Simple sharedInstance pattern in objective-c. http://www.daveoncode.com/2011/12/19/fundamental-ios-design-patterns-sharedinstance-singleton-objective-c/

sharedInstance

+(id)sharedInstance {
    static dispatch_once_t p = 0;
    
    __strong static id _sharedObject = nil;
    
    dispatch_once(&p, ^{
        _sharedObject = [[self alloc] init];
    });
    
    return _sharedObject;
}