【Kotlin 초보자】DatePicker로 일시 표시

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구현

package com.example.datapicker

import android.app.AlertDialog
import android.app.DatePickerDialog
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.widget.TextView

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

        val dataPiker = findViewById<TextView>(R.id.date_picker)
        dataPiker.setOnClickListener {
            showDataPicker()
        }

    }

    private fun showDataPicker() {

        val dataPiker = findViewById<TextView>(R.id.date_picker)
        val datePickerDialog = DatePickerDialog(this,
                AlertDialog.THEME_HOLO_DARK,
                DatePickerDialog.OnDateSetListener() { view, year, month, dayOfMonth->
                    dataPiker.text = "${year}/${month + 1}/${dayOfMonth}"
                }, 2020, 12, 1)
        datePickerDialog.show()
    }
}

<?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/date_picker"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="ここをタップして"
        android:gravity="center"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

끝에