Simple example of using RecyclerView in Kotlin | Android

Though Kotlin has lot of massive features to speedup the development time, here is the simple example of using RecyclerView in Android. In Kotlin we don't need to declare and initialize RecyclerView. We can simply access the id of RecyclerView from xml.

Ex :

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.guna.kotlinapplication.MainActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyclerView"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>
</android.support.constraint.ConstraintLayout>

We can simply access id of RecyclerView from above xml in kotlin file.

recyclerView.layoutManager = LinearLayoutManager(this)
recyclerView.setHasFixedSize(true)
recyclerView.adapter = Adapter(myStrings, this)

Here is the Adapter.kt class.

import android.support.v7.widget.RecyclerView
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import kotlinx.android.synthetic.main.item.view.*

/**
 * Created by Guna on 30-11-2017.
 */
class Adapter(val strings: Array<String>, val activity: MainActivity) : RecyclerView.Adapter<Adapter.ViewHolder>() {

    override fun onBindViewHolder(holder: ViewHolder?, position: Int) {
        holder?.bind(strings[position], activity)
    }

    override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): ViewHolder =
            ViewHolder(LayoutInflater.from(parent?.context).inflate(R.layout.item, parent, false))

    override fun getItemCount(): Int = strings.size

    class ViewHolder(itemView: View?) : RecyclerView.ViewHolder(itemView) {
        fun bind(string: String, activity: MainActivity) {
            itemView.textView.setText(string)
            itemView.setOnClickListener{
                view ->  activity.fromAdapter(string)
            }
        }
    }
}

And Here is the MainActivity.kt class.

import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.graphics.Rect
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.support.v7.widget.LinearLayoutManager
import android.support.v7.widget.RecyclerView
import android.support.v7.widget.helper.ItemTouchHelper
import android.widget.Toast
import android.widget.Toast.LENGTH_LONG
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {
    val myStrings = arrayOf("One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten",
            "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen")
    val paint = Paint()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        recyclerView.layoutManager = LinearLayoutManager(this)
        recyclerView.setHasFixedSize(true)
        recyclerView.adapter = Adapter(myStrings, this)

        val callback = object : ItemTouchHelper.SimpleCallback(ItemTouchHelper.UP or ItemTouchHelper.DOWN, ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT) {
            override fun onMove(recyclerView: RecyclerView?, viewHolder: RecyclerView.ViewHolder?, target: RecyclerView.ViewHolder?): Boolean {
                val fromPosition = viewHolder!!.adapterPosition
                val toPosition = target!!.adapterPosition

                val fromString = myStrings[fromPosition]
                val toString = myStrings[toPosition]

                myStrings[fromPosition] = toString
                myStrings[toPosition] = fromString
                recyclerView?.adapter?.notifyItemMoved(fromPosition, toPosition)
                return true
            }

            override fun onSwiped(viewHolder: RecyclerView.ViewHolder?, direction: Int) {
                if (direction == ItemTouchHelper.LEFT) {
                    Toast.makeText(this@MainActivity, myStrings[viewHolder!!.adapterPosition] + " Left Swiped", LENGTH_LONG).show()
                } else if (direction == ItemTouchHelper.RIGHT) {
                    Toast.makeText(this@MainActivity, myStrings[viewHolder!!.adapterPosition] + " Right Swiped", LENGTH_LONG).show()
                }
                recyclerView.adapter.notifyDataSetChanged()
            }

            override fun onChildDraw(c: Canvas?, recyclerView: RecyclerView?, viewHolder: RecyclerView.ViewHolder?, dX: Float, dY: Float, actionState: Int, isCurrentlyActive: Boolean) {
                if (dX > 0) {
                    paint.color = Color.GREEN
                    val rect = Rect(viewHolder!!.itemView!!.left, viewHolder.itemView.top, dX.toInt(), viewHolder.itemView.bottom)
                    c?.drawRect(rect, paint)
                } else if (dX < 0) {
                    paint.color = Color.RED
                    val rect = Rect(viewHolder!!.itemView.right.plus(dX).toInt(), viewHolder.itemView.top, viewHolder.itemView.right, viewHolder.itemView.bottom)
                    c?.drawRect(rect, paint)
                }
                super.onChildDraw(c, recyclerView, viewHolder, dX, dY, actionState, isCurrentlyActive)
            }
        }

        val helper = ItemTouchHelper(callback)
        helper.attachToRecyclerView(recyclerView)
    }

    fun fromAdapter(string: String) {
        Toast.makeText(this, string, LENGTH_LONG).show()
    }
}

Did you noticed, we never declare recyclerView nor initialized, instead we directly accessed it from activity_main.
In ViewHolder also, we didn't declare and initialize textView

And also, unless Java, default constructor will come with class name itself; like Adapter(val strings: Array, val activity: MainActivity).

So, If you think this is the right time to start with Kotlin, then start alongside with your current project. Yes, you can use Java and Kotlin in same project. If your current Activity is java, don't worry, you can create your next Activity in kotlin. Or you can convert your current Activity also to Kotlin and then continue learning Kotlin in same Activity.

Here is the full video tutorial

Interesting right?

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