WWDC 2016 Notes

Introducing Apple File System

• Both a volume manager and a filesystem
• inode is extensible: we have optional fields and fields can be added in the future without breaking backward compatibility
• Optimized for latency rather than throughput: apps read many small files
• With space sharing, free space reported is the entire free space on the container
• Copy-on-write cloning for files and directories (useful for packages)
• Copy-on-write snapshots are read-only
• Directory size is stored on the filesystem as an additional record, not in the inode
• renamex_np syscall allows atomically replacing a directory with another one
  • New syscall was needed because POSIX does not allow these semantics
  • This is useful for saving documents that are packages
• Encryption can be one key per volume, file, or extent (part of a file)
  • Unifies iOS and Mac encryption models by supporting both of them
• Migration can be done in-place for HFS+ volumes
  • Writes new APFS metadata into HFS+ free space
  • Converter is not completely atomic, but likely will not lose data
• APFS as default on all devices in 2017

Advances in iOS Photography

• New class: AVCapturePhotoOutput
• RAW capture
  • 10-bit sensor, 14 bits per pixel
  • JPEG advantages: multiple image fusion, which you can’t get through RAW capture
  • RAW + JPEG capture support
  • Uses Adobe Digital Negative (DNG) format
• Wide color capture
  • Display P3 (P3-D65) has the same gamma and white point as sRGB, so it’s more compatible than DCI-P3 (superset of sRGB)
  • 420f (“full”) capture modes support wide capture
  • AVCaptureSession has an option to set wide capture automatically (won’t do it for videos)
  • Videos don’t do wide by default because most video playback software isn’t color managed
  • Shared photos are converted to Apple Wide Color Sharing Profile, where a custom profile is generated so the data can be stored as sRGB but converted back to P3 for color managed devices

Protocol and Value Oriented Programming in UIKit Apps

• How to use value types in UIKit apps
• Local reasoning: just look at the code in front of you, don’t have to think about the context (what is the rest of the code in the program doing)
• Model layer: use struct to store and pass information about dreams
• Views: layout code is just math, so put it into a struct. The UITableViewCell subclass will hold a Layout and use it to calculate stuff
  • Easier to write unit tests against the layout algorithm when it’s not part of a UIView subclass
  • Want the layout class to work with unrelated types? Get polymorphism from a protocol so you can take both UIView and SKNode
  • “Customize through composition rather than inheritance” – don’t have to think about what superclasses & subclasses are doing
• Put all your model structs into a Model object, and set it on the view controller. When updating the model, diff the model and apply changes to the view.

Unified Logging and Activity Tracking

• Design goals
  • Unify kernel and user mode logging into one system
  • Collect a lot of information with minimum cost, by deferring work away from log time
  • Can we turn a ton of logging on all the time, instead of increasing the logging after a problem happens?
• New console app
  • Track an activity as it’s passed through different proceses
• New file formats
  • Log database format is .tracev3 – no longer text, so need to use Apple’s tools instead of grep
  • .logarchive for sharing the logs
• On a fault or error, in-memory logs are collected and preserved in special log files
• Architecture
  • Logs are written to memory shared with the logd daemon, or messages are IPC’d to diagnosticd for realtime streaming