ReactiveCocoa for swift

ViewController.swift

import UIKit
import ReactiveCocoa

class ViewController: UIViewController {
    @IBOutlet var textInput: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        var textSignal = self.textInput.rac_textSignal()
        textSignal.map {
            (text: AnyObject!) -> AnyObject! in
            var textString = text as! NSString
            if 3 >= textString.length {
                return UIColor.lightGrayColor()
            }
            return UIColor.whiteColor()
        }
        .setKeyPath("backgroundColor", onObject: self.textInput)
        
    }
}