Don't use KVO in RACCommand

[mdiep]

RACCommand has always been slow/expensive to initialize. Due to some absurd slowness in our app's unit tests, I started profiling and found that in our tests we were spending a lot of time rehashing inside KVO.

This PR changes RACCommand so that it no longer uses KVO, saving on initialization and disposal. The changes here make our app's unit tests run ~15% faster. (We create a lot of RACCommands.) I'm sure the savings won't be as noticeable in the real world, but I think these changes should still improve things.

[joshaber]

This seems legit to me, but I'd love to have a second set of 👀.

/cc @ReactiveCocoa/reactivecocoa

[neilpa]

Haven't looked at the code closely yet, but this would technically be a breaking change for RAC3. This should probably be targeted at the swift2 branch for now (which I may rename RAC4 depending on the outcome of #2327).

[ashfurrow]

Personally, it feels strange to remove a feature – possibly a breaking change – solely for the sake of unit test run duration.

[mdiep]

How is this a breaking change? It changes the implementation, but maintains the current behavior and interface.

[ashfurrow]

allowsConcurrentExecution is public property that someone may be observing. It's unlikely, given that there's a subject to use as well, but it seems like the kind of subtle change that could drive someone nuts trying to figure out why their code doesn't work anymore.

[mdiep]

allowsConcurrentExecution is public property that someone may be observing. It's unlikely, given that there's a subject to use as well, but it seems like the kind of subtle change that could drive someone nuts trying to figure out why their code doesn't work anymore.

Ah, okay. I just added that back (with a test). That's not an important part of the PR—the important part is not using RACObserve internally.

Edit: Actually, that still works without changes. But there's a test now.

[ashfurrow]

Coooool – looks good 💯

I'm not the most judicious code-reviewer, though. Someone else should double-check juuuust to be sure.

[neilpa]

This looks right to me too so here goes nothing

[mdiep]

💖