Events

Similar to an exception, an event is a (hardware) condition that is signaled. Unlike an exception, an event is a normal, expected condition. Common events are from input devices, such as

• mouse events - mouse up, mouse down, mouse move
• keyboard events - key up, key down
• network events - packet arrival
• window events - window resized, window dragged, focus lost

Special hardware detects and signals these events. Each event has additional information. For example a mouse movement event has an x and y coordinate. In a Graphical User Interface (GUI), there exist widgets which are high-level events constructed from low-level events in specific areas.

• button down - mouse down, mouse up in a particular screen area
• check-box - mouse down, mouse up in a rectangular area
• radio-box - mouse down, mouse up in a rectangular area
• text-area - keyboard events in a scrolling text field

There is special hardware support for graphics. At a higher-language level, operations (typically function calls or methods on a graphics object) are provided. In a language that supports exception handlers, flow-of-control switches to the handlers when an exception is raised, and the the normal flow-of-control resumes. In event-driven programming, there is essentially no normal flow-of-control! Only the event (exception) handlers exist. Flow-of-control is entirely driven by responding to events. The event dispatcher is called the main loop.

while (TRUE) {  // loop forever

  while (empty(event.queue));  // wait for an event

  event = pop(event.queue);
  event.type(event.information);

}

// a mouseDown event handler
public boolean mouseDown(Event event, int int x, int y) {
   // what to do on a mouse down
} 

// a buttonDown event handler
buttonDown(button b) {
   // what to do on a button down
}

Here is a Java example. Notice that the main event loop has been completely eliminated. The action method controls the dispatching of an event.

/** Respond to user actions on controls. */
           public boolean action(Event e, Object arg) {
               if (e.target instanceof TextField) {
                   setSliderValue(getValue());
                   controller.convert(this);
                   return true;
               }
               if (e.target instanceof Choice) {
                   controller.convert(this);
                   return true;
               } 
               return false;
           }

           /** Respond to the slider. */
           public boolean handleEvent(Event e) {
               if (e.target instanceof Scrollbar) {
                   textField.setText(String.valueOf(slider.getValue()));
                   controller.convert(this);
               } 
               return super.handleEvent(e);
           }

Source of Information

These lecture notes are based on Chapter 14 in "Programming Languages, 6ed" by Robert Sebesta.