Programming iOS 9

Chapter 6. View Controllers

An iOS app’s interface is dynamic, and with good reason. On the desktop, an application’s windows can be big, and there can be more than one of them, so there’s room for lots of interface. With iOS, everything needs to fit on a single display consisting of a single window, which in the case of the iPhone can be almost forbiddingly tiny. The iOS solution is to introduce, at will, completely new interface — a new view, possibly with an elaborate hierarchy of subviews — replacing or covering the previous interface.

For this to work, regions of interface material — often the entire contents of the screen — must come and go in an agile fashion that is understandable to the user. There will typically be a logical, structural, and functional relationship between the view that was present and the view that replaces or covers it, and this relationship will need to be maintained behind the scenes, in your code, as well as being indicated to the user: multiple views may be pure alternatives or siblings of one another, or one view may be a temporary replacement for another, or views may be like successive pages of a book. Animation is often used to emphasize and clarify these relationships as one view is superseded by another. Navigational interface and a vivid, suggestive gestural vocabulary give the user an ability to control what’s seen and an understanding of the possible options: a tab bar whose buttons summon alternate views, a back button or a swipe gesture ...