Programming iOS 13

Chapter 6. View Controllers

An iOS app’s interface is dynamic, and with good reason. The entire interface has to fit into a single display consisting of a single window, which in the case of the iPhone can be forbiddingly tiny. The solution is to change the interface. In response to some event — typically a user action — a new view, possibly with an elaborate hierarchy of subviews, replaces or covers 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 showing and the view that replaces or covers it. This relationship will need to be maintained behind the scenes, in your code.

The relationship between views also needs to be 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 Animation is often used to emphasize and clarify these relationships as one view is superseded by another.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 for returning to a previously visited view, a tap on an interface element to dive deeper ...