Registry pattern for singleton factory

registry.m

+ (instancetype)sharedInstance {
    static dispatch_once_t once;
    static NSMutableDictionary *sharedInstances;

    dispatch_once(&once, ^{
        // Creating of the container for shared instances for different classes
        sharedInstances = [NSMutableDictionary new];
    });

    id sharedInstance = nil;
    @synchronized(self) { /* Critical section for Singleton-behavior */
        // Getting of the shared instance for exact class
        sharedInstance = sharedInstances[NSStringFromClass(self)];
        if (!sharedInstance) {
            // Creating of the shared instance if it's not created yet
            sharedInstance = [self new];
            sharedInstances[NSStringFromClass(self)] = sharedInstance;
        }
    }
    return sharedInstance;
}