public interface IDeletable {

	public boolean isDeleteable();
	public void delete();
}

public class DeleteProperyTester extends PropertyTester {

	public static final String IS_DELETE = "delete";

	@Override
	public boolean test(Object receiver, String property, Object[] args,
			Object expectedValue) {
		if (property.equals(IS_DELETE))
			return ((IDeletable) receiver).isDeleteable();
		
		return false;
	}

}

public class DeleteHandler extends AbstractHandler{

	@Override
	public Object execute(ExecutionEvent event) throws ExecutionException {
		IWorkbenchPart edi = HandlerUtil.getActivePart(event);

		if (null != edi) {
			if (edi instanceof IDeletable) {
				IDeletable deletable = (IDeletable) edi;
				deletable.delete();
			}
		}
		return null;
	}

}

   <extension
         point="org.eclipse.ui.handlers">
      <handler
            class="command.DeleteHandler"
            commandId="test">
         <activeWhen>
            <with
                  variable="activePart">
               <test
                     forcePluginActivation="true"
                     property="test.delete">
               </test>
            </with>
         </activeWhen>
      </handler>
   </extension>

public class Editor2 extends EditorPart implements IDeletable{

	private Text text;

	
	@Override
	public void createPartControl(Composite parent) {
		GridLayout gridLayout = new GridLayout();
		gridLayout.numColumns = 4;
		parent.setLayout(gridLayout);
		text = new Text(parent,SWT.BORDER);


		text.addModifyListener(new ModifyListener() {
			
			@Override
			public void modifyText(ModifyEvent e) {
				// fire Event which triggers the PropertyTester
				// to test the isDeletable again
				
			}
		});

	}
	@Override
	public void delete() {
		// TODO Auto-generated method stub
		
	}

	@Override
	public boolean isDeleteable() {
		System.out.println("deletable");
		return !text.getText().isEmpty();
	}

}

		final IEvaluationService evaluationService = (IEvaluationService) getSite()
		.getWorkbenchWindow().getWorkbench().getService(
		IEvaluationService.class);
		evaluationService.requestEvaluation("test.delete");