Combining RAC signals and updating UI

I'm using Swift 2.1 and ReactiveCocoa 4.0.4 alpha 1. I'm combining multiple signals to enable a button based on if they have valid inputs. The following code works correctly, but I'm wondering if there is a more elegant and efficient way of accomplishing this with the ReactiveCocoa APIs, as well as if this is the proper way to be using the API in Swift.

let sig0 = firstTextField.rac_textSignal()
        .max(self.firstTextField, max: maxLength)
        .map{ next in
            if let str = next as? String {
                return str.characters.count > 0
            } else {
                return false
            }
    }

    let sig1 = secondTextField.rac_textSignal()
        .max(self.secondTextField, max: maxLength)
        .map{ next in
            if let str = next as? String {
                return str.characters.count > 0
            } else {
                return false
            }
    }

    let combined = RACSignal.combineLatest([sig0, sig1])
    combined.subscribeNext { (obj: AnyObject!) -> Void in
        let tup = obj as! RACTuple
        let valid = (tup.first as! Bool) && (tup.second as! Bool)
        self.button.enabled = valid
    }

I'm familiar with RxSwift, not RAC. :-( That said, I think I can say something constructive about this code...

sig0 and sig1 need more descriptive names.

I would be inclined to break up the combined.subscribeNext function. Something more like:

let buttonEnabled = RACSignal.combineLatest([sig0, sig1])
    .map { obj in 
        let tup = obj as! RACTuple // Wow RAC doesn't return type safe values?
        return (tup.first as! Bool) && (tup.second as! Bool)
    }

buttonEnabled.subscribeNext { obj in 
    let valid = obj as! Bool
    self.button.enabled = valid 
}

I strongly suspect that there is a function for mapping the buttonEnabled signal directly to the button though, maybe with an Action? You should look into that.

StackExchange Code Review Q#110188, answer score: 2