하고 싶은 일

화면 구현

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView1"
        android:layout_width="268dp"
        android:layout_height="94dp"
        android:text="Hello World!"
        android:textSize="36sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/btn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="32dp"
        android:text="Button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

메인 프로세싱 구현

package com.example.yamato200605b

import android.graphics.Color
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.os.Handler
import android.view.View
import android.widget.Button
import android.widget.Toast
import androidx.appcompat.app.AlertDialog
import kotlinx.android.synthetic.main.activity_main.*

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

        val getButton = findViewById(R.id.btn) as Button
        getButton.setOnClickListener(object : View.OnClickListener {
            override
            fun onClick(view: View) {
                AlertDialog.Builder(this@MainActivity)
                    .setTitle("文字色の変更")
                    .setMessage("テキストの色を赤に変更しますか？")
                    .setPositiveButton(
                        "赤にする", { dialog, which ->
                            textView1.setTextColor(Color.RED)
                        }
                    )
                    .setNegativeButton(
                        "キャンセル", { dialog, which ->
                            textView1.setTextColor(Color.BLUE)
                        }
                    )
                    .show()

            }
        })
    }
}