php
    // load configuration file
    require_once('config.php');
    // load error handling module
    require_once('error_handler.php');
    // class that contains server-side chat functionality
    class Chat
    {
      // database handler
      private $mMysqli;  
      // constructor opens database connection
      function __construct() 
      {   
        // connect to the database
        $this->mMysqli = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, 
                                                      DB_DATABASE);      
      }
      // destructor closes database connection  
      public function __destruct() 
      {
        $this->mMysqli->close();
      }
      // truncates the table containing the messages
      public function deleteMessages()
      { 
        // build the SQL query that adds a new message to the server
        $query = 'TRUNCATE TABLE chat'; 
        // execute the SQL query
        $result = $this->mMysqli->query($query);      
      }
      /*
       The postMessages method inserts a message into the database
       - $name represents the name of the user that posted the message
       - $messsage is the posted message
       - $color contains the color chosen by the user
      */ 
      public function postMessage($name, $message, $color)
      {  
    
    
        // escape the variable data for safely adding them to the database
        $name = $this->mMysqli->real_escape_string($name);
        $message = $this->mMysqli->real_escape_string($message);
        $color = $this->mMysqli->real_escape_string($color);
        // build the SQL query that adds a new message to the server
        $query = 'INSERT INTO chat(posted_on, user_name, message, color) ' .
                 'VALUES (NOW(), "' . $name . '" , "' . $message . 
                 '","' . $color . '")'; 
        // execute the SQL query
        $result = $this->mMysqli->query($query);      
      }
      /*
       The retrieveNewMessages method retrieves the new messages that have 
       been posted to the server. 
       - the $id parameter is sent by the client and it
       represents the id of the last message received by the client. Messages
       more recent by $id will be fetched from the database and returned to
       the client in JSON format.
      */
      public function retrieveNewMessages($id=0) 
      {
        // escape the variable data 
        $id = $this->mMysqli->real_escape_string($id);
        // compose the SQL query that retrieves new messages
        if($id>0)
        {
          // retrieve messages newer than $id
          $query = 
          'SELECT chat_id, user_name, message, color, ' . 
          '       DATE_FORMAT(posted_on, "%Y-%m-%d %H:%i:%s") ' . 
          '       AS posted_on ' .
          ' FROM chat WHERE chat_id > ' . $id . 
          ' ORDER BY chat_id ASC'; 
        }
        else
        {
          // on the first load only retrieve the last 50 messages from server
          $query = 
          ' SELECT chat_id, user_name, message, color, posted_on FROM ' .
          '    (SELECT chat_id, user_name, message, color, ' . 
          '       DATE_FORMAT(posted_on, "%Y-%m-%d %H:%i:%s") AS posted_on ' .
          '     FROM chat ' .
          '     ORDER BY chat_id DESC ' .
          '      LIMIT 50) AS Last50' . 
          ' ORDER BY chat_id ASC';
        } 
        // execute the query
        $result = $this->mMysqli->query($query);  
        // initialize the response array
        $response = array();        
        // add the clear flag to the response
        array_push( $response,array('clear'=> $this->isDatabaseCleared($id)));
        // initialize the results array
        $results=array();
        // check to see if we have any results
        if($result->num_rows)
        {      
          // loop through all the fetched messages to build the results array
          while ($row = $result->fetch_array(MYSQLI_ASSOC)) 
          {
            $id = htmlspecialchars($row['chat_id']);
            $color = htmlspecialchars($row['color']);
            $userName = htmlspecialchars($row['user_name']);
            $time = htmlspecialchars($row['posted_on']);
            $message = htmlspecialchars($row['message']);
            array_push($results,array('id' => $id , 
    
    
                         'color' => $color ,
                         'time' => $time ,
                         'name' => $userName ,
                         'message' => $message ));
          }      
          // close the database connection as soon as possible
          $result->close();
        }
        // add the results to the response
        array_push($response, array('results' => $results));
        return $response;    
      }
      /*
        The isDatabaseCleared method checks to see if the database has been 
        cleared since last call to the server
        -   the $id parameter contains the id of the last message received by 
        the client
      */
      private function isDatabaseCleared($id)
      {
        if($id>0)
        {
          // by checking the number of rows with ids smaller than the client's 
          // last id we check to see if a truncate operation was performed in 
          // the meantime            
          $check_clear = 'SELECT count(*) old FROM chat where chat_id<=' . $id;
          $result = $this->mMysqli->query($check_clear);
          $row = $result->fetch_array(MYSQLI_ASSOC);      
          // if a truncate operation occured the whiteboard needs to be reset
          if($row['old']==0)
            return 'true';     
        }
        return 'false';
      }
    }
    ?>
  8. Create another file named get_color.php and add this code to it:
    <?php
    // the name of the image file
    $imgfile='palette.png';
    // load the image file
    $img=imagecreatefrompng($imgfile);
    // obtain the coordinates of the point clicked by the user
    $offsetx=$_GET['offsetx'];
    $offsety=$_GET['offsety'];
    // get the clicked color
    $rgb = ImageColorAt($img, $offsetx, $offsety);
    $r = ($rgb >> 16) & 0xFF;
    $g = ($rgb >> 8) & 0xFF;
    $b = $rgb & 0xFF;
    // return the color code
    printf('#%02s%02s%02s', dechex($r), dechex($g), dechex($b));
    ?>
  9. Let's deal with the client now. Start by creating chat.css and add this code to it:
    body
    {
      font-family: Tahoma, Helvetica, sans-serif;
      margin: 1px;
      font-size: 12px;
      text-align: left   
    }
    
    #content
    {
      border: DarkGreen 1px solid;
      margin-bottom: 10px
    }
    input
    {
      border: #999 1px solid;  
      font-size: 10px 
    }
    #scroll 
    {
      position: relative; 
      width: 340px;
      height: 270px; 
      overflow: auto
    }
    .item
    {
      margin-bottom: 6px
    }
    #colorpicker
    {
      text-align:center  
    }
  10. Create a new file named index.html, and add this code to it:
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
    <head>
      <title>AJAX Chat</title>
      <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
      <link href="chat.css" rel="stylesheet" type="text/css" />
      <script type="text/javascript" src="json.js" ></script>
      <script type="text/javascript" src="chat.js" ></script> 
    </head>
      <body onload="init();">
        <noscript>
          Your browser does not support JavaScript!!
        </noscript>
        <table id="content">
          <tr>
            <td>
              <div id="scroll">
              </div>
            </td>
            <td id="colorpicker">
              <img src="palette.png" id="palette" alt="Color 
                   Palette" border="1" onclick="getColor(event);"/>
              <br />
              <input id="color" type="hidden" readonly="true" value="#000000" />
              <span id="sampleText">
                (text will look like this)
              </span>
            </td>
          </tr>
        </table>
        <div>
          <input type="text" id="userName" maxlength="10" 
                 size="10" onblur="checkUsername();"/>
          <input type="text" id="messageBox" maxlength="2000" size="50" 
                 onkeydown="handleKey(event)"/>
    
    
          <input type="button" value="Send" onclick="sendMessage();" />
          <input type="button" value="Delete All" onclick="deleteMessages();" />
        </div>
      </body>
    </html>
  11. Create another file named chat.js and add this code to it:
    /* chatURL - URL for updating chat messages */
    var chatURL = "chat.php";
    /* getColorURL - URL for retrieving the chosen RGB color */
    var getColorURL = "get_color.php";
    /* create XMLHttpRequest objects for updating 
       the chat messages and getting the selected color */
    var xmlHttpGetMessages = createXmlHttpRequestObject();
    var xmlHttpGetColor = createXmlHttpRequestObject();
    /* variables that establish how often to access the server */
    // how many miliseconds to wait to get new message
    var updateInterval = 1000;
    // when set to true, display detailed error messages
    var debugMode = true;
    /* initialize the messages cache */
    var cache = new Array();
    /* lastMessageID - the ID of the most recent chat message */
    var lastMessageID = -1; 
    /* mouseX, mouseY - the event's mouse coordinates */
    var mouseX,mouseY;
    /* creates an XMLHttpRequest instance */
    function createXmlHttpRequestObject() 
    {
      // will store the reference to the XMLHttpRequest object
      var xmlHttp;
      // this should work for all browsers except IE6 and older
      try
      {
        // try to create XMLHttpRequest object
        xmlHttp = new XMLHttpRequest();
      }
      catch(e)
      {
        // assume IE6 or older
        var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                        "MSXML2.XMLHTTP.5.0",
                                        "MSXML2.XMLHTTP.4.0",
                                        "MSXML2.XMLHTTP.3.0",
                                        "MSXML2.XMLHTTP",
                                        "Microsoft.XMLHTTP");
        // try every prog id until one works
        for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++) 
        {
          try 
          { 
            // try to create XMLHttpRequest object
            xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
          } 
          catch (e) {}
        }
      }
      // return the created object or display an error message
      if (!xmlHttp)
        alert("Error creating the XMLHttpRequest object.");
      else 
        return xmlHttp;
    }
    
    
    /* this function initiates the chat; 
       it executes when the chat page loads */
    function init() 
    {
      // get a reference to the text box
      // where the user writes new messages
      var oMessageBox = document.getElementById("messageBox");
      // prevents the autofill function from starting
      oMessageBox.setAttribute("autocomplete", "off");    
      // references the "Text will look like this" message
      var oSampleText = document.getElementById("sampleText");
      // set the default color to black
      oSampleText.style.color = "black";    
      // ensures our user has a default random name when the form loads
      checkUsername();
      // initiates updating the chat window
      requestNewMessages();
    }
    // function that ensures that the username is never empty and if so 
    // a random name is generated
    function checkUsername()
    {
      // ensures our user has a default random name when the form loads
      var oUser=document.getElementById("userName");
      if(oUser.value == "")
        oUser.value = "Guest" + Math.floor(Math.random() * 1000);
    }
    /* function called when the Send button is pressed */
    function sendMessage()
    {
      // save the message to a local variable and clear the text box
      var oCurrentMessage = document.getElementById("messageBox");
      var currentUser = document.getElementById("userName").value;
      var currentColor = document.getElementById("color").value;
      // don't send void messages
      if (trim(oCurrentMessage.value) != "" &&
          trim(currentUser) != "" && trim (currentColor) != "")
      {
        // if we need to send and retrieve messages
        params =  "mode=SendAndRetrieveNew" +
                  "&id=" + encodeURIComponent(lastMessageID) + 
                  "&color=" + encodeURIComponent(currentColor) + 
                  "&name=" + encodeURIComponent(currentUser) + 
                  "&message=" + encodeURIComponent(oCurrentMessage.value);
        // add the message to the queue
        cache.push(params);
        // clear the text box
        oCurrentMessage.value = "";
      }
    }
    /* function called when the Delete Messages button is pressed */
    function deleteMessages()
    {
      // set the flag that specifies we're deleting the messages
      params = "mode=DeleteAndRetrieveNew";  
      // add the message to the queue
      cache.push(params);
    }
    /* makes asynchronous request to retrieve new messages,
       post new messages, delete messages */
    function requestNewMessages()
    {  
      // retrieve the username and color from the page
      var currentUser = document.getElementById("userName").value;
      var currentColor = document.getElementById("color").value;
      // only continue if xmlHttpGetMessages isn't void
      if(xmlHttpGetMessages)
    
    
      {
        try
        {
          // don't start another server operation if such an operation 
          //   is already in progress 
          if (xmlHttpGetMessages.readyState == 4 || 
              xmlHttpGetMessages.readyState == 0) 
          {
            // we will store the parameters used to make the server request
            var params = "";
            // if there are requests stored in queue, take the oldest one
            if (cache.length>0)
              params = cache.shift();
            // if the cache is empty, just retrieve new messages        
            else
              params = "mode=RetrieveNew" +
                       "&id=" +lastMessageID;
            // call the server page to execute the server-side operation
            xmlHttpGetMessages.open("POST", chatURL, true);
            xmlHttpGetMessages.setRequestHeader("Content-Type", 
                                       "application/x-www-form-urlencoded");
            xmlHttpGetMessages.onreadystatechange = handleReceivingMessages;
            xmlHttpGetMessages.send(params);
          }
          else
          {
            // we will check again for new messages 
            setTimeout("requestNewMessages();", updateInterval);
          }
        }
        catch(e)
        {
          displayError(e.toString());
        }
      }
    }
    /* function that handles the http response when updating messages */
    function handleReceivingMessages() 
    {
      // continue if the process is completed
      if (xmlHttpGetMessages.readyState == 4) 
      {
        // continue only if HTTP status is "OK"
        if (xmlHttpGetMessages.status == 200) 
        {
          try
          {
            // process the server's response
            readMessages();
          }
          catch(e)
          {
            // display the error message
            displayError(e.toString());
          }
        } 
        else
        {
          // display the error message
          displayError(xmlHttpGetMessages.statusText);
        }
      }
    }
    /* function that processes the server's response when updating messages */
    function readMessages()
    {  
    
    
      // retrieve the server's response 
      var response = xmlHttpGetMessages.responseText;
      // server error?
      if (response.indexOf("ERRNO") >= 0 
          || response.indexOf("error:") >= 0
          || response.length == 0)
        throw(response.length == 0 ? "Void server response." : response);
      // retrieve the JSON object correspondig to the responseText element
      responseJSON = xmlHttpGetMessages.responseText.parseJSON();
      // retrieve the flag that says if the chat window has been cleared 
      clearChat = responseJSON[0].clear;
      // if the clear flag is set, we need to clear the chat window 
      if(clearChat == "true")
      {
        // clear chat window and reset the id
        document.getElementById("scroll").innerHTML = "";
        lastMessageID = -1;
      }    
      // initialize the arrays
      idArray = new Array();
      colorArray = new Array();
      nameArray = new Array();
      timeArray = new Array();
      messageArray = new Array();
      // retrieve the arrays
      for(i=0;i<responseJSON[1].results.length;i++)
      {
        // retrieve the arrays from the server's response     
        idArray[i] = responseJSON[1].results[i].id;
        colorArray[i] = responseJSON[1].results[i].color;
        nameArray[i] = responseJSON[1].results[i].name;
        timeArray[i] = responseJSON[1].results[i].time;
        messageArray[i] = responseJSON[1].results[i].message;
      }
      // add the new messages to the chat window
      displayMessages(idArray, colorArray, 
                      nameArray, timeArray, messageArray);
      // the ID of the last received message is stored locally
      if(idArray.length>0)
        lastMessageID = idArray[idArray.length - 1];  // restart sequence
      setTimeout("requestNewMessages();", updateInterval);
    }
    /* function that appends the new messages to the chat list  */
    function displayMessages(idArray, colorArray, nameArray, 
                             timeArray, messageArray)
    {
      // each loop adds a new message
      for(var i=0; i<idArray.length; i++)
      {
        // get the message details
        var color = colorArray[i];
        var time = timeArray[i];
        var name = nameArray[i];
        var message = messageArray[i];
        // compose the HTML code that displays the message
        var htmlMessage = "";
        htmlMessage += "<div class=\"item\" style=\"color:" + color + "\">"; 
        htmlMessage += "[" + time + "] " + name + " said: <br/>";
        htmlMessage += message.toString();
        htmlMessage += "</div>";
        // display the message
        displayMessage (htmlMessage);
      }
    }
    // displays a message
    function displayMessage(message)
    
    
    {
      // get the scroll object
      var oScroll = document.getElementById("scroll");
      // check if the scroll is down
      var scrollDown = (oScroll.scrollHeight - oScroll.scrollTop <= 
                        oScroll.offsetHeight );
      // display the message
      oScroll.innerHTML += message;
      // scroll down the scrollbar
      oScroll.scrollTop = scrollDown ? oScroll.scrollHeight : oScroll.scrollTop;
    }
    // function that displays an error message
    function displayError(message)
    {
      // display error message, with more technical details if debugMode is true
      displayMessage("Error accessing the server! "+
                     (debugMode ? "<br/>" + message : ""));
    }
    /* handles keydown to detect when enter is pressed */
    function handleKey(e) 
    {
      // get the event
      e = (!e) ? window.event : e;
      // get the code of the character that has been pressed        
      code = (e.charCode) ? e.charCode :
             ((e.keyCode) ? e.keyCode :
             ((e.which) ? e.which : 0));
      // handle the keydown event       
      if (e.type == "keydown") 
      {
        // if enter (code 13) is pressed
        if(code == 13)
        {
          // send the current message  
          sendMessage();
        }
      }
    }
    /* removes leading and trailing spaces from the string */
    function trim(s)
    {
        return s.replace(/(^\s+)|(\s+$)/g, "")
    }
    /* function that computes the mouse's coordinates in page */
    function getMouseXY(e) 
    {
      // browser specific 
      if(window.ActiveXObject)
      {
        mouseX = window.event.x + document.body.scrollLeft;
        mouseY = window.event.y + document.body.scrollTop;    
      }
      else
      {
        mouseX = e.pageX;
        mouseY = e.pageY;
      }
    }
    /* makes a server call to get the RGB code of the chosen color */
    function getColor(e)
    {
      getMouseXY(e);
      // don't do anything if the XMLHttpRequest object is null
      if(xmlHttpGetColor)
      {
    
    
        // initialize the offset position with the mouse current position 
        var offsetX = mouseX;
        var offsetY = mouseY;
        // get references 
        var oPalette = document.getElementById("palette");
        var oTd = document.getElementById("colorpicker");  
        // compute the offset position in our window
        if(window.ActiveXObject)
        {
          offsetX = window.event.offsetX;
          offsetY = window.event.offsetY;  
        } 
        else 
        {
          offsetX -= oPalette.offsetLeft + oTd.offsetLeft;
          offsetY -= oPalette.offsetTop + oTd.offsetTop;
        }  
        // call server asynchronously to find out the clicked color
        try
        {
          if (xmlHttpGetColor.readyState == 4 || 
              xmlHttpGetColor.readyState == 0) 
          {
            params = "?offsetx=" + offsetX + "&offsety=" + offsetY;        
            xmlHttpGetColor.open("GET", getColorURL+params, true);    
            xmlHttpGetColor.onreadystatechange = handleGettingColor;
            xmlHttpGetColor.send(null);    
          }
        }
        catch(e)
        {
          // display error message
          displayError(xmlHttp.statusText);
        }
      }
    }
    /* function that handles the http response */
    function handleGettingColor() 
    {
      // if the process is completed, decide to do with the returned data
      if (xmlHttpGetColor.readyState == 4) 
      {
        // only if HTTP status is "OK"
        if (xmlHttpGetColor.status == 200) 
        {
          try
          {
            //change the color
            changeColor();
          }
          catch(e)
          {
            // display error message
            displayError(xmlHttpGetColor.statusText);
          }
        } 
        else
        {
          // display error message
          displayError(xmlHttpGetColor.statusText);
        }        
      }
    }
    /* function that changes the color used for displaying messages */
    function changeColor()
    {
    
    
      response=xmlHttpGetColor.responseText;
      // server error?
      if (response.indexOf("ERRNO") >= 0 
          || response.indexOf("error:") >= 0
          || response.length == 0)
        throw(response.length == 0 ? "Can't change color!" : response);
      // change color
      var oColor = document.getElementById("color");
      var oSampleText = document.getElementById("sampleText");
      oColor.value = response;
      oSampleText.style.color = response;
    }
  12. If you haven't already done so, copy json.js to your project's folder.

What Just Happened?

First, make sure the application works well. Load http://localhost/ajax/chat/index.html with a web browser, and you should get a page that looks like the one in Figure 5.1.

Technically, the application is split in two smaller applications that build the final solution:

  • The chat application: Here, we use AJAX for passing the messages between the client and the server. The chat window contacts the server periodically to send any messages that have been typed by the user, and retrieve the newly posted messages from the server.
  • The code for choosing a color: Here, we use AJAX for calling the PHP script that can tell us the color that was clicked by the user on the color image. We use a palette containing the entire spectrum of colors to allow the user to pick a color for the text he or she writes. When clicking on the palette, the mouse coordinates are sent to the server, which obtains the color code.

If you analyze the code for a bit, the details will become clear. Everything starts with index.html. The only part that is really interesting in index.html is a scroll region that can be implemented in DHTML. A little piece of information regarding scrolling can be found here.

Basically, the idea for having a part of the page with a scrollbar next to it is to use nested layers. In our example, the div scroll and its inner layers do the trick. The outer layer is scroll. It has a fixed width and height, and the most useful property of it is overflow. Generally, the content of a block box is confined to the content edges of the box. In certain cases, a box may overflow, meaning its content lies partly or entirely outside of the box. In CSS, this property specifies what happens when an element overflows its area. For more details, please see the overflow's specification.

The chat.js script contains the JavaScript part for our application. This file can be divided in two parts: the one that handles choosing a color, and the other that handles retrieving and sending chat messages.

We will start with the color choosing functionality. This part, which in the beginning might seem pretty difficult, proves to be easy to implement. Let's have a panoramic view of the entire process.

We have a palette image that contains the entire spectrum of visible colors. PHP has two functions that will help us in finding the RGB code of the chosen color: imagecreatefrompng and imagecolorat. These two functions allow us to obtain the RGB code of a pixel, given the x and y position in the image. The position of the pixel is retrieved in the getMouseXY function in the JavaScript code.

The getColor function retrieves the RGB code of the color chosen by the user when clicking the palette image. First of all, it retrieves the mouse coordinates from the event. Then, it computes the coordinates where the click event has been produced as relative values within the image. Using this information, getColor initiates an asynchronous request to the get_color.php script, which is supposed to return the color code associated with the clicked coordinate. The callback function handleGettingColor is executed when the state of the request is changed.

The handleGettingColor function checks to see when the request to the server is completed and if no errors occurred, the changeColor function is called. This function changes the color of the sample text "text will look like this" with the given code.

Now, let's now see how the chat works. By default, when the page initializes and the OnBlur event occurs, the checkUsername function is called. This function ensures that the name of the user isn't empty, by generating an arbitrary username.

On pressing the Send button, the sendMessage function is called. This function adds the current message to the message queue to be sent to the server. Before adding it into the queue, the function trims the message by calling the trim function, and we encode the message using encodeURIComponent to make sure it gets through successfully.

The handleKey function is called whenever a keydown event occurs. When the Enter key is pressed, the sendMessage function is called so that both pressing the Send button and pressing Enter within the MessageBox control have the same effect.

The deleteMessages function adds the delete message to the messages to be sent to the server.

The requestNewMessages function is responsible for sending chat messages. It retrieves a message from the queue and sends it to the server. The change of state of the HTTP request object is handled by the handleReceivingMessages function.

The handleReceivingMessages checks to see when the request to the server is completed, and if no errors occurred, then the readMessages function is called.

The readMessages function checks to see if someone else erased all the chat messages, and if so, the client's chat window is also emptied. In order to append new messages to the chat, we call the displayMessages function. This function takes as parameters the arrays that correspond to the new messages. It composes the new messages as HTML, and it appends them to those already in the chat, by calling the displayMessage function. In the beginning, the displayMessage function checks to see if the scroll bar is at the bottom of the list of messages. This is necessary in order to reposition it at the end of the function so that the focus is now on the last new messages.

The last function presented is the init function. Its role is to retrieve the chat messages, to ensure that the username is not null, to set the text's color to black, and to turn off the auto complete functionality.

For the error handling part, we use the displayError function, which calls the displayMessage function in turn with the error message as parameter.

Let's move on to the server side of the application by first presenting the chat.php file. The server deals with clients' requests like this:

  • Retrieves the client's parameters.
  • Identifies the operations that need to be performed.
  • Performs the necessary operations.
  • Sends the results back to the client.

The request includes the mode parameter that specifies one of the following operations to be performed by the server:

  • SendAndRetrieve: First, the new messages are inserted in the database, and then all new messages are retrieved and sent back to the client.
  • DeleteAndRetrieve: All messages are erased, and the new messages that might exist are fetched and sent back to the client.
  • Retrieve: The new messages are fetched and sent back to the client.

The business logic behind chat.php lies in the chat.class.php script, which contains the Chat class.

The deleteMessages method truncates the data table, erasing all the information.

The postMessages method inserts all the new messages into the database.

The isDatabaseCleared method checks to see if all the messages have been erased. Basically, by providing the ID of the last message retrieved from the server and by checking if it still exists, we can detect if all messages have been erased.

The retrieveNewMessages method gets all the new messages since the last message (identified by its ID) retrieved from the server during the last request (if a last request exists; or all messages in other cases), and also checks to see if the database has been emptied by calling the isDatabaseCleared method. This function composes the response for the client and sends it.

The config.php file contains the database configuration parameters, and the error_handler.php file contains the module for handling errors.

Summary

At the beginning of the chapter, we saw why one can face problems when communicating with other people in a dynamic way over the Internet. We saw what the solutions for these problems are and how AJAX chat solutions can bring something new, useful, and ergonomic. After seeing some other AJAX chat implementations, we started building our own solution. Step by step, we have implemented our AJAX chat solution, keeping it simple, easily extensible, and modular.

After reading this chapter, you can try improving the solution, by adding new features like:

  • Chat rooms
  • Simple command lines (joining/leaving a chat room, switching between chat room)
  • Private messaging

AJAX and PHP: Building Responsive Web Applications

AJAX is a complex phenomenon that means different things to different people. Computer users appreciate that their favorite websites are now friendlier, and feel more responsive. Web developers learn new skills that empower them to create sleek web applications with little effort. Indeed, everything sounds good about AJAX!

At its roots, AJAX is a mix of technologies that lets you get rid of the evil page reload, which represents the dead time when navigating from one page to another. Eliminating page reloads is just one step away from enabling more complex features into websites, such as real-time data validation, drag and drop, and other tasks that weren't traditionally associated with web applications. Although the AJAX ingredients are mature (the XMLHttpRequest object, which is the heart of AJAX, was created by Microsoft in 1999), their new role in the new wave of web trends is very young, and we'll witness a number of changes before these technologies will be properly used to the best benefit of the end users. At the time of writing this book, the "AJAX" name is about just one year old.

AJAX isn't, of course, the answer to all the Web's problems, as the current hype around it may suggest. As with any other technology, AJAX can be overused, or used the wrong way. AJAX also comes with problems of its own: you need to fight with browser inconsistencies, AJAX‑specific pages don't work on browsers without JavaScript, they can't be easily bookmarked by users, and search engines don't always know how to parse them. Also, not everyone likes AJAX. While some are developing enterprise architectures using JavaScript, others prefer not to use it at all. When the hype is over, most will probably agree that the middle way is the wisest way to go for most scenarios.

In AJAX and PHP: Building Responsive Web Applications, we took a pragmatic and safe approach by teaching relevant patterns and best practices that we think any web developer will need sooner or later. We teach you how to avoid the common pitfalls, how to write efficient AJAX code, and how to achieve functionality that is easy to integrate into current and future web applications, without requiring you to rebuild the whole solution around AJAX. You'll be able to use the knowledge you learn from this book right away, into your PHP web applications.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Mohan Raphel
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Comments and Discussions

 
QuestionI have a problem PinmemberMember 1034116717-Oct-13 7:18 
GeneralGreate work. PinmemberMohammad Dayyan2-Sep-08 11:08 
Generalthis is a gr8 example on ajax+php PinmemberHari Om Prakash Sharma1-May-07 0:56 
GeneralThank so lot Pinmemberanhoa28-Sep-06 5:29 
Generalgreat example about ajax & php Pinmembermauxanhlam9-Sep-06 15:51 
GeneralTurn ASP.NET controls into AJAXenabled controls PinmemberOmegaCD8-Jul-06 20:21 
QuestionPHP in the C# section? PinmemberJon Rista5-Jul-06 17:03 
AnswerRe: PHP in the C# section? PinmemberCristian Darie8-Jul-06 3:51 
GeneralRe: PHP in the C# section? PinmemberMember 302189318-Sep-08 0:47 
Questionc# version ? PinmemberTrance Junkie27-Jun-06 22:45 

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