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How to pass data between two view controllers

Swift version: 5.2

Paul Hudson    @twostraws   

If you have a value in one view controller and want to pass it to another, there are two approaches: for passing data forward you should communicate using properties, and for passing data backwards you can either use a delegate or a block.

Passing data forward is used when you want to show some information in a detail view controller. For example, view controller A might contain a list of names that the user can select, and view controller B might show some detailed information on a single name that the user selected. In this case, you would create a property on B like this:

class ViewControllerB: UIViewController {
    var selectedName: String = "Anonymous"
}

How you set that property depends on how are you showing the detail view controller. For example, if you're using a UINavigationController and want to push the new view controller onto the stack, you would write this:

let viewControllerB = ViewControllerB()
viewControllerB.selectedName = "Taylor Swift"
navigationController?.pushViewController(viewControllerB, animated: true)

If you're using segues, you'll want to use code like this instead:

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "showDetail" {
        if let indexPath = self.tableView.indexPathForSelectedRow {
            let controller = segue.destination as! ViewControllerB
            controller.selectedName = objects[indexPath.row]
        }
    }
}

To pass data back, the most common approach is to create a delegate property in your detail view controller, like this:

class ViewControllerB: UIViewController {
    var selectedName: String = "Anonymous"
    weak var delegate: ViewControllerA!
}

When creating your detail view controller, make sure you set up its delegate property, like this:

let viewControllerB = ViewControllerB()
viewControllerB.selectedName = "Taylor Swift"
viewControllerB.delegate = self
navigationController?.pushViewController(viewControllerB, animated: true)

With this set up complete, you can now create a method in your master view controller that should be called by the detail view controller. For example, you might have something like this:

func updatedSelectedName(newName: String) {
    // do something with newName
}
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