item gcd singleton

item_gcd.m

#import "Item.h"

static id _instance;

@implementation Item

+ (id)allocWithZone:(struct _NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [super allocWithZone:zone];
    });
    return _instance;
}

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

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

@end