Fragmentの値の保存と復元

fragment_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"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/frameLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainFragment">

    <Button
        android:id="@+id/button1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:text="Button"
        app:layout_constraintEnd_toStartOf="@+id/button2"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView2" />

    <Button
        android:id="@+id/button2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginEnd="8dp"
        android:text="Button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toEndOf="@+id/button1"
        app:layout_constraintTop_toTopOf="@+id/button1" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="TextView"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView1" />
</androidx.constraintlayout.widget.ConstraintLayout>

MainFragment.kt

package com.example.myapplication

import android.os.Bundle
import android.util.Log
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.Button
import android.widget.TextView
import androidx.annotation.VisibleForTesting

class MainFragment : Fragment() {
    @VisibleForTesting
    var value1: Int = DEFAULT_VALUE1

    @VisibleForTesting
    var value2: String = DEFAULT_VALUE2

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_main, container, false)
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putInt("value1", value1)
        outState.putString("value2", value2)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        value1 = savedInstanceState?.getInt("value1") ?: DEFAULT_VALUE1
        value2 = savedInstanceState?.getString("value2") ?: DEFAULT_VALUE2

        view.findViewById<Button>(R.id.button1).setOnClickListener {
            value1 = 0
            value2 = "foo"
            Log.d("MainFragment", "button1 clicked")
            updateViews()
        }

        view.findViewById<Button>(R.id.button2).setOnClickListener {
            value1 = 1
            value2 = "bar"
            Log.d("MainFragment", "button2 clicked")
            updateViews()
        }
    }

    override fun onResume() {
        super.onResume()
        Log.d("MainFragment", "onResume")
        updateViews()
    }

    private fun updateViews() {
        Log.d("MainFragment", "updateViews: $value1, $value2")
        view?.findViewById<TextView>(R.id.textView1)?.text = "$value1"
        view?.findViewById<TextView>(R.id.textView2)?.text = value2
    }

    companion object {
        const val DEFAULT_VALUE1 = 0
        const val DEFAULT_VALUE2 = "foo"
    }
}

MainFragmentTest.kt

package com.example.myapplication

import androidx.fragment.app.testing.launchFragmentInContainer
import androidx.test.espresso.Espresso.onView
import androidx.test.espresso.action.ViewActions.click
import androidx.test.espresso.matcher.ViewMatchers.withId
import androidx.test.ext.junit.runners.AndroidJUnit4
import org.junit.Assert.*
import org.junit.Test
import org.junit.runner.RunWith

@RunWith(AndroidJUnit4::class)
class MainFragmentTest {
    @Test
    fun `初期状態のテキスト表示`() {
        val scenario = launchFragmentInContainer<MainFragment>()
        scenario.onFragment() {
            assertEquals(0, it.value1)
            assertEquals("foo", it.value2)
        }
    }

    @Test
    fun `ボタン2をクリックしたときのテキスト表示`() {
        val scenario = launchFragmentInContainer<MainFragment>()

        onView(withId(R.id.button2)).perform(click())
        scenario.onFragment() {
            assertEquals(1, it.value1)
            assertEquals("bar", it.value2)
        }
    }

    @Test
    fun `ボタン2をクリックしたあとでフラグメントが再生成`() {
        val scenario = launchFragmentInContainer<MainFragment>()

        onView(withId(R.id.button2)).perform(click())
        scenario.recreate()

        scenario.onFragment() {
            assertEquals(1, it.value1)
            assertEquals("bar", it.value2)
        }
    }
}