In support to blog post https://codewithshabib.com/2017/02/25/singletons-pattern-or-anti-pattern/

Singleton.m

@implementation Singleton

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

@end