完全单例

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

    return _instance;
}

+(instancetype)shareManager
{
    return [[self alloc] init];
}

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

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

#if __has_feature(objc_arc)
// ARC
#else
// MRC
-(oneway void)release
{
}
-(instancetype)retain
{
    return _instance;
}
-(NSUInteger)retainCount
{
    return MAXFLOAT;
}