SharedManager by Objective-C

AnyManager.m

@implementation AnyManager

+ (instancetype)sharedManager {
    static AnyManager *_sharedManager = nil;
    static dispatch_once_t oncePredicate;
    dispatch_once(&oncePredicate, ^{
        _sharedManager = [[self alloc] init];
    });

    return _sharedManager;
}

@end

References

https://github.com/AFNetworking/AFNetworking/blob/555385d2b8c6eb736fc2f5e3aeac46ec2d5ffddd/AFNetworking/AFNetworkReachabilityManager.m#L116-L126

https://www.toyship.org/archives/1770

これは簡易版であり、 alloc/copy を呼び出し側で使わない注意が必要。
allocWithZone copyWithZone を実装することで Singleton の実装になる。