How to Create a Desktop Event Dispatcher

Creating a custom event dispatcher can help you address two use cases:

• [Dispatch] Extending an EventDispatcher is used to dispatch the events. For example, the FocusEventDispatcher will send the events to the widget owning the focus.

• [Handle] Overriding the desktop is used to directly trigger a behavior. For example “BACK” command shows the previous page.

To create a custom event dispatcher, a new class should be created, extending the EventDispatcher class. Event dispatcher subclasses have to implement a method and may override optional methods, as explained in the following sections.

Dispatching the events to the widgets

The dispatchEvent() method is called by the MWT framework in order to dispatch a MicroUI event to the widgets of the desktop. The getDesktop() method may be called in order to retrieve the desktop with which the event dispatcher is associated. This is useful in order to browse the widget hierarchy of the desktop, for example by using the getWidget() and getWidgetAt() methods of Desktop.

In order to send an event to one of the widgets of the hierarchy, the sendEventToWidget() method should be used. The dispatchEvent() method should return whether or not the event was dispatched and consumed by a widget.

For example, the following snippet dispatches every event to the widget of the desktop:

@Override
public boolean dispatchEvent(int event) {
        Widget desktopWidget = getDesktop().getWidget();
        if (desktopWidget != null) {
                return sendEventToWidget(desktopWidget, event);
        } else {
                return false;
        }
}

In addition to dispatching the provided events, an event dispatcher may generate custom events. This may be done by using a DesktopEventGenerator. Its buildEvent() method allows to build an event which may be sent to a widget using the sendEventToWidget() method.

Initializing and disposing the dispatcher

EventDispatcher subclasses may override the initialize() and dispose() methods in order to allocate and free the necessary resources.

For example, the initialize() method may be overridden to create an event generator and to add it to the system pool of MicroUI:

@Override
public void initialize() {
        this.eventGenerator = new DesktopEventGenerator();
        this.eventGenerator.addToSystemPool();
}

Likewise, the dispose() method may be overridden to remove the event generator from the system pool of MicroUI:

@Override
public void dispose() {
        this.eventGenerator.removeFromSystemPool();
}