> I would imagine this means writing TouchBar (autocorrect on my iPhone hasn't learned that one yet) support into MacOS apps means developing a watchOS extensio

> I would imagine this means writing TouchBar (autocorrect on my iPhone hasn't learned that one yet) support into MacOS apps means developing a watchOS extension to bundle with the app, which seems really simple and elegant. Nope. Apps customize the Touch Bar using AppKit inside their app's code; there's no third-party code running on the Touch Bar's processor at any point. The Touch Bar has a little bit of intelligence (it's responsible for the Touch ID and Apple Pay UIs, and supposedly can display traditional function keys independent of the OS for use in dual-boot situations), but it appears to be mostly just receiving and displaying a framebuffer that's rendered and provided to it by the host machine, then forwarding touch events back to the host machine for interaction with the UI.