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Observer Design Pattern

Pattern that allows objects to notify others about changes in their state.

About Observer Design Pattern

The Observer pattern is a behavioral design pattern that allows an object (subject) to notify other objects (observers) when its state changes. In iOS, this pattern is implemented through mechanisms like NotificationCenter, KVO (Key-Value Observing), and Combine.

Key Features

  • One-to-many communication
  • Loose coupling
  • Runtime registration/deregistration
  • Broadcast notifications
  • Filter-based observation

Code Example

// Observer pattern examples in iOS

// 1. NotificationCenter
// Register for notifications
NotificationCenter.default.addObserver(
    self,
    selector: #selector(handleDataChange(_:)),
    name: Notification.Name("DataDidChangeNotification"),
    object: nil
)

// Post a notification
NotificationCenter.default.post(
    name: Notification.Name("DataDidChangeNotification"),
    object: nil,
    userInfo: ["updatedData": newData]
)

// Handle notification
@objc func handleDataChange(_ notification: Notification) {
    if let data = notification.userInfo?["updatedData"] {
        // Update UI with new data
    }
}

// Deregister when done
deinit {
    NotificationCenter.default.removeObserver(self)
}

// 2. Key-Value Observing (KVO)
// Start observing
var observation: NSKeyValueObservation?
observation = object.observe(\.property) { object, change in
    print("Property changed to: \(object.property)")
}

// Stop observing
observation?.invalidate()

// 3. Combine (iOS 13+)
import Combine

class ViewModel {
    @Published var counter: Int = 0
}

// Using the publisher
let viewModel = ViewModel()
var cancellables = Set<AnyCancellable>()

viewModel.$counter
    .sink { newValue in
        print("Counter changed to: \(newValue)")
    }
    .store(in: &cancellables)

// Changing the value
viewModel.counter += 1

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