What are the advantages and disadvantages of using the Singleton pattern?

Advantages:

• Guaranteed single instance of the class.
• Global access point to the instance.
• Lazy loading (on-demand initialization).

Disadvantages:

• Violates the Single Responsibility Principle, as the class is responsible for both creating the instance and its functionality.
• Complicates testing, especially unit testing, as it is difficult to substitute the instance with a mock or stub.
• Can lead to implicit dependencies and complicate refactoring.
• Potential issues with multithreading if thread-safe initialization is not implemented.
• May hide excessive coupling between system components.

Example of thread-safe Singleton implementation in Swift:

final class Singleton {
    // Single instance accessible via static property
    static let shared = Singleton()

    // Private initializer to prevent creation of other instances
    private init() {
        // Initialization
    }

    // Example class method
    func doSomething() {
        print("Singleton is doing something")
    }
}