In Chapter 11, you saw how fields could have constraints placed on them to limit possible input, such as numeric-only or phone-number-only. These sorts of constraints help users "get it right" when entering information, particularly on mobile devices with cramped keyboards.
Of course, the ultimate in constrained input is to allow selection only from a set of items, such as a group of radio buttons. Classic UI toolkits have list boxes, combo boxes, drop-down lists, and the like for that very purpose. Android provides many of the same sorts of widgets, plus others of particular interest for mobile devices (e.g., the
Gallery for examining saved photos).
Moreover, Android offers a flexible framework for determining ...