Many mobile applications are self-contained programs (such as a calculator app) and have no need to interact with other services or systems. You will realize, however, that as you build more and more, it will start becoming necessary to integrate your apps with external vendors and systems in order to keep your users happy. The recent trend towards integrating Facebook's Like buttons and the ability to tweet from within an app are good examples of this.
In this chapter, we are going to concentrate on talking to a variety of different service providers in a number of common ways, including basic authorization, open authorization, and using a service provider such as Parse, coupled with some PHP code, to make push notifications work on our iOS device.