In order to make a GUI, we need a strategy. For example, how do we control that a certain button should have the red color? How do we ensure that the background color changes when the user taps on the button? How do we position our different UI elements so that they look reasonable across different devices with different screen sizes, such as the iPhone and iPad? In order to answer all these questions, we need a way to control the UI. Apple offers us some tools and patterns that have been developed over time. Some of the concepts are similar to other platforms and some are specific when you develop applications for Apple devices.