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Singleton vs Dependency Injection in iOS Swift With A Simple Example



Singleton: - The Singleton is a Design Pattern. That only on single instance of a class. A Singleton can be accessed from anywhere in the program, but it cannot be modified from anywhere. It can only be modified from within the Singleton. It is, therefore, a means to protect globals.

A Singleton class supports inheritance, while a Static class is a sealed class, and therefore cannot be inherited. A Singleton class can inherit from other classes, while a Static class cannot (not even from other static classes).

  • The singleton pattern increases coupling.
Disadvantages of singleton in Swift
  • The global nature leads to dependency hiding.
  • It can be difficult to unit test the code.
  • It can lead to tightly coupled code.
  • If the single Instance of the object becomes corrupted, the entire system is compromised

A singleton class itself is not thread safe. Multiple threads can access the singleton same time and create multiple objects, violating the singleton concept. The singleton may also return a reference to a partially initialised object.

Example: - 

class MySingleton {

static let shared = MySingleton() // Singleton instance

private init() {
// Private initializer to prevent external instantiation

func doSomething() {
print("Singleton: Doing something")

// Usage:

Dependency Injection

Dependency Injection:- Dependency injection reduces coupling  

  • Improves code reusability.
  • Eases the unit testing of applications through mocking/stubbing injected dependencies.
  • Reduces boilerplate code because dependencies are initialized by their injector component.
  • Decouples component logic.
  • Makes it easier to extend the application classes.


protocol MyDependency {
func performAction()

class MyDependencyImplementation: MyDependency {
func performAction() {
print("Dependency: Performing an action")

class MyClass {
let dependency: MyDependency

init(dependency: MyDependency) {
self.dependency = dependency

func useDependency() {

// Usage:
let dependency = MyDependencyImplementation()
let myClass = MyClass(dependency: dependency)



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