public Object execute(ExecutionEvent arg0) throws ExecutionException {
		Canvas canvas = getCanvas();
		if (listener == null) {
			listener = new CanvasListener() {
				@Override
				public void canvasZoomed(DrawCanvasEvent event) {
					Object[] listeners = getListeners();
					HandlerEvent handlerEvent = new HandlerEvent(ZoomInHandler.this, true, false);
					for (int i = 0; i < listeners.length; i++) {
						final IHandlerListener listener = (IHandlerListener) listeners[i];
						listener.handlerChanged(handlerEvent);
					}
					ZoomInHandler.this.fireHandlerChanged(handlerEvent);
				}
			};
		}
		canvas.addCanvasListener(listener);
		return null;
	}

<extension point="...handlers"
  <handler class="MyHandler" commandId="my.command.id"
    <enabledWhen>
      <and>
        <count="+">
        </count>
        <iterate>
          <adapt type="MyOtherClass">
          </adapt>
        </iterate>
      </and>
    </enabledWhen>
  </handler>
</extension>

ICommandService cService = (ICommandService) Platform.getWorkbench().getService(ICommandService.class);
Command myCommand = cService.getCommand("my.command.id");

List<Object> objectsToEvaluate = new ArrayList<Object>();
objectsToEvaluate.add(new ObjectThatAdapts());
ISelection selection = new StructuredSelection(objectsToEvaluate);

EvaluationContext context = new EvaluationContext(null, selection);
context.addVariable(ISources.ACTIVE_CURRENT_SELECTION_NAME, selection);

ExecutionEvent event = new ExecutionEvent(myCommand, Collections.EMPTY_MAP, null, context);
myCommand.executeWithChecks(event);