ASP has always provided the opportunity to execute components on the server, and these components can generate sections of the page that is returned to the user. ASP+ extends this concept through server controls. All that's required to turn any HTML element into a server control is the addition of an extra attribute: runat="server".
Any HTML element in a page can be marked this way, and ASP+ will then process the element on the server and can generate output that suits this specific client. And, as a by-product, we can do extra tricks – in particular with HTML <form> and the associated form control elements, where we can create the code to manage state during round trips to the server. This makes the programming experience less monotonous and dramatically more productive.
While the concept of having HTML elements that execute on the server may at first seem a little strange, as you'll see it adds a whole new layer of functionality to the pages, and makes them easier to write at the same time. What more could a programmer want?
The Problems with Maintaining State
One of the most cumbersome tasks when creating interactive Web sites and applications is managing the values passed to the server from HTML form controls, and maintaining the values in these controls between page requests. So one of the core aims of ASP+ is to simplify this programming task. This involves no extra effort on the part of the programmer, and works fine on all browsers that support basic HTML and above.
Take a look at the following section of code. This creates a simple form using HTML controls where the user can enter the name of a computer and select an operating system. OK, so this isn't a terribly exciting example in itself, but it illustrates a pretty common scenario used by almost every web application out there today. When the form page is submitted to the server, the values the user selected are extracted from the Request.Form collection and displayed with the Response.Write method. The important parts of the page are highlighted in the code listing:
<html>
<body>
<%
If Len(Request.Form("selOpSys")) > 0 Then
strOpSys = Request.Form("selOpSys")
strName = Request.Form("txtName")
Response.Write "You selected '" & strOpSys _
& "' for machine '" & strName & "'."
End If
%>
<form action="pageone.asp" method="post">
Machine Name:
<input type="text" name="txtName">
<p />
Operating System:
<select name="selOpSys" size="1">
<option>Windows 95</option>
<option>Windows 98</option>
<option>Windows NT4</option>
<option>Windows 2000</option>
</select>
<p />
<input type="submit" value="Submit">
</form>
</body>
</html>
Although this is an ASP page (the file extension is .asp rather than .aspx), it will work just the same under ASP+ if we changed the extension to .aspx. Remember that the two systems can quite freely co-exist on the same machine, and the file extension just determines whether ASP or ASP+ processes it.
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This screenshot shows what it looks like in Internet Explorer 5. When the user clicks the Submit button to send the values to the server, the page is reloaded showing the selected values. Of course, in a real application, some the values would probably be stored in a database, or be used to perform some application-specific processing – for this example we're just displaying them in the page: |
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One problem is that the page does not maintain its state, in other words the controls return to their default values. The user has to re-enter them to use the form again. You can see this is the next screenshot: |
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To get round this situation, we have to add extra ASP code to the page to insert the values into the controls when the page is reloaded. For the text box, this is just a matter of setting the value attribute with some inline ASP code, using the HTMLEncode method to ensure that any non-legal HTML characters are properly encoded. However, for the <select> list, we have to do some work to figure out which value was selected, and add the selected attribute to that particular <option> element. The changes required are highlighted below:
<html>
<body>
<%
If Len(Request.Form("selOpSys")) > 0 Then
strOpSys = Request.Form("selOpSys")
strName = Request.Form("txtName")
Response.Write "You selected '" & strOpSys _
& "' for machine '" & strName & "'."
End If
%>
<form action="pageone.asp" method="post">
Machine Name:
<input type="text" name="txtName"
value="<% = Server.HTMLEncode(Request("txtName"))
%>">
<p />
Operating System:
<select name="selOpSys" size="1">
<option
<% If strOpSys = "Windows 95" Then Response.Write "
selected" %>
>Windows 95</option>
<option
<% If strOpSys = "Windows 98" Then Response.Write "
selected" %>
>Windows 98</option>
<option
<% If strOpSys = "Windows NT4" Then Response.Write "
selected" %>
>Windows NT4</option>
<option
<% If strOpSys = "Windows 2000" Then Response.Write "
selected" %>
>Windows 2000</option>
</select>
<p />
<input type="submit" value="Submit">
</form>
</body>
</html>
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Now, when the page is reloaded, the controls maintain their state and show the values the user selected: |
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This page, named pageone.asp, is in the Chapter01 directory of the samples available for the book. You can download all the sample files from our Web site at http://www.wrox.com.
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