kotlin recyclerview data not showing

Question

Our question is how to show Parent and Child data from two SQLite DB tables?

We have two tables that are added to two ArrayLists childList and par

Answer 1

The way I would do it is flatten the list edit: like this in the constructor

val items : MutableList = mutableListOf()    

init() {
   parents //parents should be passed as a constructor argument
         .asSequence() //this is only needed if you want to also order them
         .sortedBy { it.idD } //again only if you want to sort them
         .forEach { 
              items.add(it)
              items.addAll(it.children)
          }
}

I would create a List < Any > (or if both parent and child implement an interface a List< IItem > ) that would contain all the data as you wish to see it, so for example it could look like this:

val items : List = listOf(parent1, child11,child12,child13,parent2,child12,child13,child14.....etc)

then I would implement a single adapter with multiple view types like this:

override fun getItemCount(): Int {
    return items.size
}

fun getItem(position: Int) : Any { //or the interface 
    return items[position]
}

override getItemViewType (position: Int) : Int {
    if (getItem(position) is ModelParent)
        return 0
    return 1
}

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
    var view : View? = null
    if (viewType == 0) {
        view = LayoutInflater.from(parent.context).inflate(R.layout.parent_layout,parent,false)
        return ParentViewHolder(view);
    } else {
       view = LayoutInflater.from(parent.context).inflate(R.layout.child_layout,parent,false)
        return ChildViewHolder(view);
    }
}

and then I would use two view holders to represent how the data is shown for the specific item

 inner class ParentViewHolder(itemView : View) : RecyclerView.ViewHolder(itemView){ ...}
 inner class ChildViewHolder(itemView : View) : RecyclerView.ViewHolder(itemView){ ...}

after that I could perform different bindings by getting the type of the view ,something like this:

override fun onBindViewHolder(holder: ViewHolder, position: Int) {
    if (getItemType(position) == 0) {
        (holder as ParentViewHolder).bindData(....)
    } else {
        (holder as ChildViewHolder).bindData(....)
    }
}

edit: Here is the complete adapter I built, it is based on two layout files:

list_item_child:

list_item_parent:

and the adapter would look something like this:

import android.content.Context
import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView

data class ModelParent(val id: Int, val children: List)
data class ModelChild(val id: Int)


class JoinAdapter(internal var context: Context, val parents: List) : RecyclerView.Adapter() {
    val items = mutableListOf()

    init {
        parents //parents should be passed as a constructor argument
                .forEach {
                    items.add(it)
                    items.addAll(it.children)
                }
    }

    override fun getItemCount(): Int = items.size;


    fun getItem(position: Int): Any = items[position]

    override fun getItemViewType(position: Int): Int = if (getItem(position) is ModelParent) 0 else 1

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
        var view: View? = null
        if (viewType == 0) {
            view = LayoutInflater.from(parent.context).inflate(R.layout.rv_list_item_parent, parent, false)
            return ParentViewHolder(view!!)
        } else {
            view = LayoutInflater.from(parent.context).inflate(R.layout.rv_list_item_child, parent, false)
            return ChildViewHolder(view!!)
        }
    }

    override fun onBindViewHolder(holder: MyViewHolder, position: Int) = holder.bindData(position, getItem(position))

    inner abstract class MyViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        abstract fun bindData(position: Int, item: Any)

    }

    inner class ParentViewHolder(view: View) : MyViewHolder(view) {
        var parentDept: TextView = view.findViewById(R.id.parent_department) as TextView

        override fun bindData(position: Int, item: Any) {
            val parent = item as? ModelParent ?: return
            parentDept.text = parent.dept
        }
    }

    inner class ChildViewHolder(view: View) : MyViewHolder(view) {
        var childItem: TextView = view.findViewById(R.id.child_item) as TextView

        override fun bindData(position: Int, item: Any) {
            val child = item as? ModelChild ?: return
            childItem.text = child.item
        }
    }

}

this when used on a single recyclerview will display all children under their parent in a list