Example Code: Sending Data From Apple Watch To iPhone

WatchToPhoneDemo.swift

// WatchToPhoneDemo/WatchToPhoneDemo WatchKit Extension/InterfaceController.swift

class InterfaceController: WKInterfaceController {

    var str: String = "Hello iPhone!"
    
    @IBAction func button() {
        let dict: Dictionary = ["message": str]
        
        WKInterfaceController.openParentApplication(dict, reply: {(reply, error) -> Void in
            println("Reply recieve from iPhone app.")
        })
    }
}


// WatchToPhoneDemo/WatchToPhoneDemo/AppDelegate.swift

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    
    func application(application: UIApplication, handleWatchKitExtensionRequest userInfo: [NSObject : AnyObject]?, reply: (([NSObject : AnyObject]!) -> Void)!) {
        
        NSNotificationCenter.defaultCenter().postNotificationName("WatchKitReq", object: userInfo)
    }
}


// WatchToPhoneDemo/WatchToPhoneDemo/ViewController.swift

class ViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        NSNotificationCenter.defaultCenter().addObserver(self, selector: Selector("handleWatchKitNotification:"), name: "WatchKitReq", object: nil)
    }
    
    func handleWatchKitNotification(notification: NSNotification) {
        if let userInfo = notification.object as? [String : String] {
            label.text = userInfo["message"]
        }
    }
}

Updated for Xcode 8.3 Swift Update