单例的实现方式之GCD方法

+ (singletonobject *)shareinstance;

+ (singletonobject *)shareinstance {

    static singletonobject *__singletion;

    static dispatch_once_t oncetoken;

    dispatch_once(&oncetoken, ^{

        __singletion=[[self alloc] init];

    });

    return __singletion;

}