16. WML Event: onenterbackward

The onenterbackward event is triggered if a user goes back to a previous card through the WAP browser's URL history, and the WML code placed in the event handler will be executed.

The following WML example shows a situation in which the onenterbackward event has to be used. It is used to prevent a user from going back to a particular card. What we want to do is like this: We want to place an advertisement in an intermediate card between the table of contents and the chapters of a WML tutorial. If a user navigates to a chapter from the table of contents, an advertisement will appear for a few seconds. Later if the user presses the Back button to go back to the table of contents, the WAP browser will not display the advertisement again so as to reduce the disturbance to the user.

(onenterbackwardEg1.wml)

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.3//EN" "http://www.wapforum.org/DTD/wml13.dtd">

<wml>
  <card id="tutorial_tc" title="WML Tutorial">
    <p>
      Hello, welcome to our WML tutorial.<br/><br/>
      Table of Contents:<br/>

      <a href="#tutorial_ch1_ad">Chapter 1: WML Introduction</a><br/>

      <a href="#tutorial_ch2_ad">Chapter 2: WML Deck and Card</a><br/><br/>

      ...
    </p>
  </card>

  <card id="tutorial_ch1_ad" title="Advertisement">
    <onevent type="onenterbackward">
      <prev/>
    </onevent>
    <onevent type="ontimer">
      <go href="#tutorial_ch1"/>
    </onevent>
    <timer value="30"/>
    <p>
      <a href="http://12345site.com/">Click here to download our free ringtones, wallpapers, mobile games and MP3 songs now.</a><br/><br/>
      <a href="#tutorial_ch1">Skip</a>
    </p>
  </card>

  <card id="tutorial_ch2_ad" title="Advertisement">
    <onevent type="onenterbackward">
      <prev/>
    </onevent>
    <onevent type="ontimer">
      <go href="#tutorial_ch2"/>
    </onevent>
    <timer value="30"/>
    <p>
      <a href="http://67890site.com/">Click here to buy WAP-enabled cell phones online now. Free shipping.</a><br/><br/>
      <a href="#tutorial_ch2">Skip</a>
    </p>
  </card>

  <card id="tutorial_ch1" title="WML Tutorial Ch1">
    <p>
      <em>Chapter 1: WML Introduction</em><br/><br/>
      ...<br/>
      <anchor>
        <prev/>
        Go Back
      </anchor>
    </p>
  </card>

  <card id="tutorial_ch2" title="WML Tutorial Ch2">
    <p>
      <em>Chapter 2: WML Deck and Card</em><br/><br/>
      ...<br/>
      <anchor>
        <prev/>
        Go Back
      </anchor>
    </p>
  </card>
</wml>

Below shows the screenshots of the first card in some mobile phone browsers. This card contains the table of contents of the tutorial.

If you select the "Chapter 1: WML Introduction" anchor link, you will see an advertisement in the mobile phone browser, like this:

If you do not select the "Skip" anchor link, the mobile phone browser will display Chapter 1 of the tutorial automatically after 3 seconds:

If you select the "Go Back" anchor link, the onenterbackward event will be triggered when the advertisement card is loaded. The WML code associated with the onenterbackward event, that is <prev/>, will be executed. So, the mobile phone browser will go backwards once more and you will see the table of contents instead of the advertisement.

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