Composites - When you need to group widgets

When developing an user interface it's quite common to need components more complex than the ones that are available to us. By example the textfield widget does not have a label by default and every time we need to put a text field on a form, we also need to put a label with it.

And if we could create a new widget that was composed of a label and a textfield?

That's the main purpose of the Composite class. The Composite is a widget composed of widgets.

You may create a LabelTextBox composite, that has a label and a textfield. This way you don't need anymore to create each time a label and a textbox (and maybe some panel to layout them). Now you just need to create a LabelTextBox.

Also, using the composite helps you to keep your code clean and organized, by encapsulating all the logic and configuration of those widgets inside the composite class.

Here is an example of a composite:

public class LabelTextField extends Composite {

private HorizontalPanel hPanel;

private Label lblField;

private TextBox txtField;

public LabelTextField(Label lbl, TextBox txt) {

lblField = lbl;

txtField = txt;

hPanel = new HorizontalPanel();

hPanel.add(lblField);

hPanel.add(txtField);

initWidget(hPanel);

}

public String getValue(){

return txtField.getText();

}

public String getLabel(){

return lblField.getText();

}

}

One of the rules to use the composite class is that we must associate another widget as the main widget of this composite, through the method initWidget(Widget w).

In this case the main widget is a HorizontalPanel.

Also note that we hided the TextBox and the Label objects from the external access, and only exposed the getText() methods of the objects.