Event Queues

• Device is monitored by an asynchronous process.
• Upon change in status of device, this process places a record into an event queue.
• Application can request read-out of queue:
  • Number of events
  • 1st waiting event
  • Highest priority event
  • 1st event of some category
  • All events
• Application can also
  • Specify which events should be placed in queue
  • Clear and reset the queue
  • Etc.

• Queue reading may be blocking or non-blocking
• Processing may be through callbacks
• Events may be processed interruptively
• Events can be associated with more than devices

Without interrupts, the application will engage in an event loop

• not a tight loop
• a preliminary of register event actions followed by a repetition of test for event actions.

For more sophisticated queue management,

• application merely registers event-process pairs
• queue manager does all the rest

  ``if event XXX then invoke process YYY.''

• The cursor is usually bound to a pair of valuators, typically MOUSE_X and MOUSE_Y.
• Events can be restricted to particular areas of the screen, based on the cursor position.
• Events can be very general or specific:
  • A mouse button or keyboard key is depressed.
  • A mouse button or keyboard key is released.
  • The cursor enters a window.
  • The cursor has moved more than a certain amount.
  • An Expose event is triggered under X when a window becomes visible.
  • A Configure event is triggered when a window is resized.
  • A timer event may occur after a certain interval.
• Simple event queues just record a code for event (Iris GL).
• Better event queues record extra information such as time stamps
  (X windows).