exercises - 8. implementation support

EXERCISE 8.1

In contrasting the read-evaluation loop and the notification-based paradigm for interactive programs, construction of a pre-emptive dialog was discussed. How would a programmer describe a pre-emptive dialog by purely graphical means? (Hint: Refer to the discussion in Section 8.5 concerning the shift from external and independent dialog management to presentation control of the dialog.)

answer

Creating a pre-emptive dialog by graphical means is quite simple. In a graphics-based interaction, it is assumed that the user can only interact with parts of the system that are visible. In a windowing system, for example, the user can only direct input to a single, currently selected window, and the only means of changing the selected window would be by indicating with some gesture within that window. To create a pre-emptive dialog, the system can produce a window that covers the entire screen and make it the currently selected window. All user input would then be directed to that window and the user would have no means of selecting another window. The 'covering' window thus pre-empts any other user action except that which it is defined to support.