Programming iOS 6, 3rd Edition

Chapter 36. Persistent Storage

The device on which your app runs contains flash memory that functions as the equivalent of a hard disk, holding files that survive the device’s being powered down (persistent storage). Apps can store files to, and retrieve them from, this virtual hard disk. Apps can also define document types in which they specialize and can hand such documents to one another; apps can also share documents into the cloud (iCloud), so that multiple copies of the same app can retrieve them on different devices.

User preferences can be maintained in NSUserDefaults (Chapter 13), and visible state can be maintained through the iOS 6 state saving and restoration mechanism (Chapter 19), but the data that constitutes your app’s model in the model–view–controller architecture is neither preferences nor state, so if your app has such data, and if that data is to persist between uses, your app will probably save the data to disk as a file or files. At a minimum, you’ll probably save your data to disk when your app goes into the background (Chapter 11), so that if your app is terminated in the background, you can load your data from disk when your app launches. More proactively, you might save your data to disk more often, for extra safety, and, if your data is large, you might also release it when your app goes into the background, so as to use less memory in the background and reduce your chances of being terminated while suspended; in that case, you’ll load your data from ...