RecyclerViewとCoroutineで非同期フォトギャラリーを作る

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

plugins {
    ...
    id 'kotlin-kapt'
}

android {
    ...
    dataBinding {
        enabled = true
    }
}

dependencies {
    ...
    implementation 'androidx.activity:activity-ktx:1.2.2'
    implementation 'androidx.fragment:fragment-ktx:1.3.3'

    def koin_version = "2.2.2"
    implementation "org.koin:koin-android:$koin_version"
    implementation "org.koin:koin-android-scope:$koin_version"
    implementation "org.koin:koin-android-viewmodel:$koin_version"

    implementation "com.squareup.picasso:picasso:2.8"
}

data class PhotoGalleryItem(
    val uri: Uri /* 現状はURIのみ保持する */
) {
    companion object {
        /* DiffUtilの定義 */
        val DIFF_UTIL = object: DiffUtil.ItemCallback<PhotoGalleryItem>() {
            override fun areItemsTheSame(oldItem: PhotoGalleryItem, newItem: PhotoGalleryItem)
                    : Boolean {
                return oldItem.uri == newItem.uri
            }

            override fun areContentsTheSame(oldItem: PhotoGalleryItem, newItem: PhotoGalleryItem)
                    : Boolean {
                return oldItem == newItem
            }
        }

    }
}

class PhotoGalleryViewModel(app: Application) : AndroidViewModel(app) {
    /* 写真のリスト */
    val photoList = MutableLiveData<List<PhotoGalleryItem>>()

    /* 写真のリストを読み込む */
    fun loadPhotoList() {
        /* IOスレッドでMediaStoreから写真を検索する */
        viewModelScope.launch(Dispatchers.IO) {
            val list = mutableListOf<PhotoGalleryItem>()

            /* 読み込む列の指定 */
            val projection = arrayOf(
                MediaStore.Images.Media._ID, /* ID : URI取得に必要 */
                MediaStore.Images.Media.DATE_ADDED  /* 撮影日時 */
            )
            val selection = null /* 行の絞り込みの指定。nullならすべての行を読み込む。*/
            val selectionArgs = null /* selectionの?を置き換える引数 */
            /* 並び順の指定 : 撮影日時の新しい順 */
            val sortOrder = "${MediaStore.Images.Media.DATE_ADDED} DESC"

            getApplication<Application>().contentResolver.query(
                MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
                projection, selection, selectionArgs, sortOrder
            )?.use { cursor -> /* cursorは、検索結果の各行の情報にアクセスするためのオブジェクト。*/
                /* 必要な情報が格納されている列番号を取得する。 */
                val idColumn = cursor.getColumnIndexOrThrow(MediaStore.Images.Media._ID)
                while (cursor.moveToNext()) { /* 順にカーソルを動かしながら、情報を取得していく。*/
                    val id = cursor.getLong(idColumn)
                    /* IDからURIを取得してリストに格納 */
                    val uri = ContentUris.withAppendedId(
                        MediaStore.Images.Media.EXTERNAL_CONTENT_URI, id)
                    list.add(PhotoGalleryItem(uri))
                }
            }

            /* メインスレッドでphotoListにセットする */
            photoList.postValue(list)
        }
    }

    /* 指定したインデックスのPhotoGalleryItemを取得 */
    fun getPhotoItem(index: Int) = photoList.value?.getOrNull(index)
}

@JvmField
val appModule = module {
    viewModel {
        PhotoGalleryViewModel(androidApplication())
    }
}

class MainApplication: Application() {
    override fun onCreate() {
        super.onCreate()

        startKoin {
            androidContext(this@MainApplication)
            modules(appModule)
        }
    }
}

<application
    android:name=".MainApplication"
    ... >
</application>

class PhotoGalleryActivity : AppCompatActivity() {

    private val viewModel: PhotoGalleryViewModel by viewModel()

}

class PhotoGalleryFragment : Fragment() {

    private val viewModel: PhotoGalleryViewModel by sharedViewModel()

}

    /* Permission承認されたかどうか */
    val isPermissionGranted = MutableLiveData<Boolean>().apply { value = false }
    /* Permission拒否されたかどうか */
    val isPermissionDenied = MutableLiveData<Boolean>().apply { value = false }

companion object {
    private const val REQ_PERMISSION = Manifest.permission.READ_EXTERNAL_STORAGE
}

/* Permissionの結果を受け取る */
private val permissionRequest = registerForActivityResult(
        ActivityResultContracts.RequestPermission()
) { granted ->
    if (granted) {
        /* 承認された。*/
        viewModel.isPermissionGranted.value = true
    } else {
        /* 拒否された */
        if (shouldShowRequestPermissionRationale(REQ_PERMISSION)) {
            /* Permissionの必要性を説明するダイアログを表示する */
            showRationaleDialog()
        } else {
            /* 拒否（ファイナルアンサー）*/
            viewModel.isPermissionDenied.value = true
        }
    }
}

/* Permissionの必要性を説明するダイアログを表示する */
private fun showRationaleDialog() {
    RationaleDialog().show(childFragmentManager, viewLifecycleOwner) {
        if (it == RationaleDialog.RESULT_OK) {
            /* Permissionを要求 */
            permissionRequest.launch(REQ_PERMISSION)
        } else {
            /* あきらめる */
            viewModel.isPermissionDenied.value = true
        }
    }
}

class RationaleDialog: DialogFragment() {
    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        return activity?.let {
            AlertDialog.Builder(it)
                    .setMessage("デバイス内の写真を表示するには、アクセスを許可してください。")
                    .setPositiveButton("OK") { _, _ ->
                        setFragmentResult(REQUEST_KEY, bundleOf(Pair(RESULT_KEY, RESULT_OK)))
                    }
                    .setNegativeButton("Cancel") { _, _ ->
                        setFragmentResult(REQUEST_KEY, bundleOf(Pair(RESULT_KEY, RESULT_CANCEL)))
                    }
                    .create()
        } ?: throw IllegalStateException("Activity cannot be null")
    }

    fun show(manager: FragmentManager, lifecycleOwner: LifecycleOwner, callback: (Int)->Unit) {
        val listener = FragmentResultListener { _, result ->
            val resultValue = result.getInt(RESULT_KEY)
            callback(resultValue)
        }
        manager.setFragmentResultListener(REQUEST_KEY, lifecycleOwner, listener)
        show(manager, TAG)
    }

    companion object {
        private const val TAG = "TAG_RATIONALE_DIALOG"
        private const val REQUEST_KEY = "RATIONALE_DIALOG_REQUEST_KEY"
        private const val RESULT_KEY = "RATIONALE_DIALOG_RESULT_KEY"
        const val RESULT_OK = 1
        const val RESULT_CANCEL = -1
    }
}

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    context?.let {
        /* 必要なPermissionが与えられているかどうかを確認 */
        val result = PermissionChecker.checkSelfPermission(it, REQ_PERMISSION)
        if (result == PermissionChecker.PERMISSION_GRANTED) {
            /* 承認済み */
            viewModel.isPermissionGranted.value = true
        } else {
            /* 要求する */
            permissionRequest.launch(REQ_PERMISSION)
        }
    }
}

companion object {
    private const val REQ_PERMISSION = Manifest.permission.READ_EXTERNAL_STORAGE
}

override fun onResume() {
    super.onResume()
    /* Permission承認済みなら、デバイスの写真にアクセスする */
    if (viewModel.isPermissionGranted.value == true){
        viewModel.loadPhotoList()
    }
}

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <import type="android.view.View" />
        <variable
            name="viewModel"
            type="net.engawapg.app.photogallery.gallery.PhotoGalleryViewModel" />
    </data>

    <androidx.constraintlayout.widget.ConstraintLayout
        ... >

        <TextView
            android:visibility="@{viewModel.isPermissionDenied ? View.VISIBLE : View.INVISIBLE}"
            android:id="@+id/permissionExplanation"
            ...
            android:text="デバイス内の写真を表示するには、アプリの設定画面でアクセスを許可してください。"
            ... />

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/recyclerView"
            ...
            android:visibility="@{viewModel.isPermissionGranted ? View.VISIBLE : View.INVISIBLE}"
            ... />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

private var _binding: FragmentPhotoGalleryBinding? = null
private val binding get() = binding!!

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View {
    _binding = DataBindingUtil.inflate(
        inflater, R.layout.fragment_photo_gallery, container, false)
    binding.viewModel = viewModel
    binding.lifecycleOwner = viewLifecycleOwner
    return binding.root
}

override fun onDestroyView() {
    super.onDestroyView()
    _binding = null
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    /* RecyclerViewの設定 */
    val imageAdapter = ImageAdapter()
    /* リストは後から更新する */
    viewModel.photoList.observe(viewLifecycleOwner, Observer {
        imageAdapter.submitList(it)
    })
    binding.recyclerView.apply {
        setHasFixedSize(true)
        layoutManager = GridLayoutManager(this@PhotoGalleryFragment.context, SPAN_COUNT)
        adapter = imageAdapter
    }
}

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <variable
            name="viewModel"
            type="net.engawapg.app.photogallery.gallery.PhotoGalleryViewModel" />
        <variable
            name="item"
            type="net.engawapg.app.photogallery.gallery.PhotoGalleryItem" />
    </data>

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

        <ImageView
            android:id="@+id/imageView"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:contentDescription="Image in the device"
            app:layout_constraintDimensionRatio="1"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:onClick="@{() -> viewModel.onClick(item)}"
            tools:ignore="HardcodedText" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</layout>

inner class ImageAdapter
    : ListAdapter<PhotoGalleryItem, ImageViewHolder>(PhotoGalleryItem.DIFF_UTIL) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ImageViewHolder {
        val layoutInflater = LayoutInflater.from(parent.context)
        val binding = DataBindingUtil.inflate<ViewPhotoGalleryImageBinding>(
                layoutInflater, R.layout.view_photo_gallery_image, parent, false)
        return ImageViewHolder(binding)
    }

    override fun onBindViewHolder(holder: ImageViewHolder, position: Int) {
        val item = viewModel.getPhotoItem(position)

        holder.binding.viewModel = viewModel
        holder.binding.item = item
        holder.binding.executePendingBindings()

        item?.let {
            /* Picassoライブラリを使って非同期に画像を読み込む */
            Picasso.get().load(it.uri)
                .fit()
                .centerCrop()
                .into(holder.binding.imageView)
        }
    }
}

inner class ImageViewHolder(val binding: ViewPhotoGalleryImageBinding)
    : RecyclerView.ViewHolder(binding.root)

val onSelect = MutableLiveData<Event<Uri>>()

fun onClick(item: PhotoGalleryItem) {
    /* URIをイベントとして渡す */
    onSelect.value = Event(item.uri)
}

class PhotoGalleryActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        ...
        /* ViewModelでURIが選択されたイベントを受信し、Activityの結果としてセットする */
        viewModel.onSelect.observe(this, EventObserver {
            setResult(RESULT_OK, Intent().putExtra(INTENT_URI, it.toString()))
            finish()
        })
    }

    companion object {
        const val INTENT_URI = "Uri"
    }
}

class PhotoGalleryActivity : AppCompatActivity() {
    /* ActivityResultContractのカスタマイズ。このActivityのResultとしてURIを返す */
    class ResultContract: ActivityResultContract<Unit, Uri?>() {
        override fun createIntent(context: Context, input: Unit?) =
            Intent(context, PhotoGalleryActivity::class.java)

        override fun parseResult(resultCode: Int, intent: Intent?): Uri? {
            return if (resultCode == RESULT_OK) {
                intent?.getStringExtra(INTENT_URI)?.let {
                    Uri.parse(it)
                }
            } else {
                null
            }
        }
    }
}

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    /* PhotoGalleryActivityの結果をURIで受け取る */
    private val launcher = registerForActivityResult(PhotoGalleryActivity.ResultContract()) {
        binding.imageView.setImageURI(it)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = DataBindingUtil.setContentView(this, R.layout.activity_main)

        binding.button.setOnClickListener {
            launcher.launch()
        }
    }
}