单例模式:
//static id _instace;
//
//+ (id)allocWithZone:(struct _NSZone *)zone
//{
// static dispatch_once_t onceToken;
// dispatch_once(&onceToken, ^{
// _instace = [super allocWithZone:zone];
// });
// return _instace;
//}
//
//
//- (id)copyWithZone:(NSZone *)zone
//{
// return _instace;
//}
//
//+ (instancetype)sharedInstance
//{
// static dispatch_once_t onceToken;
// dispatch_once(&onceToken, ^{
// _instace = [[self alloc] init];
// });
// return _instace;
//}