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Date: 27 January 2006
Author: David Leangen http://www.leangen.net

Overview

This article shows how to display read-only information from a database. The text is based mostly on information provided on the mailing list by Igor Vaynberg.

Use Case

We want to search a CRM database for contacts. Contacts are provided based on search criteria. In this application, contact information is read-only.

Wicket Solution

The page that we create will contain a repeater of some kind (see also repeater examples). Since the results will be different for each page view, it is not sufficient to initialize the repeater from within the page constructor. Since in Wicket a page is only constructed once, in this case the data would never change.

Rather, what we need is a repeater that refreshes itself each time the page renders. This is exactly what ListView is for!

When working with a database, it is often a good choice to use IDataProvider/Dataview repeaters. Such repeaters are
specifically tailored to work with data retrieved from a database.

However, for the sake of simplicity, in this example we use the ListView, which is the "simplest" approach.

Upon each page render, ListView reads a List (which just happens to be its model object) and renders its list items based on that List.

The following is a very basic example of how to go about this.

  Note: this code has not been tested. It is only intended to show the concepts.
  
  SearchPage extends WebPage
  {
      String criteria;
      List results;
  
      public SearchPage()
      {
          ListView view = 
              new ListView ( "list", new PropertyModel(this, "results"))
              {
                   protected void populateItem( ListItem item )
                   {
                       String result = (String) item.getModelObject();
                       item.add( new Label( "item", result ) );
                   }
               };
  
          Form form = new SearchForm(....);
          form.add( new TextField( "textField", new PropertyModel(this, "criteria" ) );
          form.add
          ( 
              new Button("button")
              {
                  public void onSubmit()
                  {
                      results = getResultsFromCriteria(criteria);
                  }
              }
          );
          add(form);
      }
  }

The principle is this: every time the page renders, the ListView will populate itself with the inserted List, that also happens to be a page property. Every time the form is submitted (i.e. there is a new request for a search), the List is updated based on the results obtained using the search string.

In other words, all the objects are created once and only once in the Page constructor, but the data is retrieved dynamically upon each Page render.

For additional discussion on ListView see:
http://thread.gmane.org/gmane.comp.java.wicket.user/7763


ListView vs DataProvider

DataProvider is made for working with large data sets where it is too expensive to load the enire dataset into memory at once. DataProvider allows you to only retrieve the window of the dataset you are going to display.

If you have a list of 40 items and it is not expensive to load it you might as well use a ListView .

[From a post of Igor's to the Wicket-user list - Gwyn 20:21, 12 Apr 2006 (BST)]

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