kotlin abstract static fun in companion objects

Question

I learn the use of ViewHolder from an offical sample named UserViewHolder.

public class UserViewHolder extends RecyclerView.ViewHolder {

static UserViewHolder create(LayoutInflater inflater, ViewGroup parent) {
    UserItemBinding binding = UserItemBinding
        .inflate(inflater, parent, false);
    return new UserViewHolder(binding);
  } 

  private UserItemBinding mBinding;

  private UserViewHolder(UserItemBinding binding) {
    super(binding.getRoot());
    mBinding = binding;
  }

  public void bindTo(User user) {
    mBinding.setUser(user);
    mBinding.executePendingBindings();
  }

}

I'm going to write many ViewHolder classes, so I hope I can write an abstract class. In Java, it looks like:

public abstract  static class BaseViewHolder {

abstract static BaseViewHolder create()

abstract void bindTo()

}

I try to write it using Kotlin , but finally I find that it's not as simple as it in Java.

abstract class BaseViewHolder(itemView: View):RecyclerView.ViewHolder(itemView) {

abstract fun bindTo(viewModel: BaseViewModel)

}

In Kotlin, if I want a static function, I need to write the function in "companion objects". But it can't be a "abstract".

In Java, a abstract class with abstract classes is common.

But how can I write it in Kotlin?

update:

I have wrote my own SleepViewHolder. I'm going to write lots of ViewHolder, such as AppleViewHolder, BananaViewHolder and so on. So I want to build a BaseViewHolder as a pattern. My question is that, in that case, what's the best way to write the pattern BaseViewHolder? Should I change the constrcuter of it, or make the create function public?

open class SleepViewHolder private constructor(private val binding: ItemSleepBinding)
: RecyclerView.ViewHolder(binding.root) {

companion object {
    @JvmStatic
    fun create(inflater: LayoutInflater, parent: ViewGroup): SleepViewHolder {

        val binding: ItemSleepBinding
                = DataBindingUtil.inflate(inflater, R.layout.fragment_base, parent, false)

        return SleepViewHolder(binding)
    }
}

open fun bindTo(viewmodel: SleepViewModel) {
    binding.vm = viewmodel
    binding.executePendingBindings()
}

}

Answer 1

In Kotlin, unlike Java or C#, classes do not have static methods. In most cases, it's recommended to simply use package-level functions instead.

If you need to write a function that can be called without having a class instance but needs access to the internals of a class (for example, a factory method), you can write it as a member of an object declaration inside that class.

Even more specifically, if you declare a companion object inside your class, you'll be able to call its members with the same syntax as calling static methods in Java/C#, using only the class name as a qualifier.

This is how you can write a companion class

class MyClass {
   companion object { } // will be called "Companion"
}
fun MyClass.Companion.foo() { // ...
}

this is how you call foo() function...

MyClass.foo()