Android: SortedList with duplicates

Question

I have some problems understanding RecyclerViews SortedList.

Lets say I have a very simple class only having a very simple class holding data:<

Answer 1

I had a similar issue when creating a chat app which I need to update messages by their IDs and sort them by their dates. The support lib's SortedList does not do that or at least I had little time to dive into its source code and test. So, I created a small component, MultiSortedList:

import android.support.v7.widget.RecyclerView

/**
 * Created by abduaziz on 6/14/18.
 *
 *   MultiSortedList is a wrapper component to ArrayList that keeps its elements in a sorted order
 *   using UpdateCallbackInterface. It is intended to be used inside recycler view adapters.
 *
 * */

class MultiSortedList(var updateCallback: UpdateCallback, var adapter: RecyclerView.Adapter<*>? = null) {

    companion object {
        val TAG = "SORTEDLIST"
    }

    // internal list to hold elements by sortBy() -> visible to user
    private val list: ArrayList = arrayListOf()

    // internal list to hold elements by updateBy() -> not visible
    private val uList: ArrayList = arrayListOf()

    // add adapter from ui
    fun addAdapter(adapter: RecyclerView.Adapter<*>?) {
        this.adapter = adapter
    }

    /*
    * 1. Search for existing element that satisfies updateBy()
    * 2. Remove the existing element if found
    * 3. Add the new item with sortBy()
    * 4. Notify if adapter is not null
    * */
    fun add(newItem: T) {
        remove(newItem)

        // save to internal list by updateBy()
        var toBeStoredPosition = uList.binarySearch { updateCallback.updateBy(it, newItem) }
        if (toBeStoredPosition < 0) toBeStoredPosition = -(toBeStoredPosition + 1)
        uList.add(toBeStoredPosition, newItem)

        // save to UI list and notify changes
        var sortPosition = list.binarySearch { updateCallback.sortBy(it, newItem) }
        if (sortPosition < 0) sortPosition = -(sortPosition + 1)
        list.add(sortPosition, newItem)
        adapter?.notifyItemInserted(sortPosition)
    }

    /*
    * Remove and notify the adapter
    * */
    fun remove(removeItem: T) {
        val storedElementPosition = uList.binarySearch { updateCallback.updateBy(it, removeItem) }
        if (storedElementPosition >= 0 && storedElementPosition < uList.size) {

            // remove from internal list
            val itemTobeRemoved = uList[storedElementPosition]
            uList.removeAt(storedElementPosition)

            // remove from ui
            val removePosition = list.binarySearch { updateCallback.sortBy(it, itemTobeRemoved) }
            if (removePosition >= 0 && removePosition < list.size) {
                list.removeAt(removePosition)
                adapter?.notifyItemRemoved(removePosition)
            }
        }
    }

    // can be accessed -> list.get(position) or list[position]
    operator fun get(pos: Int): T {
        return list[pos]
    }

    // for adapter use
    fun size(): Int {
        return list.size
    }

    inline fun forEachIndexed(action: (Int, T) -> Unit) {
        for (index in 0 until size()) {
            action(index, get(index))
        }
    }

    /*
    * UpdateCallback is the main interface that is used to compare the elements.
    *   - sortBy() is used to locate new elements passed to SortedList
    *   - updateBy() is used to update/remove elements
    *
    * Typical example would be Message model class which we want to:
    *   - Sort messages according to their dates
    *   - Update/Remove messages according to their randomIDs or IDs.
    * */
    interface UpdateCallback {
        fun sortBy(i1: T, i2: T): Int
        fun updateBy(oldItem: T, newItem: T): Int
    }
}

The usage is explained here: https://medium.com/@abduazizkayumov/sortedlist-with-recyclerview-part-2-64c3e9b1b124