metal:run

(metal:run ctx fn &key fps)

Draws fn on a timer: one frame immediately, then fps a second (60 by default). The clock is appkit:timer, an NSTimer on the main thread, which is where Metal wants the frame anyway. Answers the timer, so (objc:send timer "invalidate") stops the loop. Part of the metal package, a Metal drawing surface written in rontolisp over objc and loaded on first use: macOS (java -jar, the rontolisp binary, or a compiled .class / .jar; never a .wasm), with a display. See the macOS GUI guide.

CL-USER> (metal:run *ctx* (lambda (encoder) (draw encoder)) :fps 30)
#<objc __NSCFTimer>