What methods can be used to implement the Singleton pattern in programming?

The Singleton pattern guarantees that a class has only one instance and provides a global point of access to it. In iOS (Swift/Objective-C), there are several ways to implement Singleton:

1. Using a static constant (Swift)

class MySingleton {
    static let shared = MySingleton()
    private init() { }
}

This method is lazy and thread-safe because static let is initialized once.

2. Using dispatch_once (Objective-C)

+ (instancetype)sharedInstance {
    static MySingleton *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

3. Lazy initialization with check (not recommended due to thread safety issues)

class MySingleton {
    static var shared: MySingleton = {
        return MySingleton()
    }()
    private init() { }
}

4. Using enum (Swift)

enum MySingleton {
    static let shared = MySingleton()
}

But most often, the first method with static let is used.

Important: the constructor should be private to prevent creating other instances.