HFS+ and its predecessor HFS are more than 30 years old. These file systems were developed in an era of floppy disks and spinning hard drives, when file sizes were calculated in kilobytes or megabytes.
Today, people commonly store hundreds of gigabytes and access millions of files on high-speed, low-latency flash drives. People carry their data with them, and they demand that sensitive information be secure.
Apple File System is a new, modern file system for iOS, macOS, tvOS, and watchOS. It is optimized for Flash/SSD storage and features strong encryption, copy-on-write metadata, space sharing, cloning for files and directories, snapshots, fast directory sizing, atomic safe-save primitives, and improved file system fundamentals.
A Developer Preview of Apple File System is available in macOS Sierra. Apple plans to release Apple File System as a bootable file system in 2017.
To understand how your app interacts with the file system, read File System Programming Guide.