Đối với các ứng dụng iOS, nó rất quan trọng để biết liệu ứng dụng của bạn đang chạy trong nền trước hoặc nền. Vì nguồn lực hạn chế trên các thiết bị IOS, một ứng dụng phải cư xử khác nhau trong nền hơn ở mặt trước. Hệ điều hành cũng hạn chế ứng dụng của bạn có thể làm trong nền để cải thiện tuổi thọ pin | CHAPTER 3 App States and Multitasking For iOS apps it is crucial to know whether your app is running in the foreground or the background. Because resources are more limited on iOS devices an app must behave differently in the background than in the foreground. The operating system also limits what your app can do in the background in order to improve battery life and to improve the user s experience with the foreground app. The operating system notifies your app whenever it moves between the foreground and background. These notifications are your chance to modify your app s behavior. While your app is in the foreground the system sends touch events to it for processing. The UIKit infrastructure does most of the hard work of delivering events to your custom objects. All you have to do is override methods in the appropriate objects to process those events. For controls UIKit simplifies things even further by handling the touch events for you and calling your custom code only when something interesting happens such as when the value of a text field changes. As you implement your app follow these guidelines Required Respond appropriately to the state transitions that occur. Not handling these transitions properly can lead to data loss and a bad user experience. For a summary of how to respond to state transitions see Managing App State Changes page 35 . Required When moving to the background make sure your app adjusts its behavior appropriately. For guidelines about what to do when your app moves to the background see Being a Responsible Background App page 58 . Recommended Register for any notifications that report system changes your app needs. When an app is suspended the system queues key notifications and delivers them when the app resumes execution. Apps should use these notifications to make a smooth transition back to execution. For more information see Processing Queued Notifications at Wakeup Time page 47 . Optional Ifyourapp needs to do actual work while in
