1.7. Design Patterns

Besides using predefined objects in your code, you’ll also use several predefined design patterns^[4] to help you design and implement your apps according to Apple’s guidelines (Fig. 1.8). Like a pattern a dressmaker uses to create clothing, a design pattern provides programmers with an architectural template for designing and implementing apps.

^[4] Some books you’ll want to consult on design patterns are the seminal “gang of four” book, Design Patterns: Elements of Reusable Object-Oriented Software, by Gamma, Helm, Johnson and Vlissides, ©1994, Addison Wesley, and Cocoa Design Patterns, by Buck and Yacktman, ©2010, Addison Wesley.

Fig. 1.8. Design patterns used in iPhone for Programmers.

Design Pattern	Where it’s used	How it’s used
Abstract Factory	Introduced in Chapter 4’s Tip Calculator app; used in every later app.	Many Foundation framework classes (Fig. 1.9) allow programmers to use one familiar interface to interact with different data structures.
Chain of Responsibility	Introduced in Chapter 7’s Spot-On app; seen in several later apps.	Built into Cocoa as the mechanism for dealing with events.
Command	Introduced in Chapter 5’s Favorite Twitter Searches app; used in most later apps.	To bind GUI components to actions (i.e., event handlers) that are triggered in response to events.
Composite	Introduced in Chapter 5’s Favorite Twitter Searches app; used in most later apps.	To create a hierarchy of objects that can all be manipulated through the root object.
Decorator	Introduced in Chapter 6’s Flag Quiz app; used in most later apps.	To add new functionality to an existing class without subclassing.
Facade	Introduced in Chapter 6’s Flag Quiz app.	To provide a simple interface for the behaviors of a complex subsystem.
Model View Controller	Introduced in Chapter 4’s Tip Calculator app; used in every later app.	To separate app data (contained in the model) from graphical presentation (the view) and input-processing logic (the controller).
Memento	Introduced in Chapter 5’s Favorite Twitter Searches app; used in every later app that needs to save data.	To represent an object as a bit stream so it can be saved to a file or transferred over a network (also called “serialization”).
Singleton	Introduced in Chapter 5’s Favorite Twitter Searches app.	To ensure that only one object of a class is created. Other objects in the app can share the singleton object.
Template Methods	Introduced in Chapter 4’s Tip Calculator app; used in every later app.	To define an algorithm in a superclass, parts of which a subclass can override.