How to Pass information Back in iOS when reversing a Segue using Swift?

Question

I have two view controllers, One and Two. I go from VC One to VC Two. On VC Two, I select some data that I store in an array. When I press the "Back" button on the navigation bar, I would like to send that array back to VC One.

What's the best way to do this using Swift & Storyboards?

Thanks!

Answer 1

Here is my solution in Swift 3

1) Create a protocol inside the SecondController.swift file. We preferably create the protocol from where we will be getting data from.

protocol Protocol {
        func passingDataBack(withString: String)
}   

2) Create a variable of type Protocol

var proto: Protocol!

3) Switch to the ViewController.swift file and inherit the Protocol we made from the SecondController.swift file.

class ViewController: UIViewController, Protocol {

}   

4) We then want to conform to the Protocol we made by creating the function we made

    func passingDataBack(withString: String) {
//        withString will return the value that has been passed from our SecondController class
        self.title = withString

    }

5) Use the prepareForSegue method and segue to the SecondController class

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        let vc = segue.destination as? SecondController
        vc?.proto = self //This line will instantiate the protocol to our ViewController class
    }

6) Go back to our SecondController.swift file and use the didSelectRow method and pass our data

 override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

    proto.passingDataBack(withString: items[indexPath.row]) //Call the protocol and the function then pass our data.
    _ = self.navigationController?.popViewController(animated: true) //This will pop back to our previous controller.

}

* Important Things To Remember!!! *

You must set the protocol from controllerB to instantiate to controllerA when switching from controllerA to controllerB

In our example, we moved from ViewController to SecondController. We instantiate our protocol from our SecondController by doing

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    let vc = segue.destination as? SecondController
    vc?.proto = self //This line will instantiate the protocol to our ViewController class
}

If you do not do this, you will get a Thread 001 error on this line

proto.passingDataBack(withString: items[indexPath.row]) //Call the protocol and the function then pass our data.

Source code Github

Answer 2

If you were presenting a modal view with Done and Cancel buttons (sort of like a picker), grabbing the value during an unwind segue method would probably be the easiest.

Given that you want to use the navigation controller's native Back button, the best practice would probably be to implement a protocol that VC One can conform to, and then update VC One as soon as the data on VC Two is selected. Something like:

In VCTwo.swift:

protocol VCTwoDelegate {
    func updateData(data: String)
}

class VCTwo : UIViewController {
    var delegate: VCTwoDelegate?
    ...
    @IBAction func choiceMade(sender: AnyObject) {
        // do the things
        self.delegate?.updateData(self.data)
    }
    ...
}

and in VCOne.swift:

class VCOne: ViewController {
    ...
    override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
        if segue.identifier == "VCTwoSegue" {
            (segue.destinationViewController as VCTwo).delegate = self
        }
    }
    ...
}

extension VCOne: VCTwoDelegate {
    func updateData(data: String) {
        self.internalData = data
    }
}