5.2 Exclusive-Choice Lists
An exclusive-choice list is a screen of choices that always has one, and only one, element selected.
The minimum size of an exclusive-choice list is two elements.
5.2.1 Selection in an Exclusive-Choice List
A selection makes a highlighted item become On (selected) and any previously selected element become Off (not selected).
Make sure an exclusive-choice list always has an element selected. Situations where the list will not have an element selected include when the application adds the first element to the list, provides all the list elements to the list constructor, or deletes the selected element. At these times, choose an element and select it. Do not display the list with no element selected.
To have a particular element selected, set the selection explicitly, preferably before the list screen is visible. For example, set the selection immediately after creating the exclusive-choice list or just before deleting the currently selected element.
5.2.2 Abstract Commands on Exclusive-Choice List Screens
Give exclusive-choice screens at least one abstract command. As shown in the left screen of Figure, an exclusive-choice screen needs an abstract command to handle the user's selection and move the user to another screen in the application.
6. Exclusive-Choice Lists
Give exclusive-choice screens more than one abstract command so that users can either make a selection or move to another screen without making a choice. This applies the design consideration, "Make Everything Interruptible" on page 13.
For example, the right screen of Figure shows a screen with two abstract commands: one is type Screen (labeled Save) and the other is type Back (labeled Back). (See "Types of Abstract Commands" on page 166 for more information.)
The abstract command with the Save label sets the coverage area to the chosen value and moves the user to another screen in the application. The abstract command with the Back label returns the user to the previous screen. If the screen contained only the abstract command labeled Save, users would be forced to choose a setting in order to move to another screen.
The screen could have abstract commands for other tasks too.
5.2.3 Exclusive-Choice List Alternatives
To enable users to choose among options on a form, use a choice group instead of an exclusive-choice list. See "Choice Groups" on page 116 for more information.
Use an implicit list instead of an exclusive-choice list if you want a screen that operates like a menu. See "Implicit Lists" on page 48 for more information.