He has gone in deep on performant low-level audio code, and how it integrates with the system APIs. He’s done a lot of interesting stuff with programmatic drawing of icons in his apps. He did a lot of good caching work back in the Instapaper days, when cell connections were almost like dialup.

He seemed to be reluctant to learn Swift, because Swift would have gotten him…what? I think he thought that Objective-C was mature, tested, comprehensible, and battle-tested in production. And it wasn’t going to change out from underneath him…which you sure couldn’t say about Swift for the first few years. You eventually had to adopt it, as Apple is moving to Swift-only, but I think Objective-C let him accomplish his goals, and a lot of the good security stuff in Swift is maybe not super relevant to his app development.

(Ugh, PHP, though…)