Obtaining the Control with Focus

How to obtain the name of the form control that currently has focus.

About this technique	Details
Category	Java Client Techniques> General techniques for controls
Description	You'll learn about: Basic steps for adding a focus listener Adding an interface Adding the listener Processing the focus event and getting the control name Removing the listener
	You can run this technique code from: Database: Examples3_Java Form: frmControlWithFocus NOTE   First make sure that database is running on your localhost SilverStream Server
Related reading	See the chapter on form basics in the Programmer's Guide For more about listeners, see Adding a Listener to a Form Control

The example implements an AWT listener for a focus event. By adding the listener to the form, it responds to a focus change for any component on the form. When the user tabs or clicks among the form controls, the name of the control with focus is displayed in the text field at the bottom of the form.

Basic steps for adding a focus listener   

1. In the Programming Editor, add the Java Interface java.awt.event.AWTEventListener.

2. In the formActivate() method, call the addAWTEventListener() using the FOCUS_EVENT_MASK.

3. In the eventDispatched event, check for an instance of a focus event, and handle the component as required. This example just displays the name of the component.

4. In the windowClosing event, remove the listener.

Adding an interface   

In the Programming Editor, you specify that the form implements an interface by selecting File>Java Interfaces. You must specify the fully qualified package name for the interface. For AWT events, it is java.awt.event.AWTEventListener.

Be sure to check Create stubs for interface methods so that the methods you must implement for the interface are added to the form's code.

Adding the listener   

Toolkit is the abstract superclass of AWT containers. The AWTEventListener is a special-purpose listener for low-level events.

  Toolkit.getDefaultToolkit().addAWTEventListener( 
     (AWTEventListener) this, AWTEvent.FOCUS_EVENT_MASK); 

Notes on the code

• The listener is associated with the form, referred to by the keyword this.

Processing the focus event and getting the control name   

This code from the eventDispatched event checks the type of the event object and looks for the event ID FOCUS_GAINED. Then it displays the name of the component in the fldWithFocus control.

  if (aWTEvent1 instanceof FocusEvent &&  
     aWTEvent1.getID() == FocusEvent.FOCUS_GAINED) 
  { 
     FocusEvent focusEvent = (FocusEvent) aWTEvent1; 
     Component componentFocus = (Component) aWTEvent1.getSource(); 
     fldWithFocus.setText(componentFocus.getName()); 
  } 

Notes on the code

• The sample code from the application checks whether the aWTEvent1 object is actually a FocusEvent object. It also checks for a specific event ID.

• The getID() and getSource() methods are methods of the AWTEvent object passed to the event. Individual event IDs for specific events are defined in subclasses of AWTEvent. The event IDs we are interested here are constants defined in FocusEvent.

• The getName() method for a component object reports the name of the component's instance variable.

Removing the listener   

This code removes the listener before the window closes.

  Toolkit.getDefaultToolkit().removeAWTEventListener( 
     (AWTEventListener) this); 

Notes on the code

• The listener is associated with the form, referred to by the keyword this.