创建单例

gistfile1.m

static dispatch_once_t predicate;

+ (id)sharedUser
{
    static id shareUser = nil;
    dispatch_once(&predicate, ^{
        shareUser = [[super allocWithZone:NULL] init];
    });
    return shareUser;
}

+ (id)allocWithZone:(NSZone *)zone
{
    return [self sharedUser];
}