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Ajax: A Beginner s Guide
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<div id="targetDiv"> <p>The fetched message will appear here</p> </div> </body>
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Referring to a file in this way will only work if the file (datatxt) is in the same directory as the web page (ajaxhtml) itself In other words, if you simply give a filename, the browser will assume that file is to be found in the same directory as the web page the browser is currently displaying Now say that the datatxt file is in data, a subdirectory of the directory that contains ajaxhtml In that case, you can refer to datatxt in a relative way as data/datatxt (you use a forward slash, /, on web servers, not the backward slash that you use in Windows to indicate subdirectories):
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<body> <H1>An Ajax example</H1> <form> <input type = "button" value = "Fetch the message" onclick = "getData('data/datatxt', 'targetDiv')"> </form> <div id="targetDiv"> <p>The fetched message will appear here</p> </div> </body>
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You can also give a complete URL for the data you want to fetch, like this:
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<body> <H1>An Ajax example</H1> <form> <input type = "button" value = "Fetch the message" onclick = "getData('http://wwwstarpowdercom/datatxt', 'targetDiv')"> </form> <div id="targetDiv"> <p>The fetched message will appear here</p> </div> </body>
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Creating Ajax Applications
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And the URL need not be the URL of a file either it could be any web resource, such as a PHP script (as you re going to see soon in this book, PHP is a programming language that runs on web servers and lets you take control of the HTML and data you send back to browsers):
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<body> <H1>An Ajax example</H1> <form> <input type = "button" value = "Fetch the message" onclick = "getData('http://wwwstarpowdercom/dataphp', 'targetDiv')"> </form> <div id="targetDiv"> <p>The fetched message will appear here</p> </div> </body>
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However, here s one thing to note: if the URL you connect to, such as http://www starpowdercom/dataphp, and the Ajax-enabled page (ajaxhtml here) that s attempting to download that URL are on different servers, you re going to have a security problem If your Ajax-enabled page attempts to download data behind the scenes from a different server, your browser is going to suspect that something underhanded is going on, and will ask permission from the user, via a dialog box, before proceeding Making the user respond to a dialog box, however, is not exactly Ajax s idea of doing things unobtrusively, behind the scenes We ll see how to get around this restriction in 4 To keep it simple, the examples in this book mostly download data from the same web server directory that the Ajax-enabled HTML page itself is in Now you ve configured the XMLHttpRequest object The next step is to get ready for the data download
Getting Ready for the Data Download
The A in Ajax stands for asynchronous, which means that you don t sit around waiting for the Ajax download to happen While the download is going on, the browser can be doing other things interacting with the user, for example That means that Ajax needs to signal the browser when the data has been downloaded and is ready to be used, and that happens with a callback function That s a function that Ajax calls when your data is downloading, or has been completely downloaded How do you tell Ajax which function you want to call You tell the XMLHttpRequest object That object has a property named onreadystatechange that you assign the callback function to As an example of how it might work, suppose you have
Ajax: A Beginner s Guide
a callback function simply named callback Here s how you could have the XMLHttpRequest object call that function, by assigning that function to the onreadystatechange property:
function getData(dataSource, divID) { if(XMLHttpRequestObject) { XMLHttpRequestObjectopen("GET", dataSource); XMLHttpRequestObjectonreadystatechange = callback() } } function callback() { }
Now when there s a change in the status of the data you re downloading, such as when the downloading is complete, the callback function will be called This works, but in Ajax, you usually take a shortcut with an anonymous function one without any name You can create an anonymous function simply with the keyword function, followed by the arguments passed to that function enclosed in parentheses (there aren t any arguments here), followed by the body of the function, enclosed in curly braces Here s what that looks like in ajaxhtml:
function getData(dataSource, divID) { if(XMLHttpRequestObject) { XMLHttpRequestObjectopen("GET", dataSource); XMLHttpRequestObjectonreadystatechange = function() { } } }
So what s going to happen here is that when there s a change in the status of the data the XMLHttpRequest object is downloading, the anonymous function is going to be called, which means that the code in the curly braces following the function keyword will be executed
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