Nicolas Frankel is an IT consultant with 10 years experience in Java / JEE environments. He likes his job so much he writes technical articles on his blog and reviews technical books in his spare time. He also tries to find other geeks like him in universities, as a part-time lecturer. Nicolas is a DZone MVB and is not an employee of DZone and has posted 217 posts at DZone. You can read more from them at their website. View Full User Profile

Separation of Concerns in Vaadin

02.23.2012
| 3727 views |
  • submit to reddit

Last week, we quickly created a Delete generated column in a table.

Although this is enough to attain our objective, two nested anonymous classes puts a strain on maintenance costs. Moreover, behavior code (the deletion) is interwoven with GUI code (column).

Correcting these mistakes can be achieved through the creation of two top-level classes, one for the deletion behavior, and the other for the column. We have to find a way to pass the data container and the item id to the behavior class: the most obvious way to do so it change the latter's structure to store both like so:

public class DeleteButtonColumnGenerator implements ColumnGenerator {
 
    @Override
    public Object generateCell(Table source, Object itemId, Object columnId) {
 
        Button button = new Button("Delete");
 
        button.addListener(new DeleteClickListener(itemId, source.getContainerDataSource()));
 
        return button;
    }
}

This design is much more decoupled and maintenance friendly than the following, but we can do better. The click listener's structure is at present tightly coupled to the parameters it needs to access. It would be nice to remove this coupling. Fortunately, Vaadin components can store data on their own. Let's rely on this feature to clean our design.

First, create a placeholder for the container and the item id:

public class ContainerItemId {
 
    private final Object itemId;
     
    private final Container container;
 
    public ContainerItemId(Container container, Object itemId) {
     
        this.itemId = itemId;
        this.container = container;
    }
 
    public Object getItemId() {
     
        return itemId;
    }
 
    public Container getContainer() {
     
        return container;
    }
}

Then, we remove all references to both container and item id from the click listener and use the Vaadin way:

public class DeleteClickListener implements ClickListener {
 
    @Override
    public void buttonClick(ClickEvent event) {
 
        Button button = event.getButton();
         
        ContainerItemId cii = (ContainerItemId) button.getData();
         
        if (cii != null) {
             
            cii.getContainer().removeItem(cii.getItemId());
        }
    }
}

Notice how we get the reference to the button in the event, it's enough!

Finally, we have to pass the data to the button in the column generator:

public class DeleteButtonColumnGenerator implements ColumnGenerator {
 
    @Override
    public Object generateCell(Table source, Object itemId, Object columnId) {
 
        Button button = new Button("Delete");
 
        ContainerItemId cii = new ContainerItemId(source.getContainerDataSource(), itemId);
         
        button.setData(cii);
         
        button.addListener(new DeleteClickListener());
 
        return button;
    }
}

This new design is much more modularized: for application that go beyond prototypes, this approach should be favored over the previous one.

From http://morevaadin.com/content/separation-concerns

Published at DZone with permission of Nicolas Frankel, author and DZone MVB.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

Tags: