OC单例

• (instancetype)sharedManager
  {
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  _sharedManger = [[super allocWithZone:NULL] init];
  });
  return _sharedManger;
  }

• (id)copyWithZone:(NSZone *)zone
  {
  return [xxxx sharedManager];
  }

• (id)allocWithZone:(struct _NSZone *)zone
  {
  return [xxxx sharedManager];
  }