[25] Design patterns

Question 1

What is the Singleton pattern and give an iOS example?

Answer 1

Ensures only one instance of a class exists. Example: UIApplication.shared, UserDefaults.standard.

Question 2

What is the downside of using Singleton in iOS?

Answer 2

Global state, makes testing harder, can cause hidden dependencies.

Question 3

Code sample of singleton pattern

Answer 3

final class Singleton {
    // 1. Shared static instance (lazy + thread-safe)
    static let shared = Singleton()
    
    // 2. Private initializer prevents external initialization
    private init() {}
    
    // Example method
    func doSomething() {
        print("Singleton instance is working.")
    }
}

Question 4

What is the Observer pattern and give an iOS example?

Answer 4

Defines a one-to-many dependency so observers are notified of changes. Example: NotificationCenter, KVO, Combine publishers.

Question 5

Sample code for notification Center

Answer 5

import Foundation

// Sender
class Poster {
    func postMessage() {
        NotificationCenter.default.post(name: .myNotification, object: nil)
    }
}

// Receiver
class Observer {
    init() {
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(handleNotification),
            name: .myNotification,
            object: nil
        )
    }
    
    @objc private func handleNotification() {
        print("Notification received!")
    }
}

// Usage
extension Notification.Name {
    static let myNotification = Notification.Name("MyNotification")
}

let observer = Observer()
let poster = Poster()
poster.postMessage()  // prints: Notification received!

Question 6

What is the Delegation pattern and how is it used in iOS?

Answer 6

One object delegates responsibility to another via a protocol. Example: UITableViewDelegate, UITextFieldDelegate.

Question 7

What is the Factory Method pattern and give an iOS example?

Answer 7

Provides an interface for creating objects but lets subclasses decide the type. Example: UIButton(type: .system).

Question 8

What is the Adapter pattern and give an iOS example?

Answer 8

Converts one interface into another expected by clients. Example: Using NSFetchedResultsController to adapt Core Data results for UITableView.

Question 9

What is MVC in iOS architecture?

Answer 9

Model-View-Controller: Apple’s classic architecture, but can lead to ‘Massive View Controller’.

Question 10

What is MVVM and how does it improve over MVC?

Answer 10

Model-View-ViewModel separates presentation logic into the ViewModel, reducing controller size. Common with Combine/SwiftUI.

Question 11

What is the Strategy pattern and give an iOS example?

Answer 11

Defines a family of interchangeable algorithms. Example: Sorting arrays with different closures.

Question 12

What is the Command pattern and give an iOS example?

Answer 12

Encapsulates an action as an object. Example: NSUndoManager commands for undo/redo.

Question 13

What is the Facade pattern and give an iOS example?

Answer 13

Provides a simplified interface to a complex subsystem. Example: URLSession as a facade over CFNetwork.

Question 14

What is the Iterator pattern and give an iOS example?

Answer 14

Provides a way to access elements of a collection sequentially without exposing internal representation. Example: Swift Sequence and IteratorProtocol.

Question 15

What is the Composite pattern and give an iOS example?

Answer 15

Treats individual objects and compositions uniformly. Example: UIView containing subviews in UIKit.

Question 16

Why is Singleton commonly used in iOS frameworks?

Answer 16

To provide global shared resources like UIApplication, FileManager, UserDefaults without multiple instances.

Question 17

How can you make a Singleton thread-safe in Swift?

Answer 17

By using a static constant which is lazily initialized and guaranteed to be thread-safe in Swift.

Question 18

What is the difference between NotificationCenter and KVO?

Answer 18

NotificationCenter broadcasts to many observers; KVO observes property changes on specific objects.

Question 19

How is the Observer pattern used in SwiftUI?

Answer 19

Via @Published properties and ObservableObject, which notify SwiftUI views automatically.

Question 20

Why is Delegation preferred over NotificationCenter in UIKit?

Answer 20

Delegation is one-to-one and enforces compile-time contracts via protocols, making it safer.

Question 21

How does Delegation reduce coupling?

Answer 21

By defining behavior in a protocol, the delegate class can change without modifying the delegating class.

Question 22

What is the advantage of Factory Method in Swift?

Answer 22

It encapsulates object creation, makes code more flexible and testable, avoids spreading init details.

Question 23

Give an example of Factory Method in Foundation or UIKit.

Answer 23

URLRequest(url:) creates a request; UIButton(type:) creates a button with style.

Question 24

When would you use the Adapter pattern in iOS?

Answer 24

When you want to integrate a legacy API with a new interface, e.g., adapting old Objective-C APIs for SwiftUI.

Question 25

What is the difference between Adapter and Facade?

Answer 25

Adapter converts one interface to another; Facade simplifies a subsystem with a unified API.

Question 26

Why can MVC lead to 'Massive View Controller' in iOS?

Answer 26

Because UIViewController often ends up handling UI, data logic, and navigation, mixing responsibilities.

Question 27

How does MVVM help testing in iOS?

Answer 27

Business logic in ViewModel can be tested independently without UI framework dependencies.

Question 28

What is the benefit of Strategy pattern in Swift?

Answer 28

It allows changing algorithms at runtime by passing different closures or strategy objects.

Question 29

Give an iOS example where Strategy is used implicitly.

Answer 29

Sorting collections with custom closures, e.g., sorted(by:).

Question 30

How does Command pattern support undo operations?

Answer 30

Each action is encapsulated as a command object that can be stored and reversed by calling undo().

Question 31

Give an example of Command pattern in UIKit.

Answer 31

UIKeyCommand and UICommand encapsulate user actions in menus and keyboard shortcuts.

Question 32

Why is URLSession a Facade?

Answer 32

It provides a simple API for network tasks while hiding lower-level CFNetwork complexities.

Question 33

How does Swift implement Iterator pattern?

Answer 33

Through Sequence and IteratorProtocol; for-in loops use them under the hood.

Question 34

Why is Iterator useful?

Answer 34

It allows sequential access to collections without exposing internal details like array indices.

Question 35

Why is the UIView hierarchy an example of Composite?

Answer 35

Both UIView and its subviews conform to the same interface, allowing recursive composition.

Question 36

How does Composite pattern simplify UI code?

Answer 36

It lets you treat individual views and groups of views uniformly, e.g., applying constraints or transforms.

Question 37

What is the Prototype pattern and give an iOS example?

Answer 37

Creates new objects by cloning existing ones. Example: NSCopying protocol.

Question 38

What is the Builder pattern and give an iOS example?

Answer 38

Separates object construction from representation. Example: URLComponents building a URL.

Question 39

What is the Chain of Responsibility pattern and give an iOS example?

Answer 39

Passes a request along a chain of handlers. Example: UIResponder chain for touch events.

Question 40

What is the Memento pattern and give an iOS example?

Answer 40

Captures and restores an object's state. Example: State restoration APIs in UIKit.

Question 41

What is the Flyweight pattern and give an iOS example?

Answer 41

Minimizes memory by sharing objects. Example: Reusing cell objects in UITableView with reuseIdentifier.

Question 42

What is the Mediator pattern and give an iOS example?

Answer 42

Defines an object that centralizes communication. Example: UICollectionView mediates between cells, layout, and data source.

Question 43

What is the Template Method pattern and give an iOS example?

Answer 43

Defines the skeleton of an algorithm, deferring steps to subclasses. Example: UIView.draw(_:) where subclasses override drawing behavior.