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.

answer

In the example, we want to allow the user to toggle between three font characteristics - bold, italics and underline. Pull-down menus and dialog boxes are the most frequently used methods for implementing such a dialog. A pull-down menu would have the three choices listed with some way of indicating whether the characteristic was active or not. Upon each invocation of the menu, the user could change the active status of one font characteristic. For example, they could turn on the bold feature or turn off the underline feature. The important feature of this dialog is that more complex tasks (changing the active status of two or more font characteristics) can only be achieved by separate invocations of the menu. And often the only way the user can check the status would be by popping up the menu again (without invoking any action). This last point shows that the menu design alone does not support immediate display of relevant state changes as discussed in Chapter 7 under the synthesizability principle.

Within a dialog box, the user can effect several changes at one invocation. Technically, it is not the dialog box interaction device that allows for this, but the use of toggle buttons. Each font characteristic is represented by a box that acts as a simple on/off switch for that characteristic. As an aid to the user, the toggle button can change its appearance so that the user can know its status (on or off) by looking at it. A similar interaction widget is the checkbox, which would be labelled with the particular font characteristic. Clicking on the checkbox would put a mark in the box to indicate that the characteristic is on. Another click would remove the mark. In fact, a dialog box need not be used to portray the set of toggle buttons or checkboxes, as they could be presented continually so that the user can always view the current font characteristic settings. The reason a dialog box might be used is that there may be many font characteristics to choose from and the dialog box will keep them grouped together and preserve screen space by allowing them to be hidden and used on an as-needed basis.

EXERCISE 8.3

This exercise is based on the nuclear reactor scenario at: www.hcibook.com/e3/scenario/nuclear/

(a) In the Seeheim model: Treating the Application Interface model and Application together, there are three main layers:

(i) presentation/lexical
(ii) dialogue/syntactic
(iii) application/semantic

For each of these three layers list at least two different items of the description of the nuclear reactor control panel that is relevant to the level (that is at least six items in total, two for each level)

(b) There are no items in the description that relate to the switch (rapid feedback) part of the Seeheim model. Why do you think this is the case?

answer available for tutors only

EXERCISE 8.4

A user has a word-processor and a drawing package open. The word-processor's window is uppermost. The user then clicks on the drawing window (see figure below). The drawing window then pops to the front.

Describe in detail the things that the window manager and applications perform during the processing of the mouse click in the above scenario. Explain any assumptions you make about the kind of window manager or application toolkits that are being used.

windows

answer available for tutors only

EXERCISE 8.5

A designer described the following interface for a save operation.
The users initially see a screen with a box where they can type the file name (see screen 1). The screen also has 'list' button that they can use to obtain a listing of all the files in the current directory (folder). This list appears in a different window. When the user clicks the 'save' button the system presents a dialogue box to ask the user to confirm the save (see screen 2).

dialog box
screen 1

dialog box
screen 2

Two programmers independently coded the interface using two different window managers. Programmer A used an event-loop style of program whereas programmer B used a notifier (callback) style.

(a) Sketch out the general structure of each program.
(b) Highlight any potential interface problems you expect from each programmer and how they could attempt to correct them.