Event Types and Priorities

In addition to events corresponding to user and windowing actions, such as button clicks, key presses, and updates, the system dispatches two kinds of internal events: timer events and explicitly queued events.

Timer events are created by instances of timer%. When a timer is started and then expires, the timer queues an event to call the timer's notify method. Like a top-level window, each timer is associated with a particular eventspace (the current eventspace as described in Creating and Setting the Eventspace) when it is created, and the timer queues the event in its eventspace.

Explicitly queued events are created with queue-callback, which accepts a callback procedure to handle the event. The event is enqueued in the current eventspace at the time of the call to queue-callback, with either a high or low priority as specified by the (optional) second argument to queue-callback.

An eventspace's event queue is actually a priority queue with events sorted according to their kind, from highest-priority (dispatched first) to lowest-priority (dispatched last):

• The highest-priority events are high-priority events installed with queue-callback.
• Timer events have the second-highest priority.
• Graphical events, such as mouse clicks or window updates, have the second-lowest priority.
• The lowest-priority events are low-priority events installed with queue-callback.

Although a programmer has no direct control over the order in which events are dispatched, a programmer can control the timing of dispatches by setting the event dispatch handler via the event-dispatch-handler parameter. This parameter and other eventspace procedures are described in more detail in Eventspaces.