What are the commonly used design pattern in iOS apps?

What design patterns are commonly utilized  and used in iOS apps?

Ans: Design pattern in iOS : Typical commonly used patterns when building iOS applications are people who Apple advocates for in their Cocoa, Cocoa Touch, Objective-C, and Swift documentation. These are the patterns that each iOS developer learns.

iOS design patterns used are

• MVC
• MVVM
• Singleton
• Delegate
• Observer.
• Fasade
• Builder

MVC

The good old Model-View-Controller is Apple’s go-to application architecture design pattern. It’s good for small/simple apps, but not sustainable within the long term. We’ll cover it in additional details within the following section. MVC is a design pattern that separates the application into three main components: Model, View, and Controller. The Model represents the data and business logic of the application, the View represents the user interface, and the Controller acts as an intermediary between the Model and the View, handling user input and updating the Model and View as necessary.

The Model-View-Controller (MVC) design pattern is a widely used architectural pattern in iOS development. It separates an app’s data (model), user interface (view), and control logic (controller) into separate components to improve the app’s maintainability, reusability, and testability.

To implement the MVC pattern in iOS, follow these steps:

1. Define the model layer. This layer is responsible for managing the app’s data and business logic. It should contain all the classes and structures that represent the app’s data.
2. Define the view layer. This layer is responsible for displaying the app’s user interface. It should contain all the classes and components that define the look and feel of the app.
3. Define the controller layer. This layer is responsible for managing the interaction between the model and view layers. It should contain all the classes that handle user input, update the model layer, and update the view layer.
4. Implement communication between the layers. The controller layer acts as a mediator between the model and view layers, passing data between them as needed. The view layer should never directly modify the model layer; instead, it should use the controller layer to update the model layer.

Here’s an example of how to implement the MVC pattern in iOS:

struct TodoItem {
    let title: String
    let completed: Bool
}

// View Layer
class TodoListView: UIView {
    // ...
}

// Controller Layer
class TodoListViewController: UIViewController {
    private var todoItems: [TodoItem] = []

    private var todoListView: TodoListView {
        return view as! TodoListView
    }

    override func loadView() {
        view = TodoListView()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        todoItems = fetchTodoItems()
        todoListView.configure(with: todoItems)
    }

    private func fetchTodoItems() -> [TodoItem] {
        // Fetch data from model layer
        return []
    }

    private func updateTodoItem(_ todoItem: TodoItem) {
        // Update data in model layer
    }
}


In this example, the model layer consists of a TodoItem struct that represents a single item in a to-do list. The view layer consists of a TodoListView class that defines the user interface for the to-do list. The controller layer consists of a TodoListViewController class that manages the interaction between the model and view layers.

The TodoListViewController fetches data from the model layer and passes it to the view layer to display. When the user interacts with the view layer, the TodoListViewController updates the model layer accordingly.

By using the MVC pattern, you can create a modular and scalable architecture for your iOS app that separates concerns and makes it easier to maintain and evolve your codebase.Regenerate response

Model-View-ViewModel

(MVVM): MVVM is a design pattern that is similar to MVC, but separates the application into three different components: Model, View, and ViewModel. The ViewModel is responsible for exposing the data and operations that the View needs to display, and for handling user input and updating the Model as necessary.

The Model-View-ViewModel (MVVM) design pattern is a popular architecture pattern that is commonly used in iOS app development. MVVM is a variant of the Model-View-Controller (MVC) pattern that separates the presentation logic from the business logic of the application.

In MVVM, the presentation logic is implemented in a ViewModel object, which interacts with the View to manage its state and handle user input. The business logic of the application is implemented in a Model object, which is responsible for fetching data from a remote API, local database or other data source. The View is responsible for displaying the UI elements on the screen and forwarding user input to the ViewModel.

Here’s an example of how you might implement the MVVM pattern in iOS using Swift:

// Define the Model
struct Person {
    var name: String
    var age: Int
}

// Define the ViewModel
class PersonViewModel {
    var person: Person
    
    init(person: Person) {
        self.person = person
    }
    
    var name: String {
        return person.name
    }
    
    var age: String {
        return "\(person.age)"
    }
    
    func update(name: String, age: Int) {
        person.name = name
        person.age = age
    }
}

// Define the View
class PersonView: UIView {
    var viewModel: PersonViewModel
    
    init(viewModel: PersonViewModel) {
        self.viewModel = viewModel
        super.init(frame: .zero)
        
        // Setup the UI elements and bind them to the ViewModel
        let nameLabel = UILabel()
        let ageLabel = UILabel()
        let nameTextField = UITextField()
        let ageTextField = UITextField()
        
        nameLabel.text = "Name:"
        ageLabel.text = "Age:"
        
        nameTextField.text = viewModel.name
        ageTextField.text = viewModel.age
        
        nameTextField.addTarget(self, action: #selector(nameTextFieldChanged), for: .editingChanged)
        ageTextField.addTarget(self, action: #selector(ageTextFieldChanged), for: .editingChanged)
        
        // Add the UI elements to the view
        addSubview(nameLabel)
        addSubview(ageLabel)
        addSubview(nameTextField)
        addSubview(ageTextField)
    }
    
    @objc func nameTextFieldChanged() {
        viewModel.update(name: nameTextField.text ?? "", age: viewModel.person.age)
    }
    
    @objc func ageTextFieldChanged() {
        if let age = Int(ageTextField.text ?? "") {
            viewModel.update(name: viewModel.person.name, age: age)
        }
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

// Usage example
let person = Person(name: "John Doe", age: 30)
let viewModel = PersonViewModel(person: person)
let view = PersonView(viewModel: viewModel)

In this example, we define a Person struct that represents a person with a name and age. We then define a PersonViewModel class that manages the presentation logic for a Person object. The PersonView class is responsible for displaying the UI elements on the screen and forwarding user input to the PersonViewModel.

When the PersonView is initialized, it sets up the UI elements and binds them to the PersonViewModel. When the user changes the text in the name or age text fields, the PersonView updates the PersonViewModel with the new values. The PersonViewModel then updates the underlying Person object with the new values.

Using the MVVM pattern in this way can help to separate the presentation logic from the business logic of your iOS app, making it easier to test and maintain your code. It also allowsRegenerate 

Singleton

This is a standard OOP design pattern where you create the one and only in- stance of a category that may be used everywhere within the application where an in- stance of that class is important.

this is often a useful design pattern but commonly overused to the purpose of becoming an anti-pattern. the most issue is that developers often use singletons to store a worldwide state which isn’t an honest idea because of race conditions and other varieties of data overrides that inevitably happen.

Singleton is a design pattern that ensures that there is only one instance of a particular object in the application. In iOS development, singletons are commonly used for managing global data or services, such as a Network Manager or Database Manager.

the Singleton design pattern can be used in iOS app development. The Singleton pattern is a creational pattern that allows you to ensure that a class has only one instance, and provides a global point of access to that instance.

In iOS development, the Singleton pattern is commonly used to provide access to global resources or to manage the state of an application. For example, you might use a Singleton to manage the user’s login status or to provide access to a shared network client.

Here’s an example of how you might implement the Singleton pattern in iOS using Swift:

class MySingleton {
    static let shared = MySingleton()
    
    private init() {
        // Perform any necessary setup here
    }
    
    func doSomething() {
        print("Hello, World!")
    }
}

// Access the Singleton instance
let singleton = MySingleton.shared
singleton.doSomething()

In this example, we define a MySingleton class with a static shared property that provides access to the Singleton instance. We make the initializer private to ensure that only the Singleton instance can create new instances of the class.

We then define a doSomething method that can be called on the Singleton instance, and use it to print a message to the console.

To access the Singleton instance, we simply use the shared property of the MySingleton class. We can then call the doSomething method on the Singleton instance to perform some action.

Using the Singleton pattern in this way can make it easy to manage global state in your iOS app, and can help to simplify your code by providing a single point of access to important resources or functionality. However, it’s important to be careful when using the Singleton pattern, since it can lead to tight coupling and make your code harder to test and maintain.

Delegate

Delegate is one in all the core Cocoa design patterns. it’s a variation of the Ob- server pattern where just one object can observe or be delegated to events from another object.

It’s a one-to-one relationship that’s implemented through protocols. Cocoa itself uses this pattern plenty with UITableViewDelegate, UITableViewDataSource, UIPickerViewDelegate, and similar proto- cols that are exposed by the framework for developers to use.

Delegation is a design pattern that allows one object to delegate some of its responsibilities to another object. In iOS development, delegation is commonly used to allow a View Controller to delegate tasks to another object, such as a Table View or Collection View.

The delegate design pattern is a common pattern used in iOS development. It allows one object to communicate with another object by passing messages to a delegate object. The delegate object acts as a mediator between the two objects and receives messages from the first object, processes them, and then passes them on to the second object.

To implement the delegate pattern in iOS, follow these steps:

1. Define a protocol that declares the methods that the delegate object will implement. For example:

protocol MyDelegate {
    func didReceiveData(data: Data)
    func didFailWithError(error: Error)
}

2. Create a class that will act as the delegate object and conform to the protocol. For example:

class MyDelegateObject: MyDelegate {
    func didReceiveData(data: Data) {
        // Process the data here
    }

    func didFailWithError(error: Error) {
        // Handle the error here
    }
}

3.In the class that needs to communicate with the delegate object, create a property to hold the delegate object and call the appropriate delegate methods when necessary. For example:

class MyClass {
    var delegate: MyDelegate?

    func fetchData() {
        // Fetch data here
        if let data = fetchData() {
            delegate?.didReceiveData(data: data)
        } else {
            delegate?.didFailWithError(error: MyError.failedToFetchData)
        }
    }
}

4,Set the delegate property of the object that needs to communicate with the delegate object to an instance of the delegate object. For example:

let myClass = MyClass()
let myDelegate = MyDelegateObject()
myClass.delegate = myDelegate

By using the delegate pattern, you can create a flexible and decoupled architecture for your iOS app that allows objects to communicate with each other without having to know about each other’s implementation details.

Observer

This pattern may be a common one in iOS. It’s a design pattern that helps objects ob- serve state changes or events in other objects without coupling that observation to internal implementation of these objects.

Developers can always implement the Observer pattern themselves, but there are two built-in implementations in Cocoa already – Delegate, one-to-one observation, and KVO (key-value ob- serving), one-to-many observation.

Observer is a design pattern that allows objects to be notified when the state of another object changes. In iOS development, the Observer pattern is commonly used with the NotificationCenter or KVO (Key-Value Observing) mechanisms to allow one object to observe changes to another object.

the Observer design pattern can be used in Swift to implement loosely coupled communication between objects. The Observer pattern allows one object (the “subject”) to notify a set of “observers” when its state changes, without having to know anything about the observers themselves.

Here’s an example of how you might use the Observer pattern in Swift:

protocol Observer {
    func update(subject: Subject)
}

class Subject {
    var observers = [Observer]()

    func addObserver(observer: Observer) {
        observers.append(observer)
    }

    func removeObserver(observer: Observer) {
        observers = observers.filter { $0 !== observer }
    }

    func notifyObservers() {
        for observer in observers {
            observer.update(subject: self)
        }
    }

    var state: String = "" {
        didSet {
            notifyObservers()
        }
    }
}

class MyObserver: Observer {
    func update(subject: Subject) {
        print("Subject state changed to: \(subject.state)")
    }
}

let subject = Subject()
let observer = MyObserver()
subject.addObserver(observer: observer)

subject.state = "New state"
// Output: Subject state changed to: New state

In this example, we have a Subject class that maintains a list of observers and a state property. When the state property changes, the Subject notifies all of its observers by calling their update method.

We also have an Observer protocol that defines the update method that the Subject calls when its state changes. Finally, we have a MyObserver class that implements the Observer protocol and simply prints out the new state of the Subject when it is notified.

Using the Observer pattern in this way can make it easy to implement communication between objects without tightly coupling them together. It can also make your code more modular and easier to test, since each object can be tested independently of the others.

Facade:

Facade is a design pattern that provides a simplified interface to a complex subsystem. In iOS development, Facade is commonly used to provide a simplified interface to a complex system or library, such as a Core Data or CloudKit.

The Facade pattern is a structural design pattern that provides a simplified interface to a complex subsystem. It allows you to encapsulate the complexities of a system and provide a simple interface for clients to interact with.

In iOS development, the Facade pattern is commonly used to provide a simplified interface to a complex system or library. For example, you could use the Facade pattern to provide a simplified interface to Core Data or CloudKit, which are both complex frameworks that require a lot of boilerplate code to use effectively.

By using the Facade pattern, you can hide the complexities of these frameworks behind a simple interface that is easy to use and understand. This can help to reduce the amount of code you need to write, improve the readability of your code, and make it easier to maintain and refactor your code over time.

Here’s an example of how you could use the Facade pattern in iOS development:

class CoreDataFacade {
    let persistentContainer: NSPersistentContainer
    
    init() {
        self.persistentContainer = NSPersistentContainer(name: "MyApp")
        self.persistentContainer.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error {
                fatalError("Failed to load Core Data stack: \(error)")
            }
        })
    }
    
    func saveContext() {
        let context = persistentContainer.viewContext
        if context.hasChanges {
            do {
                try context.save()
            } catch {
                let nserror = error as NSError
                fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
            }
        }
    }
    
    func fetch<T: NSManagedObject>(_ request: NSFetchRequest<T>) -> [T] {
        let context = persistentContainer.viewContext
        do {
            let results = try context.fetch(request)
            return results
        } catch {
            print("Error fetching objects: \(error)")
            return []
        }
    }
}

In this example, we have a CoreDataManager class that manages the Core Data stack and provides a simplified interface to it. We also have a MyDataFacade class that uses the CoreDataManager to save and fetch data, but provides a simpler interface for the rest of the app to use.

Using Facade in this way can make your code simpler, easier to understand, and easier to maintain, especially when dealing with complex subsystems.

Builder:

Builder is a design pattern that allows you to create complex objects by breaking them down into smaller, simpler objects. In iOS development, the Builder pattern is commonly used with UIKit to create complex user interfaces.

the Builder design pattern can be used in iOS app development to create complex objects by breaking them down into smaller, simpler objects. The Builder pattern can be used to separate the construction of an object from its representation, allowing you to create different representations of the same object using the same construction process.

In iOS development, the Builder pattern is commonly used with the UIKit framework to create complex user interfaces. For example, you might use the Builder pattern to create a view hierarchy by breaking it down into smaller, simpler views and then assembling them into a more complex hierarchy.

Here’s an example of how you might use the Builder pattern to create a complex user interface in iOS:

class MyViewBuilder {
    private let label = UILabel()
    private let button = UIButton()

    func withLabelText(_ text: String) -> MyViewBuilder {
        label.text = text
        return self
    }

    func withButtonTitle(_ title: String) -> MyViewBuilder {
        button.setTitle(title, for: .normal)
        return self
    }

    func build() -> UIView {
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.spacing = 16

        stackView.addArrangedSubview(label)
        stackView.addArrangedSubview(button)

        return stackView
    }
}

let myView = MyViewBuilder()
    .withLabelText("Hello, World!")
    .withButtonTitle("Tap me!")
    .build()

In this example, we have a MyViewBuilder class that allows us to create a complex view hierarchy by chaining together method calls that configure the individual components of the view. We start with a label and a button, and use the withLabelText and withButtonTitle methods to set their properties. Finally, we use the build method to assemble the components into a stack view and return it as a single UIView.

Using the Builder pattern in this way can make it easier to create complex view hierarchies, especially when the hierarchy needs to be created dynamically or in response to user input. It can also make your code more modular and easier to test, since each component can be tested independently before being assembled into the final view hierarchy.

Adapter design pattern

The Adapter design pattern is a structural pattern commonly used in iOS development. It allows incompatible classes to work together by creating an adapter that acts as a bridge between them. The adapter class converts the interface of one class into the interface expected by the client code.

To implement the Adapter pattern in iOS, follow these steps:

1. Identify the incompatible classes that need to work together. For example, you might have an existing class with a method that you want to use in a new class.

class ExistingClass {
    func existingMethod() {
        // Do something
    }
}

class NewClass {
    func newMethod() {
        // Need to call existingMethod() from here
    }
}

2. Create a protocol that defines the interface expected by the client code. This protocol will be implemented by the adapter class.

protocol NewClassProtocol {
    func newMethod()
}

3. Create an adapter class that conforms to the protocol and also has a reference to the existing class. The adapter class will implement the protocol methods and call the existing class’s method from within them.

class Adapter: NewClassProtocol {
    let existingClass = ExistingClass()

    func newMethod() {
        existingClass.existingMethod()
    }
}

4. Modify the client code to use the adapter class instead of the incompatible class.

let adapter = Adapter()
let newClass = NewClass()
newClass.delegate = adapter
newClass.newMethod() // This will call the existingMethod() from the adapter

By using the Adapter pattern, you can make incompatible classes work together without having to modify their existing code. This can be particularly useful when integrating third-party libraries or frameworks into your iOS app.Regenerate response

Red Flag:

ios design patterns :When interviewer asks this question (in one form or another) what they’re trying to find are some things besides MVC. Because MVC is that the go-to design pattern, the expectation is that each iOS developer knows what it’s. What they need to listen to from you though, is what else is usually used and available out of the box.