net qr code open source s AJAX APPLICATIONS AND EMPOWERING THE WEB USER EXPERIENCE in Visual Basic .NET

Print QR Code in Visual Basic .NET s AJAX APPLICATIONS AND EMPOWERING THE WEB USER EXPERIENCE

CHAPTER 11 s AJAX APPLICATIONS AND EMPOWERING THE WEB USER EXPERIENCE
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Figure 11-6. The image server in action
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Accessing the Image Server from HTML
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The reason why you do not see any source on this page is because there is none. The request to the server simply returns a picture. By setting the MIME type on the response, the browser knows to read the bitstream for the image and render it as a JPG. Thus, within a page, you could point an <img> tag at the page to get it to render these graphics. Here s an example:
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<!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" > <head> <title>Untitled Page</title> </head> <body> <img alt="No Image" src="GetPicture.aspx picID=100" /> <img alt="No Image" src="GetPicture.aspx picID=102" /> <img alt="No Image" src="GetPicture.aspx picID=103" /> </body> </html>
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The <img> tags don t point to a JPG or another image, just to this service. So, when you render the page in the browser, the call is made to the service for each <img> tag, and the results are rendered by the browser. You can see this in Figure 11-7.
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CHAPTER 11 s AJAX APPLICATIONS AND EMPOWERING THE WEB USER EXPERIENCE
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Figure 11-7. Viewing several pictures from the server with <img> tags
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Writing the Forward-Caching Ajax Client
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The principle behind the forward-caching Ajax client is very straightforward. You keep track of the ID of the current image and provide buttons that allow you to add and remove 1 from this ID. You then construct a URL to the image server, passing it the new ID and getting the response. Then you download the images that are 1 and 2 more than this value, as well as those that are 1 and 2 less than this value. When you call the URL and download the images, the browser will cache them. For example, if the current image is 100, and you move forward to 101, the page will construct a URL call to the server asking for 101. If this has already been called, the results will be cached, and the image loaded from the cache. It will also construct calls to 102, 103, 100, and 99, and put the responses in the browser cache. To do this, you need to use a slightly different technique from what you did in the simple example. You ll use two XMLHttpRequest objects: one with a callback that renders the image, and one that doesn t use a callback, but just gets the image and caches it. Add a new HTML page to your application and call it AJAXPictureClient.htm. Change the body of the application to contain a <div> in which the image will be displayed, and two buttons one for moving forward and one for moving backward. Here s the complete markup, including the JavaScript event-handler function calls that handle loading the page and clicking the buttons:
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<body onload="initAJAX();"> <div id="pic"><img src="GetPicture.aspx picid=100" alt="not present" /></div> <input type="button" value="<<" id="btnPrev" onclick="DoPrev();" /> <input type="button" value=">>" id="btnNext" onclick="DoNext();" /> </body>
CHAPTER 11 s AJAX APPLICATIONS AND EMPOWERING THE WEB USER EXPERIENCE
Next, add a <script> tag to the head of your page, and add the following var declarations to it. They aren t part of a function within the <script> tag, so they are common to all functions.
var var var var var ajaxRequest cachedAjaxRequest currentPic = 100; theURL; theCachedURL;
As you step through the functions, you ll see the purpose of these. The first function that you ll need to implement is the initAJAX function called by the loading event of the page body. Here it is:
function initAJAX() { try { ajaxRequest = new cachedAjaxRequest } catch(error) { ajaxRequest = new cachedAjaxRequest } ShowPic(currentPic); }
XMLHttpRequest(); = new XMLHttpRequest();
ActiveXObject("Microsoft.XMLHTTP"); = new ActiveXObject("Microsoft.XMLHTTP");
This creates two XMLHttpRequest objects. The reason for using two of them is subtle. You may think that you can just use one instance, and only provide the callback function for calls to it where you want to display the image. Unfortunately, Ajax doesn t allow this. Once you set up a connection and specify the callback, the callback will be hit for all calls to that function. This function also calls ShowPic, passing it the value of the current picture. You can see the ShowPic function here:
function ShowPic(thisPic) { theURL = "GetPicture.aspx picid=" + thisPic; ajaxRequest.open("GET", theURL);
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