WWDC 2016 Notes

My notes from Apple’s Worldwide Developers Conference

14 Jun 2016

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

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

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.

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