Passing Data Between Screens in iOS: A Complete Guide

Passing Data Between Screens in iOS: From Beginner to Pro

Everyone who starts writing for iOS eventually hits a wall: "How do I pass a string, array, or object to another screen?" This question is so popular that on Stack Overflow it has gathered thousands of views and dozens of answer options. And for good reason — iOS architecture is built so that each screen (ViewController) lives its own life, and data between them needs to be passed deliberately.

In this article, we will break down 5 main ways to pass data between view controllers in Swift. You will learn when to use a segue, and when to use delegates or closures, and why NotificationCenter is not always evil. Let's go!

Why is this even a problem?

iOS uses a strict MVC (Model-View-Controller) architecture. Each ViewController is responsible for its own piece of the screen and does not have direct access to the data of a neighboring controller. This is done for safety and modularity, but for the developer, it means you need to explicitly "pass" data from point A to point B.

Imagine: you have a list of users, and you want to show detailed information when a cell is tapped. You tap — a new screen opens. How do you pass the user ID? This is where the magic begins.

Method 1: Passing Data via Storyboard Segue (Simplest)

The most straightforward way is to use prepare(for:sender:). You simply prepare the data before the transition. Ideal for simple apps and prototypes.

// In the source ViewController\noverride func prepare(for segue: UIStoryboardSegue, sender: Any?) {\n    if segue.identifier == "showDetail" {\n        guard let destinationVC = segue.destination as? DetailViewController else { return }\n        destinationVC.userID = 42 // passing a number\n        destinationVC.userName = "Anna" // passing a string\n    }\n}\n\n// In DetailViewController\nvar userID: Int = 0\nvar userName: String = ""\n

Pros: Minimal code, everything in one place. Cons: Tight coupling to Storyboard, hard to test, easy to get confused with identifiers.

Method 2: Programmatic Navigation with Data Passing (Advanced Level)

When you get tired of segues and want to control everything from code, use a direct controller instance.

// In the source ViewController\nfunc showDetail() {\n    let storyboard = UIStoryboard(name: "Main", bundle: nil)\n    guard let detailVC = storyboard.instantiateViewController(withIdentifier: "DetailVC") as? DetailViewController else { return }\n    \n    detailVC.userID = 42\n    detailVC.userName = "Anna"\n    \n    navigationController?.pushViewController(detailVC, animated: true)\n}\n

This method gives you full control: you can load data from the network, validate it, and only then transition. Great for complex logic.

Method 3: Delegates (Delegate Pattern) — Callback

What to do if you need to pass data back? For example, a user selected a date on the second screen, and we want to return it to the first. This is where the delegate comes to the rescue.

Step 1: Create a Protocol

protocol DatePickerDelegate: AnyObject {\n    func didSelectDate(_ date: Date)\n}\n

Step 2: Add a Delegate to the Child Controller

class DatePickerViewController: UIViewController {\n    weak var delegate: DatePickerDelegate?\n    \n    @IBAction func saveButtonTapped(_ sender: UIButton) {\n        let selectedDate = datePicker.date\n        delegate?.didSelectDate(selectedDate)\n        dismiss(animated: true)\n    }\n}\n

Step 3: Implement the Delegate in the Parent Controller

class MainViewController: UIViewController, DatePickerDelegate {\n    func didSelectDate(_ date: Date) {\n        print("Received date: \\(date)")\n        // update UI\n    }\n    \n    override f