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The XMLHttpRequest Object

The XMLHttpRequest object, which is part of JavaScript, has been around for years but has really gained popularity recently thanks to the rise of Ajax. XMLHttpRequest is a class that defines the functionality for HTTP (HyperText Transfer Protocol) interactions. Even if that sounds like gibberish to you, you're actually quite familiar with the concept: When you load a Web page in your browser, you're making an HTTP request (normally). Using XMLHttpRequest, JavaScript in one page can make that same kind of transaction behind the scenes (i.e., without the browser leaving the current page). The JavaScript can use the response from the second page as needed, most likely to update the first page's content in some way.

As an example of how you might use this, say you wanted to create an iTunes-like application with a search feature. The user enters some text—a song or album title, or an artist's name—in a box, and then clicks an icon or presses Enter. This would queue the JavaScript, which would use Ajax to send the search term to another page. That page would, unbeknownst to the user, actually perform the search and return the results to the original page. The JavaScript in that page would then update the application window, showing the results of the search.

There are many ways this kind of functionality can be added to an application by using the AIR API. But it's worth knowing how to make an XMLHttpRequest using plain old JavaScript, so let's work through an example.

To use XMLHttpRequest, start by creating an object of type XMLHttpRequest:

var xhr = new XMLHttpRequest();

Next, provide to the open() method the type of request to make—normally GET or POST—and the file to be communicated with:

xhr.open('get', 'filename.ext');

This line opens a connection to filename.ext, to which it will make a GET request. If you're not familiar with what GET and POST mean, search the Web for (probably too detailed) answers. Or, for the time being, simply understand that you'll generally use GET, because it's the standard method for requesting information from a page, whereas POST is used to send information to a page.

The next step is to name the function to be called when filename.ext returns its results. The onreadystatechange property takes this value. This property is one of five important XMLHttpRequest object properties listed in Table 4.1 (remember that in object-oriented programming a property or attribute is a variable defined in a class). Assign to this property the name of the function without any parentheses or quotation marks:

xhr.onreadystatechange = callThisFunction;

Table 4.1. Performing XMLHttpRequests relies upon the XMLHttpRequest properties listed here.

XMLHttpRequest Properties

Property

Contains the...

onreadystatechange

Name of the function to be called when the readyState property changes

readyState

Current state of the request (see Table 4.2)

responseText

Returned data as a string

responseXML

Returned data as XML

status

HTTP status code returned

So the JavaScript will send the request to filename.ext, that page will send back a reply, and at that time the callThisFunction() function will be called. This function, defined shortly, will take the returned data and update the page content accordingly.

The last step in this sequence is to send the request. For GET requests, you should provide the value null as the send() method's only argument:

xhr.send(null);

That wraps up the "making the request" JavaScript; next is the handling of the returned results (what filename.ext sends back). Remember that this will be done within the callThisFunction() function. But you'll first want to confirm that the request was successful. To do so, check that the readyState is equal to 4 (see Table 4.2 for the list of readyState values):

function callThisFunction () {
    if (xhr.readyState == 4) {
       // Handle the returned data.

Table 4.2. Of the five readyState values listed here, the last one is the most important for knowing when to handle the returned data.

XMLHttpRequest readyState Values

Value

Meaning

0

uninitialized

1

loading

2

loaded

3

interactive

4

complete

The readyState attribute indicates the status of the request process. At first the ready-State value is uninitialized (which equals 0, see Table 4.2). When the request is made, the server page starts to load, making the readyState value 1. Then the server page finishes loading, making readyState 2. Some interaction will occur between the two pages, making readyState 3, and eventually the request is completed, giving readyState a value of 4. Often, these states will change very quickly, but in terms of handling the response, getting a readyState of 4 is most important.

Having created the XMLHttpRequest object, performed the transaction, and confirmed the results, the final step is to use the returned data to alter the page content. The easiest way to access that data is to refer to the responseText property. This attribute stores the result of the request, which is what the requested page would display if loaded directly in a Web browser. If the result of the request is XML data, you would use responseXML instead.

Once you have the page's response, you can use it however the application dictates. You might write the response content to the page, use it to change some existing values, and so on. To help demonstrate this concept and to provide you with some usable code, let's run through a basic example of an XMLHttpRequest.

To use XMLHttpRequest:

  1. Begin a new AIR project in your text editor or IDE.

    For this particular program, it's not necessary to include the AIRAliases.js file, although it's not a big deal if you do.

  2. Create a plain text file named message.txt that contains some text (Script 4.3).

    Script 4.3. This bit of a text (a quote from Homer Simpson, naturally) will be retrieved using an XMLHttpRequest.

    1   They have the Internet on computers now.
    

    The contents of this file will be read in by the XMLHttpRequest object and printed in the main application page.

  3. Within the body of the main HTML file, add the following (Script 4.4):
    <h1 id="response" style="color: red;"></h1>
    <button id="do" onclick="getMessage()">Get the message!</button>
    

    Script 4.4. This is the primary HTML file for the XMLHttpRequest AIR application. It performs the actual request, updating the body of the page using the results.

    1   <html><!-- Script 4.4 -->
    2      <head>
    3         <script type="text/javascript">
    4
    5            // Create an XMLHttpRequest object:
    6            var xhr = new XMLHttpRequest();
    7
    8            // This function is called when the user clicks the button:
    9            function getMessage() {
    10
    11              // Open the connection:
    12              xhr.open('get', 'message.txt');
    13
    14              // Identify the function to handle the ready state change:
    15              xhr.onreadystatechange = printMessage;
    16
    17              // Send the request:
    18              xhr.send(null);
    19
    20           } // End of getMessage() function.
    21
    22           // This function updats the page after the request is made:
    23           function printMessage() {
    24
    25              // Only do something when the readyState is complete:
    26              if (xhr.readyState == 4) {
    27                 document.getElementById( 'response' ).innerText = xhr.responseText;
    28              }
    29
    30           } // End of printMessage() function.
    31
    32        </script>
    33     </head>
    34     <body>
    35        <h1 id="response" style="color: red;"></h1>
    36        <button id="do" onclick="getMessage()">Get the message!</button>
    37     </body>
    38  </html>
    

    The body of this application is just a button (Figure 4.10) that, when clicked, invokes the XMLHttpRequest functionality. That JavaScript will update this empty H1 with the response from the text file (Figure 4.11).

    Figure 4.10

    Figure 4.10 The application as it appears when it first opens.

    Figure 4.11

    Figure 4.11 The result after clicking the button.

  4. Within the head of the main HTML file, begin a section of JavaScript and create an XMLHttpRequest object:
    <script type="text/javascript">
    var xhr = new XMLHttpRequest();
    
  5. Define the getMessage() function:
    function getMessage() {
      xhr.open('get', 'message.txt');
      xhr.onreadystatechange = printMessage;
      xhr.send(null);
    }
    

    This function will be called when the user clicks the button (see the code in step 3). The first step within the function is to invoke the open() method of xhr (that variable is accessible within the function because it was defined outside of the function, per JavaScript scope behavior). The first argument is the HTTP method to use and the second is the name of the file to request, which in this case is message.txt, created earlier.

    Next, assign to the onreadystatechange attribute the name of the function to be called when the readyState value changes. Finally, make the request by calling send().

  6. Define the printMessage() function:
    function printMessage() {
      if (xhr.readyState == 4) {
         document.getElementById( 'response' ).innerText = xhr.responseText;
         }
    }
    

    This function will be called whenever the readyState value changes (it will actually change several times). The code in this function will not do anything until readyState has a value of 4. At that time, the innerText—the value between the tags—of the H1 with an ID of response will be assigned the value of the textual response of message.txt. This will literally be the contents of that file (see step 2).

  7. Complete the JavaScript section:
    </script>
    
  8. Save, test, debug, run, and build the application.

    Make sure that the message.txt file is in the same directory as index.html and that it's included when you build the application.

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