Programming iOS 6, 3rd Edition

Model–View–Controller

In Apple’s documentation and elsewhere, you’ll find references to the term model–view–controller, or MVC. This refers to an architectural goal of maintaining a distinction between three functional aspects of a program that displays information to the user and permits the user to alter that information. The notion goes back to the days of Smalltalk, and much has been written about it since then, but informally, here’s what the terms mean:

Model

The data and its management, often referred to as the program’s “business logic,” the hard-core stuff that the program is really all about.

View

What the user sees and interacts with.

Controller

The mediation between the model and the view.

Consider, for example, a game where the current score is displayed to the user:

• A UILabel that shows the user the current score for the game in progress is view; it is effectively nothing but a pixel-maker, and its business is to know how to draw itself. The knowledge of what it should draw — the score, and the fact that this is a score — lies elsewhere. A rookie programmer might try to use the score displayed by the UILabel as the actual score: to increment the score, read the UILabel’s string, turn that string into a number, increment the number, turn the number back into a string, and present that string in place of the previous string. That is a gross violation of the MVC philosophy. The view presented to the user should reflect the score; it should not store the score.