Chapter 23. 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 between runs of your app, even if the device is powered down. This chapter is about how and where files are saved and retrieved, and about some of the additional ways in which files can be manipulated: for example, apps can define document types in which they specialize and can hand such documents to one another, and can share documents into the cloud (iCloud), so that multiple copies of the same app can retrieve them on different devices. The chapter also explains how user preferences are maintained in NSUserDefaults, and describes some specialized file formats and ways of working with their data, such as XML, SQLite, Core Data, and images.

The Sandbox

The hard disk as a whole is not open to your app’s view. A limited portion of the hard disk is dedicated to your app alone: this is your app’s sandbox. The idea is that every app, seeing only its own sandbox, is hindered from spying or impinging on the files belonging to other apps, and in turn is protected from having its own files spied or impinged on by other apps. Your app’s sandbox is thus a safe place for you to store your data. Your sandbox, and hence your data, will be deleted if the user deletes your app; otherwise, it should reliably persist. (Your app can also see some higher-level directories owned by the system as a whole, but cannot write to them.)

The sandbox ...

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