userCallback()

[embedded-creations]

I needed a callback function to know when my transfer had completed, maybe others could use this as well?

[crteensy]

I'll consider it, thanks for the suggestion!

My first pick would probably be to put it into a chip select class, so that the callback is called when a chip is deselected. Your approach is a bit more general (and thus more flexible), but adds overhead to all transfers including those that don't need a callback.

[embedded-creations]

True, but it's not much overhead to check for a null pointer. In my use case, I'm not using a chip select, I'm driving an APA102 LED strip that only needs CLK and TX pins.

It might be useful to associate the callback with a transaction and not with every transfer. I could see someone wanting a callback only on the last transfer when queueing up several.