Crafting Singletons

The UIApplication and UIDevice classes let you access information about the currently running application and the device hardware it is running on. They do so by offering singletons—that is, sole instances of classes in the current process. For example, [UIApplication sharedApplication] returns a singleton that can report information about the delegate it uses, whether the application supports shake-to-edit features, what windows are defined by the program, and so forth.

Most singleton objects act as control centers. They coordinate services, provide key information, and direct external access, among other functionality. If you have a need for centralized functionality, such as a manager that accesses a web service, a singleton approach ensures that all parts of your application coordinate with the same central manager.