Making a CelebrityCollector with Apache Tapestry: the For Component - The For component (Page 4 of 4 )
The For component does three crucially important things:
It takes a bunch of objects and iterates through them one by one. Technically, this bunch can be an Array or any representatives of the Collection interface, such as an ArrayList or a HashSet, to name a few.
Each time it picks an object from the bunch it makes that object available for our code.
It displays any markup and any components surrounded by itself as many times as there are objects in the bunch.
Let’s begin from configuring a For component in the List page specification:
<component id="eachCelebrity" type="For">
<binding name="source" value="celebrities"/>
</component>
We’ve specified a For component and given it some name (eachCelebrity). We have also provided its required and most important binding, source. This is where the bunch of objects will come from. Let’s define the getCelebrities() method in the CelebritiesList page then:
public abstract class CelebritiesList extends BasePage {
public List getCelebrities() {
return new DataSource().getCelebrities();
}
}
We can place a For component in HTML markup in the same way as we did this before, using a div or a span. However, in this specific case it is more reasonable to convert a table row (tr element) into a For component. Let’s see how we can do this:
<table width="300" cellpadding="5" border="1">
<tr>
<th>Last Name</th>
<th>First Name</th>
</tr>
<tr jwcid=”eachCelebrity”>
<td>
<a href="">Smith</a>
</td>
<td>John</td>
</tr>
<tr jwcid=”$remove$”>
<td>
<a href="">Smithson</a>
</td>
<td>Jane</td>
</tr>
<tr jwcid=”$remove$”>
<td>
<a href="">Swedenborg</a>
</td>
<td>Emmanuel</td>
</tr>
</table>
We have marked one of the table rows as an “eachCelebrity” component which we have just defined in the page specification. It should be repeated as many times as there are Celebrity objects in the bunch provided to this component. But now we have two table rows which we don’t want to see at runtime; they were written by the designer for preview purposes only.
Quite conveniently, Tapestry has a special thing which looks like a component ID but in fact just marks an HTML element for removal at runtime: jwcid=”$remove$”. So we have marked the two remaining table rows for removal.
Run the application, and this is what you should see:
It works! The rows for Jane Smithson and Emmanuel Swedenborg were removed, as required, and the row for John Smith was repeated four times – exactly the number of celebrities in our mock data source. Now we need to display the real celebrities instead of John Smith.
First of all, we shall ask the For component to expose to our code the Celebrity object it is currently iterating through. We use another binding for this purpose:
<component id="eachCelebrity" type="For">
<binding name="source" value="celebrities"/>
<binding name="value" value="currentCelebrity"/>
</component>
Each time our For component picks a Celebrity object from the List provided to it, it will pass this object to the page class by invoking the page class’ setCurrentCelebrity() method. We don’t have such a method at the moment and we don’t need to write it by hand. All we need to do is tell Tapestry to create a currentCelebrity property.
Normally, we would simply provide an abstract getter method, and Tapestry would do the rest. This time however we are not going to access the new property from our Java code. As you will see in a moment, it will be used only by components on the page. So it makes sense to specify the property in the page specification:
<page-specification
class="com.devshed.tapestry.celebrities.CelebritiesList">
<property name="currentCelebrity"/>
<component id="eachCelebrity" type="For">
<binding name="source" value="celebrities"/>
<binding name="value" value="currentCelebrity"/>
</component>
</page-specification>
Finally, we are going to use a couple of Insert components to display the first and last name of the current celebrity in the corresponding table row:
<tr jwcid="eachCelebrity">
<td>
<a href="">
<span jwcid="lastName">Smith</span>
</a>
</td>
<td>
<span jwcid="firstName">John</span>
</td>
</tr>
Since we decided to use declared components here, we need to define them in the page specification:
<component id="firstName" type="Insert">
<binding name="value" value="currentCelebrity.firstName"/>
</component>
<component id="lastName" type="Insert">
<binding name="value" value="currentCelebrity.lastName"/>
</component>
Run the application, click on the “List Celebrities” link, and you should see the correct result:
However, if you click on a link in the table, nothing will happen, while we would like to see the Details page for the selected celebrity. To implement this functionality, we need a DirectLink component, and this is exactly what we are going to deal with in the next article.
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