fs_tigre fs_tigre - 4 months ago 13
Swift Question

How to pass data between UIViewControllers with protocols/delegates

In the code below I have a

ViewController
("SenderViewController"), which passes a message to the main
ViewController
when a button is tapped. What I don't fully understand is how does
messageData()
method in the main
ViewController
know when to listen for the message.

Can someone please explain me what is triggering the
messageData()
method in the main
ViewController
?

SenderViewController:

import UIKit
protocol SenderViewControllerDelegate {
func messageData(data: AnyObject)
}
class SenderViewController: UIViewController {
@IBOutlet weak var inputMessage: UITextField!
var delegate: SenderViewControllerDelegate?

@IBAction func sendData(sender: AnyObject) {
/
if inputMessage.text != ""{
self.presentingViewController!.dismissViewControllerAnimated(true, completion: nil)
self.delegate?.messageData(inputMessage.text!)
}
}
}


Main ViewController:

import UIKit
class ViewController: UIViewController, SenderViewControllerDelegate{
@IBOutlet weak var showData: UILabel!

override func viewDidLoad() {
super.viewDidLoad()
}

@IBAction func goToView(sender: AnyObject) {
let pvc = storyboard?.instantiateViewControllerWithIdentifier("senderViewController") as! SenderViewController
pvc.delegate = self
self.presentViewController(pvc, animated:true, completion:nil)
}

// What triggers this method, how it know when to listen?
func messageData(data: AnyObject) {
self.showData.text = "\(data)"
}
}


Thanks a lot!

Answer

Objects don't exactly listen for method calls. They sit there, waiting to invoked.

The line

self.delegate?.messageData(inputMessage.text!)

From your SenderViewController is a function call. (The term method and function are pretty much interchangeable, although the method is usually used for the functions of objects.) It invokes the function messageData in ViewController.

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