Passing Values from ASP to WML

A common question asked by developers new to WML is how can they pass values between WML client-side variables and ASP server-side variables. The following example illustrates the case:

 

<% Response.ContentType = "text/vnd.wap.wml" %>

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"

                     "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

   <card id="card1" title="Card 1">

      <p>

         x is 500 (in ASP)

         <% x = 500 %>

         <do type="accept" label="Card 2">

            <go href="#card2" method="get">

               <setvar name="x" value="<% =x %>" />

            </go>

         </do>

      </p>

   </card>

 

   <card id="card2" title="Card 2">

      <p>

         The value of x is $(x) (in WML)

      </p>

   </card>

</wml>

 

 

Here, I'm passing the value of x in ASP to the variable in x in WML. I make use of a <setvar> element to pass the value from ASP to WML.

Passing Values from WML to ASP

How about the other way round?

 

<% Response.ContentType = "text/vnd.wap.wml" %>

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"

                     "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

   <card id="card1" title="Card 1">

      <p>

         How many stars do you want to display?

         <select name="loop">

            <option value="1">1 star</option>

            <option value="2">2 stars</option>

            <option value="3">3 stars</option>

            <option value="4">4 stars</option>

         </select>

         <do type="accept" label="Display">

            <go href="#card2" method="get">

               <postfield name="loop" value="$loop" />

           </go>

         </do>

      </p>

   </card>

 

   <card id="card2" title="Card 2">

      <p>

         <% loopcount = Request.QueryString("loop")

            For i= 1 To loopcount

               Response.Write "*"

            Next

         %>

      </p>

   </card>

</wml>

 

 

In this case, I'm using the <postfield> element to pass the value to card2:

 

               <postfield name="loop" value="$loop"/>

 

And in card2, I've used the Request.QueryString collection to retrieve and assign the value to the variable loopcount in ASP:

 

         <% loopcount = Request.QueryString("loop")

Tips for Using ASP to Generate WML Content

When generating WML content using ASP, bear in mind that a deck can contain multiple cards. If your content for each card is generated dynamically, it may have an impact on the efficiency of your web server. To illustrate this, consider the following example:

 

<% Response.ContentType = "text/vnd.wap.wml" %>

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN"

                     "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

   <!-- First Card -->

   <card id="card1" title="Card1">

      <p>

         Time on card 1 is : <% =time %>

         <do type="accept" label="Card2">

            <go href="#card2" />

         </do>

      </p>

   </card>

 

   <!-- Second Card -->

   <card id="card2" title="Card2">

      <p>

         Time on card 2 is : <% =time %>

         <do type="accept" label="Card1">

            <go href="#card1" />

         </do>

      </p>

   </card>

</wml>

 

When the ASP parser parses the above deck, both cards will be 'given' the same time. When the user moves from card1 to card2, they will see the same time on both cards. This may not be what we intended - we might want the cards to contain the different times that they were displayed on the WAP device.

 

To solve this problem, I need to make sure that caching is turned off on the WAP device. For this, I can use the Response.Expires property, as described earlier in the paper. However, this causes the deck to be parsed twice by the ASP parser, as the WAP device will have to reload the same deck from the origin server to display the second card.

 

 

Remember: caching is at deck level. When a user moves from a card to another within a deck, using a Response.Expires property will not cause a reload of the deck from the origin server.

 

Hence, when you have dynamic content on multiple cards within a deck, it is important that these cards be separated into different decks. The diagram showing the request and response between the WAP device and origin server is valid provided caching can be done at the card level.

 

As can be seen, every time the WAP device requests the same deck, the ASP server spends time parsing it. If the deck contained a lot of cards, the time needed would be substantial: this is inefficient. Instead, by breaking the cards down into multiple decks, the time spent parsing individual decks is reduced.

Common Errors

Finally, let's briefly examine some of the common errors that developers may encounter when using ASP to generate WML content.

 

Not setting the correct MIME type

The WML MIME type must be set using the Response.ContentType property, like this:

 

<% Response.ContentType = "text/vnd.wap.wml" %>

 

Cookies are not supported

When using cookies or the Session object, always ensure that the target platform is capable of supporting cookies.

 

The POST method may not work correctly on some WAP devices

When using a <postfield> element, be aware that some WAP devices (the Nokia 7110, for instance) have problems in supporting the POST method.

 

You need a web server to run ASP!

Very often, people just run ASP documents straight from their local hard disk. ASP documents require a web server (in particular, the ASP parser) to process them, so that the resulting page can be sent to the client. Also important point to note is that you need to use the http:// syntax, and not the physical filename.