A major task that a GUI designer needs to do is to determine what will happen when a GUI is invoked. Every GUI component may generate different kinds of “events” when a user makes access to it using his mouse or keyboard. E.g. if a user moves his mouse on top of a button, an event of that button will be generated to the Windows system. E.g. if the user further clicks, then another event of that button will be generated (actually it is the click event). For any event generated, the system will first check if there is an event handler, which defines the action for that event. For a GUI designer, he needs to develop the event handler to determine the action that he wants Windows to take for that event.