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android kotlin - ListView ViewHolder example

MainActivity.kt

package com.cfsuman.kotlintutorials

import android.app.Activity
import android.os.Bundle
import android.widget.*


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

        // get the widgets reference from XML layout
        val listView = findViewById<ListView>(R.id.listView)


        // Get an instance of base adapter
        val adapter = ColorBaseAdapter()

        // Set the list view adapter
        listView.adapter = adapter
    }
}
activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rootLayout"
    android:background="#DCDCDC"
    android:padding="12dp">

    <ListView
        android:id="@+id/listView"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="16dp"
        android:divider="@null"
        android:dividerHeight="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>
ColorBaseAdapter.kt

package com.cfsuman.kotlintutorials

import android.app.Activity
import android.graphics.Bitmap
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.BaseAdapter
import android.widget.ImageView
import android.widget.TextView
import android.widget.Toast
import androidx.cardview.widget.CardView


class ColorBaseAdapter : BaseAdapter(){
    private val list = colors()

    override fun getView(position:Int, convertView: View?,
                         parent: ViewGroup?):View{
        val viewHolder:ViewHolder
        var convertView = convertView

        if(convertView==null){
            // If convert view is null then inflate a
            // custom view and use it as convert view
            convertView = LayoutInflater.from(parent?.context).
            inflate(R.layout.custom_view,parent,false)

            // Create a new view holder instance using convert view
            viewHolder = ViewHolder(convertView)

            // Set the view holder as convert view tag
            convertView.tag = viewHolder
        }else{
            /*
                If convert view is not null then
                initialize the view holder using convert view tag.
            */
            viewHolder = convertView.tag as ViewHolder
        }


        // Display the current color name and value on view holder
        viewHolder.colorName.text = list[position].first
        viewHolder.imageView.setImageBitmap(
            drawCircle(list[position].second))


        // Set a click listener for card view
        viewHolder.card.setOnClickListener {
            // Show selected color in a toast message
            Toast.makeText(parent?.context,
                "Clicked : ${list[position].first}",
                Toast.LENGTH_SHORT).show()

            // Get the activity reference from parent
            val activity = parent?.context as Activity

            // Get the activity root view
            val viewGroup = activity.findViewById<ViewGroup>(
                android.R.id.content).getChildAt(0)

            // Change the root layout background color
            viewGroup.setBackgroundColor(list[position].second)
        }

        // Finally, return the convert view
        return convertView!!
    }


    override fun getItem(position: Int): Any? {
        return list[position]
    }


    override fun getItemId(position: Int): Long {
        return position.toLong()
    }



    override fun getCount(): Int {
        return list.size
    }



    // Custom method to generate list of color name value pair
    private fun colors():List<Pair<String,Int>>{
        return listOf(
            Pair("INDIANRED", Color.parseColor("#CD5C5C")),
            Pair("LIGHTCORAL",Color.parseColor("#F08080")),
            Pair("SALMON",Color.parseColor("#FA8072")),
            Pair("DARKSALMON",Color.parseColor("#E9967A")),
            Pair("LIGHTSALMON",Color.parseColor("#FFA07A")),
            Pair("CRIMSON",Color.parseColor("#DC143C")),
            Pair("RED",Color.parseColor("#FF0000")),
            Pair("FIREBRICK",Color.parseColor("#B22222")),
            Pair("DARKRED",Color.parseColor("#8B0000")),

            Pair("PINK",Color.parseColor("#FFC0CB")),
            Pair("LIGHTPINK",Color.parseColor("#FFB6C1")),
            Pair("HOTPINK",Color.parseColor("#FF69B4")),
            Pair("DEEPPINK",Color.parseColor("#FF1493")),
            Pair("MEDIUMVIOLETRED",Color.parseColor("#C71585")),
            Pair("PALEVIOLETRED",Color.parseColor("#DB7093")),

            Pair("LIGHTSALMON",Color.parseColor("#FFA07A")),
            Pair("CORAL",Color.parseColor("#FF7F50")),
            Pair("TOMATO",Color.parseColor("#FF6347")),
            Pair("ORANGERED",Color.parseColor("#FF4500")),
            Pair("DARKORANGE",Color.parseColor("#FF8C00")),
            Pair("ORANGE",Color.parseColor("#FFA500")),

            Pair("GOLD",Color.parseColor("#FFD700")),
            Pair("YELLOW",Color.parseColor("#FFFF00")),
            Pair("LIGHTYELLOW",Color.parseColor("#FFFFE0")),
            Pair("LEMONCHIFFON",Color.parseColor("#FFFACD")),
            Pair("LIGHTGOLDENRODYELLOW",Color.parseColor("#FAFAD2")),
            Pair("PAPAYAWHIP",Color.parseColor("#FFEFD5")),
            Pair("MOCCASIN",Color.parseColor("#FFE4B5")),
            Pair("PEACHPUFF",Color.parseColor("#FFDAB9")),
            Pair("PALEGOLDENROD",Color.parseColor("#EEE8AA")),
            Pair("KHAKI",Color.parseColor("#F0E68C")),
            Pair("DARKKHAKI",Color.parseColor("#BDB76B"))
        )
    }



    private fun drawCircle(color:Int=Color.WHITE,width:Int=200): Bitmap {
        // Initialize a new Bitmap object
        val bitmap:Bitmap = Bitmap.createBitmap(
            width, // Width
            width, // Height
            Bitmap.Config.ARGB_8888 // Config
        )


        // Initialize a new Canvas instance
        val canvas: Canvas = Canvas(bitmap)


        // Draw a solid color to the canvas background
        canvas.drawColor(Color.TRANSPARENT)


        // Initialize a new Paint instance to draw the Circle
        val paint: Paint = Paint()
        paint.style =Paint.Style.FILL
        paint.color = color
        paint.isAntiAlias = true


        // Calculate the available radius of canvas
        val radius:Int = Math.min(canvas.width,canvas.height/2)


        // Set a pixels value to padding around the circle
        val padding:Int = 5


        // Finally, draw the circle on the canvas
        canvas.drawCircle(
            (canvas.width / 2).toFloat(), // cx
            (canvas.height / 2).toFloat(), // cy
            (radius - padding).toFloat(), // Radius
            paint // Paint
        )


        // Return the newly created bitmap
        return bitmap
    }
}



private class ViewHolder(view:View){
    var colorName: TextView = view.findViewById(R.id.colorName)
    var imageView: ImageView = view.findViewById(R.id.imageView)
    val card: CardView = view.findViewById(R.id.cardView)
}
custom_view.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <androidx.cardview.widget.CardView
        android:id="@+id/cardView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="4dp"
        app:cardBackgroundColor="#F8F8F8"
        app:cardCornerRadius="8dp"
        app:cardElevation="4dp"
        app:cardMaxElevation="6dp"
        app:contentPadding="16dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <ImageView
                android:id="@+id/imageView"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

            <TextView
                android:id="@+id/colorName"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:fontFamily="sans-serif"
                android:gravity="center"
                android:textSize="20sp"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toEndOf="@+id/imageView"
                app:layout_constraintTop_toTopOf="parent" />

        </androidx.constraintlayout.widget.ConstraintLayout>
    </androidx.cardview.widget.CardView>
</androidx.constraintlayout.widget.ConstraintLayout>
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