In iOS application development, how can you implement the singleton design pattern to ensure that only one shared instance of a class exists throughout the app lifecycle?

Introduction / Context:
The singleton design pattern ensures that only one instance of a class is created and that a global point of access to that instance is provided. In iOS development, singletons are commonly used for managers such as networking, analytics, or configuration. Interviewers often ask how to implement a singleton correctly in Swift or Objective C to check understanding of design patterns and memory management.

Given Data / Assumptions:

• The platform is iOS, using either Swift or Objective C.
• The goal is to enforce a single shared instance of a particular class.
• We want a thread safe, easy to use implementation pattern.

Concept / Approach:
The modern way to implement a singleton in Swift is to declare a static constant property inside the class, often named shared or default, and to make the initialiser private or fileprivate. This static property is initialised lazily and only once. In Objective C, a common pattern uses a static variable and dispatch_once or similar mechanisms to ensure that the instance is created only once in a thread safe manner. In both cases, direct instantiation from outside the class is restricted to maintain the singleton guarantee.

Step-by-Step Solution:
Step 1: Define a class that you want to make a singleton, for example a NetworkManager. Step 2: Add a static property such as static let shared = NetworkManager() inside the class definition. Step 3: Mark the class initialiser as private so that code outside the class cannot create additional instances. Step 4: Access the shared instance everywhere in the app through NetworkManager.shared, rather than calling a public constructor. Step 5: Recognise that this pattern ensures only one instance is created and reused, which is the essence of the singleton pattern.

Verification / Alternative check:
To verify that the implementation is a true singleton, try to create another instance from outside the class. If the initialiser is private, the compiler will prevent this. You can also log the memory address of NetworkManager.shared from different places in the app and confirm that it is the same instance each time, which demonstrates that only one object is used.

Why Other Options Are Wrong:
Option B: Creating a new instance wherever needed is the opposite of a singleton and leads to many instances, not one. Option C: Declaring a class abstract does not create a singleton, and in Swift there is no direct abstract class keyword in the same way as some other languages. Option D: Storing multiple instances in an array and selecting one randomly guarantees that more than one instance exists, which violates the singleton pattern definition.

Common Pitfalls:
A common pitfall is to misuse singletons for almost every class, which leads to tight coupling and testing difficulties. Another mistake is to implement a singleton without making the initialiser private, which allows additional instances and breaks the pattern. Ensuring thread safety when initialising the singleton is also important, although modern Swift static properties handle this automatically.

Final Answer:
In iOS, you achieve a singleton by defining a static shared instance property and restricting direct initialisation of the class.