6. Actions, Outlets & Delegates

Core Concepts

IBOutlets and IBActions

IBOutlets connect code to UI elements; IBActions respond to UI events:

class DetailViewController: UIViewController {
    @IBOutlet weak var nameLabel: UILabel!
    @IBOutlet weak var saveButton: UIButton!
    
    @IBAction func saveTapped(_ sender: UIButton) {
        nameLabel.text = "Saved!"
        saveButton.isEnabled = false
    }
}

Target-Action Pattern

The target-action mechanism connects UI control events to methods:

let slider = UISlider()
slider.addTarget(self, action: #selector(sliderChanged(_:)),
    for: .valueChanged)

@objc func sliderChanged(_ sender: UISlider) {
    print("Slider value: \(sender.value)")
}

Protocols and Delegates

Delegation allows one object to communicate back to another:

protocol OrderDelegate: AnyObject {
    func orderDidComplete(orderId: String)
}

class OrderManager {
    weak var delegate: OrderDelegate?
    
    func processOrder() {
        // Process the order...
        delegate?.orderDidComplete(orderId: "ORD-123")
    }
}

class OrderViewController: OrderDelegate {
    let manager = OrderManager()
    
    init() {
        manager.delegate = self
    }
    
    func orderDidComplete(orderId: String) {
        print("Order \(orderId) completed!")
    }
}

Notification Center

Broadcasts messages across the app using NotificationCenter:

// Post
NotificationCenter.default.post(
    name: NSNotification.Name("UserLoggedIn"),
    object: nil,
    userInfo: ["userId": "123"])

// Observe
NotificationCenter.default.addObserver(
    self,
    selector: #selector(handleLogin),
    name: NSNotification.Name("UserLoggedIn"),
    object: nil)

KVO (Key-Value Observing)

Observes property changes on objects:

class MyObject: NSObject {
    @objc dynamic var value: Int = 0
}

// Observe
observation = object.observe(\.value, options: [.new]) {
    object, change in
    print("New value: \(change.newValue ?? 0)")
}