Converting a generic RecyclerView Adapter to Kotlin

Question

Currently, I have a list I want to populate, but the list items can have different data. I have two data classes and to accommodate this for the RecyclerView, I have two different ViewHolders extending a basic base ViewHolder. The different ViewHolders are required as different layouts are used for the different data classes.

I have converted the view holders to Kotlin, however I have an issue with the adapter.

The base ViewHolder in Kotlin:

abstract class BaseViewHolder<T> internal constructor(view: View) : RecyclerView.ViewHolder(view) {
    abstract fun bind(item: T)
}

A ViewHolder that implements the base ViewHolder in Kotlin:

class StandardViewHolder(view: View): BaseViewHolder<Standard>(view) {

    private val _eventView      : TextView = view.findViewById(R.id.eventTextView)
    private val _dateView       : TextView = view.findViewById(R.id.dateTextView)

    override fun bind(item: Standard) {
        _eventView.text     = item.event
        _dateView.text      = item.date.toString()
    }
}

In Java, I can create an adapter that uses these ViewHolders:

public class ListAdapter extends RecyclerView.Adapter<BaseViewHolder> {

    private List<Object> _items;
    private Context _context;

    public ListAdapter(List<Object> items, Context context){
        _items      = items;
        _context    = context;
    }

    @NonNull
    @Override
    public BaseViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        if(viewType == R.layout.item_standard){
            return new StandardViewHolder(LayoutInflater.from(_context).inflate(
                    R.layout.item_standard, parent, false));
        }

        return new AdvancedViewHolder(LayoutInflater.from(_context).inflate(
                R.layout.item_advanced, parent, false));
    }

    @Override
    @SuppressWarnings("unchecked")
    public void onBindViewHolder(@NonNull BaseViewHolder holder, int position) {
        holder.bind(_items.get(position));
    }

    @Override
    public int getItemCount() {
        return _items.size();
    }

    @Override
    public int getItemViewType(int position) {
        if(_items.get(position) instanceof Standard){
            return R.layout.item_standard;
        }

        return R.layout.item_advanced;
    }
}

However, if I convert this to Kotlin, I get an error on the holder.bind, being:

Out-projected type 'BaseViewHolder<*>' prohibits the use of 'public abstract fun bind(item: T)'

class ListAdapter(private val _items: List<Any>, private val _context: Context) : RecyclerView.Adapter<BaseViewHolder<*>>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BaseViewHolder<*> {
        return if (viewType == R.layout.item_standard) {
            StandardViewHolder(LayoutInflater.from(_context).inflate(
                    R.layout.item_standard, parent, false))
        } else {
            AdvancedViewHolder(LayoutInflater.from(_context).inflate(
                    R.layout.item_advanced, parent, false))
        }
    }

    override fun onBindViewHolder(holder: BaseViewHolder<*>, position: Int) = holder.bind(_items[position])

    override fun getItemCount() = _items.size

    override fun getItemViewType(position: Int): Int {
        return if (_items[position] is Standard) {
            R.layout.item_standard
        } else {
            R.layout.item_advanced
        }
    }
}

How do I use these generic ViewHolders in Kotlin?

Answer 1

For Generic Recycler view use the following code:

Step 1: Extend Adapter with RecyclerView.Adapter() e.g.

class MyAdapter(private val recentList: ArrayList<CollectionModel>) :
        RecyclerView.Adapter<**RecyclerView.ViewHolder**>() {
}

Step 2: set View type for all views e.g.

override fun getItemViewType(position: Int): Int {
        return if (recentList[position].type == 0) CollectionModel.COLLECTION_HEADER else CollectionModel.COLLECTION_DETAIL
    }

Step 3: inside onCreateViewHolder define proper view and ViewHolder e.g.

if (viewType == CollectionModel.COLLECTION_HEADER) {
            val view = LayoutInflater.from(parent.context).inflate(R.layout.signle_row_header, parent, false)
            return HeaderCollectionHolder(view)
        } else {
            val view = LayoutInflater.from(parent.context).inflate(R.layout.single_row_collection_history, parent, false)
            return DetailHolder(view)
        }

Step 4: Inside onBindViewHolder bind data based on view type and type cast the view holder using as Keyword e.g.

val collectionModel = recentList[position]

        if (recentList[position].type == CollectionModel.COLLECTION_HEADER) {
            (holder as HeaderCollectionHolder).apply {
                header.text = collectionModel.date
            }
        } else {
            (holder as DetailHolder).apply {
                name.text = collectionModel.name
                phone.text = collectionModel.phone
                collectedAmount.text = collectionModel.lastCollectAmount.toInt().toString()
                balanceAmount.text = collectionModel.pendingBalance.toInt().toString()
                Utils.setImage(collectionModel.picUrl, imageView)
            }
        }

Step 5: Create two view holder class as below:

inner class DetailHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
    var name: TextView = itemView.findViewById(R.id.name)
    var phone: TextView = itemView.findViewById(R.id.phone)
    var collectedAmount: TextView = itemView.findViewById(R.id.total_amount)
    var balanceAmount: TextView = item`enter code here`View.findViewById(R.id.balance_amount)
    var imageView: CircularImageView = itemView.findViewById(R.id.user_image)

}

inner class HeaderCollectionHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
    val header: TextView = itemView.findViewById(R.id.text)
}

Thats it.

Answer 2

Nice one… Actually i had some rework on that and see what i’ve done.. I’ve made it the most dynamic adapter one can ever make.

https://medium.com/@sandipsavaliya/tired-with-adapters-4240e5f45c24

Answer 3

The problem is that, even though you know that the item being bound matches the type of the ViewHolder that was created for it, you can't prove it at compile-time, so the compiler yells at you. You can't call bind on BaseViewHolder<*> because then the argument would have to be of type * which can't happen. What you need there BaseViewHolder<Any> but you can't make the adapter RecyclerView.Adapter<BaseViewHolder<Any>> because that will break onCreateViewHolder. And I've tried BaseViewHolder<out Any>, that doesn't work either.

So here is what you do: Use BaseViewHolder<*>, but then inside onBindViewHolder cast it to BaseViewHolder<Any>. The compiler will complain "Hey! That's an unchecked cast! You shouldn't do that!", so tell it to shut up with @Suppress("UNCHECKED_CAST").

class ListAdapter(
        private val _items: List<Any>,
        private val _context: Context
) : RecyclerView.Adapter<BaseViewHolder<*>>() {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): BaseViewHolder<*> {
        return if (viewType == R.layout.item_standard) {
            StandardViewHolder(LayoutInflater.from(_context).inflate(
                    R.layout.item_standard, parent, false))
        } else {
            AdvancedViewHolder(LayoutInflater.from(_context).inflate(
                    R.layout.item_advanced, parent, false))
        }
    }

    @Suppress("UNCHECKED_CAST")
    override fun onBindViewHolder(holder: BaseViewHolder<*>, position: Int) {
        (holder as BaseViewHolder<Any>).bind(_items[position])
    }

    override fun getItemCount() = _items.size

    override fun getItemViewType(position: Int): Int {
        return if (_items[position] is Standard) {
            R.layout.item_standard
        } else {
            R.layout.item_advanced
        }
    }
}