WML and WMLScript

Inheritance and Templates

Wouldn't it be nice to inherit something? Don't you sometimes wish that some rich old uncle died somewhere and left you a few million dollars? Well, this is what inheritance is all about. The only difference is that no body has to die - yet!

The yet after a pause is significant. It is because the person who may die will be you - if you do not understand this chapter well. So pay close attention to all that goes on, because the changes are so slight that you may miss the significance until it is too late.

Take for instance, the situation in the French revolution. The revolution was at its peak, and the revolutionists brought out these three fine lads of the King. The first, Raj, was in charge of the computer systems imported into the country. When asked how he would like to die, Raj chose to do so with his head down. He was ashamed of all the second rate computers he had sold his countrymen, and so could not face the crowd.

The crowd jeered and cried for blood. "Execute him!" yelled the party chief. The executioner released the cable, and down came the guillotine. But it stopped just two inches from Raj's neck. Feeling it was divine intervention because he had repented from the bottom of his heart, the crowd let him go.

The second, Har, was incharge of group software and networking. He too chose to look down in shame. After all, he had worked out such great deals for himself and even started another company under the guise of sharing some of the revenue that he was making with the public at large. But always managed to keep the lion's share for himself. Repentant, he too did not want to face the crowd. Again, the guillotine was raised, and then let go. And for the second time, it stopped a scant 2 inches from it's victim's neck. Not wanting to anger the Lord, as it was he who pardoned Har, the crowd let him go as well.

And the third, Dew, who could really do nothing, however, was a different sort. Incharge of the software companies, he couldn't care less about anyone else. So what if his wig fell off as he looked to the sky. He would show the crowd that he was made of sterner stuff. He'd face the guillotine as it cut off his head.

The crowd cheered and yelled - "Off with his head!" The guillotine was raised, and just as the party head was about the give the order. Dew cried out, "Wait!" The executioner stopped. A hush fell over the crowd. Would he repent too, they wondered. But no, Dew was not going to. In fact, as usual lost in his own thoughts, and eager to prove himself smarter than all, he cried out in a loud voice. "Stop, I can see what's jamming your machine!"

Don't be too eager. Take your time to understand the basics, for it is the subtle differences that we demonstrate here, that will make big differences later. You will get your chance to be a genius later.

In this example, you will wonder why this has been repeated. This routine has 3 cards.

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

</do>

<p>

Card 1

</p>

</card>

<p>

Card 2

</p>

</card>

<p>

Card 3

</p>

</card>

</wml>


Screen 5. 1	Screen 5. 2	Screen 5. 3

Each card is given an id. The first card has a <do> and the type is given as accept. The label ‘first’ shows up on the next screen. When the program starts, the title yes will appear at the top of the screen. Selecting first will execute <go href>. This takes you to card a2. If you have followed the program sequence, you will have understood that at this point, there is no way to jump to a3.

Almost always a user would like to go back to the previous screen. Unfortunately, in WAP appliances, there is no Back button as seen in a web browser. To do so, we learnt about the <prev> tag. But then this feature has to be built into the program. If this has to be added to every card, it would be a big pain. Would it not be a good idea to have every card copy or display a certain standard user interface? For this, there is a tag called template.

Screen 5. 4

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

</do>

</template>

</do>

<p>

Card 1

</p></card>

<p>

Card 2

</p></card>

<p>

Card 3

</p>

</card>

</wml>


Screen 5. 5	Screen 5. 6	Screen 5. 7	Screen 5. 8

Don't hope to have everything done for you though, or else your job as a WAP programmer will be pretty boring. The template comes immediately after <wml>. It has to come before the first <card>. Simply, template means copy. Anything that you place in template will be used in all the cards you define later. In <template>, whatever code i.e. <do /do> you write, is automatically available in <card>. You should realise by now, that when you have a <do> in a template, it gets added on to the <do> in the card. And all the other cards as well. Hence in the first card there are two dos with two separate labels and the second and the third cards will have one do each.

While copy is a nice simple word that could be used, programmers never touch it. Hence, this is called inheritance. It means that every card that follows, will inherit all that the template has.

But inheritances rarely come without problems. What happens if two people have the same name? Who inherits what? The do, for instance, has also been given a name. If you don't specify a name, the name is inherited as well. But if you do, then it has an identity of its own. If you use the names similar to the ones in the template, then the definition in the card overwrites the definition in the template.

In the next scriptlet, we have <do /do> within a card. The type is accept, then we give a name. Within the do tags we have given a new instruction ie noop.

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

<noop/>

</do>

<p>

Card 1

</p>

</card>

</wml>

Screen 5. 9

Screen 5. 10

Noop does nothing. Those who worked their way up the programming ladder, and worked with assembly languages, will recognise this as being the term used to define No Operation. In this case, the noop effectively disables the <do>. Within the service options, first is no more visible.

In wml, you have 4 tasks - go, prev, which take you somewhere either forward or backward in the program; refresh is to display the same card over again; and noop which does nothing at all. With a noop, you will see no options displayed. You may ask why you need a noop? The reason is simple. You want to retain all the other features of the template and disable the do. You therefore can have the <prev>, <go> and other items defined, and disable the <do>s you don't want to use.

Screen 5. 11

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

</do>

</template>

<noop/>

</do>

<p>

Card 1

</p>

</card>

</wml>

Manuals hardly keep up with all the changes that programmers make to the program. This is why you keep having books come out with "Secrets to…(fill in whatever subject you like)". This example is something that we found out by experimenting. It is not there in the manual. This may get a bit confusing. So slow down a bit and stay with us. What we have is an example using an onenterforward.

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

</onevent>

<p>

Hello World!!!

</p>

</card>

</wml>

Screen 5. 12

Screen 5. 13

The onenterforward will move to #a2. The onevent also has an onenterforward which says goto a3. The documentation tells you that in the card if you put the onenterforward, it is a shorter form of saying <onevent> </onevent>. But then, which came first - the chicken or the egg? Is the onenterforward called before the onevent? Or is this the other way around? The documentation is silent about this. If in a card you have onevent; and the onevent has an event, the onevent's event takes precedence. In the example, you will see a3 gets called and not a2.

Let's add a little more spice to our already complicated plot. Often, books (as manuals do) cast off all these 'vague' questions. After all, you are not supposed to do anything extra. Well, programmers skimped on a couple of bytes a few decades ago, and you know how much was spent fixing the problem.

In a template, you can also have an onevent. The card can also have an onevent. Both the do and the onevent, as we have told you before, are same. The rule is that if you have the same onevent in the card and in the template, the one in the card will take effect first. Between the card and the template, the card gets the priority.

a11.wml

<?xml version="1.0"?>

<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://www.wapforum.org/DTD/wml_1.1.xml">

<wml>

</onevent>

</template>

<p>

Hello World!!!

</p>

</card>

</wml>

Screen 5. 14

Screen 5. 15

Our last example goes a step further and we have added a template with onevent. That means in the card you can say event, and in the template also you have an onevent. The card's event gets called first.

The same card rules apply here. To sum up - a card gets precedence over the template when the same events are used in the wml file. The rule also applies for oneventbackward event or any other event that you may use.