Firm foundation in the main hci principles, the book provides a working

Figure 5.3 Root (background) window activated by default upon system start and also serving 
as the top window for the window manager process.

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8 8
H U M A N – C O M P U T E R I N T E R A C T I O N 
directed to the target program or process), to invoke its corresponding 
handler.
Note that an event does not necessarily correspond exactly just to 
an individual physical input. The stream of raw inputs may be filtered 
and processed to form/find meaningful input sequences from the cur-
rent context. For instance, a sequence of raw inputs may form a mean-
ingful event such as a double-click command, keyboard commands 
with modifiers (e.g., ctrl-alt-del), and mouse enter/exit commands 
(e.g., detection of mouse cursor leaving a particular window).
Figure 5.5 shows the two-tier event-queuing system in greater 
detail. There is the system-level event-queuing system that dispatches 
the events at the top application level. Each application or process also 
typically manages its own event queue, dispatching them to its own 
UI objects. The proper event is captured by the UI object as it tra-
verses down the application’s hierarchical UI structure, e.g., from top 
to bottom. Figures 5.4 and 5.5 illustrate this process. Then the event 
Event 76
Event 75
Top Window
(Address Book)
Add
Dialog Box
Textbox 1
Textbox 2
Textbox 1
Handler
Textbox 2
Handler
Top Window
Mouse Left
Button
Handler
Event 74
Application
Event Queue
File Menu
Edit Menu
Figure 5.4 Event being dispatched to the right UI object handler for a given application (organized 
as a set of UI objects and associated event handlers in a hierarchical manner) from the application-
event queue.

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U S E R I N T E R FA C E L AY E R
handler (also sometimes called the callback function) associated with 
the UI object is activated in response to the event that is captured.
The events do not necessarily have to be generated externally by the 
interaction devices; indeed, sometimes they are generated internally 
for special purposes (these are sometimes called the pseudo-events). 
For instance, when a window is resized, in addition to the resizing 
event itself, the internal content of the window must be redrawn, and 
the same goes for the other windows occluded or newly revealed by 
the resized window. Special pseudo-events are enqueued and con-
veyed to the respective applications/windows. In the case of resiz-
ing/hiding/activating and redrawing of windows, it is the individual 
application’s responsibility, rather than the window manager’s, to 
update its display contents, because only the respective applications 
have the knowledge of how to update their content. Thus a special 
redraw pseudo-event is sent to the application with information about 
which region is to be updated (Figure 5.6). The window content might 
need to be redrawn not because of the window management com-
mands such as resizing and window activation, but due to the needs 
of the application itself, which can generate special pseudo-events for 
redrawing parts of its own window. More generally, UI objects can