• Create your first Android App with Kotlin

• AndroidManifest.xml
• strings.xml
• colors.xml
• MainActivity.kt

AndroidManifest.xml

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

    <TextView
            android:id="@+id/value"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="100sp"
            android:textStyle="bold"
            android:textAlignment="center"
            android:text="@string/init_value"
            android:background="@color/colorBackground"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="@id/plusBtn"
            app:layout_constraintBottom_toBottomOf="@id/minusBtn"
    />

    <Button
            android:id="@+id/plusBtn"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="50sp"
            android:text="@string/count_up"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginTop="50dp"
    />

    <Button
            android:id="@+id/minusBtn"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textSize="50sp"
            android:text="@string/count_down"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            android:layout_marginBottom="50dp"
    />

</androidx.constraintlayout.widget.ConstraintLayout>

strings.xml

<resources>
    <string name="app_name">My Application</string>
    <string name="init_value">0</string>
    <string name="count_up">นับขึ้น</string>
    <string name="count_down">นับลง</string>
</resources>

colors.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">#008577</color>
    <color name="colorPrimaryDark">#00574B</color>
    <color name="colorAccent">#D81B60</color>
    <color name="colorBackground">#ffcccc</color>
</resources>

MainActivity.kt

package com.phaisarn.myapplication

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    var id = 0

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

        value.text = id.toString()

        plusBtn.setOnClickListener {
            value.text = (++id).toString()
        }

        minusBtn.setOnClickListener {
            value.text = (--id).toString()
        }
    }
}