Add UIControl rx_subscribeEnabledTo

[ashfurrow]

Hey!

Preparing my talk for AltConf tomorrow on FRP, and I'm using RxSwift (ssssshhhhhhh don't tell anyone – I want to focus on concepts and ideas instead of "which FRP framework" I use).

Anyway, coming from ReactiveCocoa, there are a few things I've noticed we could add. I'm making implementations for them in the sample code for now, but when I have time, I'll add tests and send a PR for them.

The first one is adding rx_subscribeEnabledTo for UIControl. Something like

emailTextField.rx_text
    >- map { $0.isEmail() }
    >- submitButton.rx_subscribeEnabledTo
    >- disposeBag.addDisposable

[kzaher]

Hi Ash,

Yeah, I think there were a lot of requests for that. I think this is a good idea, so if you want to do a PR, that would be great.

I think @DTartaglia also likes this idea :)

👍