//基于gcd的单例模式

static Type *_instances;

+ (id)allocWithZone:(NSZone *)zone
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken,^{
    _instances = [super allocWithZone:zone];
});
     return _instances;
}
//创建一个获取单例的方法
+ (Type *)sharedInstances
{
    if(_instances == nil){
       _instances = [[self alloc] init];
    }
    return _instances;
}