swift两个视图控制器之间的传值

import UIKit

class ViewController: UIViewController, myDelegate {
    
    @IBOutlet weak var principalLabel: UILabel!
    
    @IBAction func mainButton(sender: UIButton) {
        
        //we got it the final instance in storyboard
        let secondController: SecondViewController = self.storyboard?.instantiateViewControllerWithIdentifier("SecondViewController") as! SecondViewController
        secondController.data = "Text from superclass"
        //who is it delegate
        secondController.delegate = self
        //we do push to navigate
        self.navigationController?.pushViewController(secondController,
            animated: true)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
    func writeDateInLabel(data:NSString){
        self.principalLabel.text = data as String
    }
    
}

当点击第二个视图控制器的按钮,就通过代理回传给第一个视图控制器的lable赋值

protocol myDelegate {
    func writeDateInLabel(date:NSString)
}

class SecondViewController: UIViewController {
    
    var data: NSString = ""
    var delegate: myDelegate?
    
    @IBOutlet weak var secondLabel: UILabel!
    
    @IBAction func secondButton(sender: AnyObject) {
        self.delegate?.writeDateInLabel("I got it!")
        self.navigationController?.popViewControllerAnimated(true)
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // Do any additional setup after loading the view.
        secondLabel.text = data as String
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    
}