A file system handles the persistent storage of data files, apps, and the files associated with the operating system itself. Therefore, the file system is one of the fundamental resources used by all processes.
APFS is the default file system in macOS, iOS, watchOS, and tvOS. APFS replaces HFS+ as the default file system for iOS 10.3 and later, and macOS High Sierra and later. macOS additionally supports a variety of other formats, as described in Supported File Systems.
Regardless of the underlying format, all of the disks attached to the device—whether they are physically plugged in or are connected indirectly through the network—contribute space to create a single collection of files. Because the number of files can easily be many millions, the file system uses directories to create a hierarchical organization. Although the basic directory structures are similar for iOS and macOS, there are differences in the way each system organizes apps and user data.
Before you begin writing code that interacts with the file system, you should first understand a little about the organization of file system and the rules that apply to your code. Aside from the basic tenet that you cannot write files to directories for which you do not have appropriate security privileges, apps are also expected to be good citizens and put files in appropriate places. Precisely where you put files depends on the platform, but the overarching goal is to make sure that the user’s files remain easily discoverable and that the files your code uses internally are kept out of the user’s way.
The iOS file system is geared toward apps running on their own. To keep the system simple, users of iOS devices do not have direct access to the file system and apps are expected to follow this convention.
For security purposes, an iOS app’s interactions with the file system are limited to the directories inside the app’s sandbox directory. During installation of a new app, the installer creates a number of container directories for the app inside the sandbox directory. Each container directory has a specific role. The bundle container directory holds the app’s bundle, whereas the data container directory holds data for both the app and the user. The data container directory is further divided into a number of subdirectories that the app can use to sort and organize its data. The app may also request access to additional container directories—for example, the iCloud container—at runtime.
These container directories constitute the app’s primary view of the file system. Figure 1-1 shows a representation of the sandbox directory for an app.