OC 中的单例模式

OC中单例模式的实现：

+ (PeriodicElements *)sharedPeriodicElements
{
    @synchronized(self) {
        static dispatch_once_t pred;
        dispatch_once(&pred, ^{ sharedPeriodicElementsInstance = [[self alloc] init]; });
    }
    return sharedPeriodicElementsInstance;
}

+ (id)allocWithZone:(NSZone *)zone {
    @synchronized(self) {
        if (sharedPeriodicElementsInstance == nil) {
            sharedPeriodicElementsInstance = [super allocWithZone:zone];
            return sharedPeriodicElementsInstance;  // assignment and return on first allocation
        }
    }
    return nil; //on subsequent allocation attempts return nil
}

- (id)copyWithZone:(NSZone *)zone {
    return self;
}