5.2. Interfaces and Abstractions

Components are laid out in the user interface hierarchically. The layout is managed by layout components, or more generally components that implement the ComponentContainer interface. Such a container is the parent of the contained components.

The getParent() method allows retrieving the parent component of a component. While there is a setParent(), you rarely need it as you usually add components with the addComponent() method of the ComponentContainer interface, which automatically sets the parent.

A component does not know its parent when the component is created, so you can not refer to the parent in the constructor with getParent(). Also, it is not possible to fetch a reference to the application object with getApplication() before having a parent. For example, the following is invalid:

public class AttachExample extends CustomComponent {
    public AttachExample() {
        // ERROR: We can't access the application object yet.
        ClassResource r = new ClassResource("smiley.jpg",
                                            getApplication());
        Embedded image = new Embedded("Image:", r); 
        setCompositionRoot(image);
    }
}

Adding a component to an application triggers calling the attach() method for the component. Correspondingly, removing a component from a container triggers calling the detach() method. If the parent of an added component is already connected to the application, the attach() is called immediately from setParent().

public class AttachExample extends CustomComponent {
    public AttachExample() {
    }
    
    @Override
    public void attach() {
        super.attach(); // Must call.
        
        // Now we know who ultimately owns us.
        ClassResource r = new ClassResource("smiley.jpg",
                                            getApplication());
        Embedded image = new Embedded("Image:", r); 
        setCompositionRoot(image);
    }
}

You can set the field value with the setValue() and read with the getValue() method defined in the Property interface. The actual value type depends on the component.

The Field interface defines a number of attributes, which you can retrieve or manipulate with the corresponding setters and getters.

Field inherits Property.ValueChangeListener to allow listening for field value changes and Property.Editor to allow editing values.

When the value of a field changes, a Property.ValueChangeEvent is triggered for the field. You should not implement the valueChange() method in a class inheriting AbstractField, as it is already implemented in AbstractField. You should instead implement the method explicitly by adding the implementing object as a listener.

AbstractField is the base class for all field components. In addition to the component features inherited from AbstractComponent, it implements a number of features defined in Property, Buffered, Validatable, and Component.Focusable interfaces.