16. dialogue notations and design


In the example of the digital watch in Section 16.3.8 (Design Focus), what would be the dangerous states? Relate the lexical issues of the buttons for a digital watch to these dangerous states and provide some design advice. Does your own digital watch satisfy these criteria?


The time and alarm setting modes are dangerous states, in that we do not want to change either accidentally. Design advice would be to include some sort of guard that makes it difficult to get accidentally into the time-setting modes, or, alternatively, once within the modes, to make the buttons that advance the time difficult to press.

The watch in the Design Focus guards these states by requiring button 'A' to be pressed for two seconds before the mode changes. It might be easy to press a watch button accidentally, but it is quite hard to hold it down. This solution would not of course work for a keyboard, where it is quite easy to hold a key down accidentally. Other watches guard these modes by insetting the mode change button or the buttons required to actually change the time. One has to press these buttons with a sharp instrument, such as a pencil - not an easy slip to make. Some digital clocks guard the mode by making you hold a button down continuously while you are changing the time. Guarding the mode is far preferable to guarding the change buttons, as the latter makes it very hard to change the time when you want to do it.

In fact, changing the time is often a problem for the reversibility of watch and clock dialogs. Typically there is a button to advance the time by one hour or one minute, but not one to put the time back. So, if you press the minute advance button once too often, you have to press it 59 more times to reverse the mistake!

Other exercises in this chapter

ex.16.1 (ans), ex.16.2 (ans), ex.16.3 (ans), ex.16.4 (ans), ex.16.5 (tut), ex.16.6 (tut)

all exercises for this chapter