Project: Student Diary App (Plan और Setup)

[versions]
agp = "8.7.3"
kotlin = "2.0.21"
ksp = "2.0.21-1.0.28"
coreKtx = "1.15.0"
appcompat = "1.7.0"
material = "1.12.0"
constraintlayout = "2.2.0"
recyclerview = "1.4.0"
viewpager2 = "1.1.0"
activityKtx = "1.9.3"
fragmentKtx = "1.8.5"
lifecycle = "2.8.7"
navigation = "2.8.5"
room = "2.6.1"
work = "2.10.0"
coroutines = "1.9.0"
retrofit = "2.11.0"
glide = "4.16.0"
junit = "4.13.2"

[libraries]
androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
androidx-appcompat = { group = "androidx.appcompat", name = "appcompat", version.ref = "appcompat" }
material = { group = "com.google.android.material", name = "material", version.ref = "material" }
androidx-constraintlayout = { group = "androidx.constraintlayout", name = "constraintlayout", version.ref = "constraintlayout" }
androidx-recyclerview = { group = "androidx.recyclerview", name = "recyclerview", version.ref = "recyclerview" }
androidx-viewpager2 = { group = "androidx.viewpager2", name = "viewpager2", version.ref = "viewpager2" }
androidx-activity-ktx = { group = "androidx.activity", name = "activity-ktx", version.ref = "activityKtx" }
androidx-fragment-ktx = { group = "androidx.fragment", name = "fragment-ktx", version.ref = "fragmentKtx" }
androidx-lifecycle-viewmodel-ktx = { group = "androidx.lifecycle", name = "lifecycle-viewmodel-ktx", version.ref = "lifecycle" }
androidx-lifecycle-livedata-ktx = { group = "androidx.lifecycle", name = "lifecycle-livedata-ktx", version.ref = "lifecycle" }
androidx-lifecycle-runtime-ktx = { group = "androidx.lifecycle", name = "lifecycle-runtime-ktx", version.ref = "lifecycle" }
androidx-navigation-fragment-ktx = { group = "androidx.navigation", name = "navigation-fragment-ktx", version.ref = "navigation" }
androidx-navigation-ui-ktx = { group = "androidx.navigation", name = "navigation-ui-ktx", version.ref = "navigation" }
androidx-room-runtime = { group = "androidx.room", name = "room-runtime", version.ref = "room" }
androidx-room-ktx = { group = "androidx.room", name = "room-ktx", version.ref = "room" }
androidx-room-compiler = { group = "androidx.room", name = "room-compiler", version.ref = "room" }
androidx-work-runtime-ktx = { group = "androidx.work", name = "work-runtime-ktx", version.ref = "work" }
kotlinx-coroutines-android = { group = "org.jetbrains.kotlinx", name = "kotlinx-coroutines-android", version.ref = "coroutines" }
retrofit = { group = "com.squareup.retrofit2", name = "retrofit", version.ref = "retrofit" }
retrofit-converter-gson = { group = "com.squareup.retrofit2", name = "converter-gson", version.ref = "retrofit" }
glide = { group = "com.github.bumptech.glide", name = "glide", version.ref = "glide" }
junit = { group = "junit", name = "junit", version.ref = "junit" }

[plugins]
android-application = { id = "com.android.application", version.ref = "agp" }
kotlin-android = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
ksp = { id = "com.google.devtools.ksp", version.ref = "ksp" }

plugins {
    alias(libs.plugins.android.application)
    alias(libs.plugins.kotlin.android)
    alias(libs.plugins.ksp)            // generates Room code
}

android {
    namespace = "com.codingeasily.studentdiary"
    compileSdk = 35

    defaultConfig {
        applicationId = "com.codingeasily.studentdiary"
        minSdk = 26
        targetSdk = 35
        versionCode = 1
        versionName = "1.0"
        testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            isMinifyEnabled = true
            isShrinkResources = true
            proguardFiles(getDefaultProguardFile("proguard-android-optimize.txt"), "proguard-rules.pro")
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_17
        targetCompatibility = JavaVersion.VERSION_17
    }
    kotlinOptions {
        jvmTarget = "17"
    }
    buildFeatures {
        viewBinding = true             // generates ActivityMainBinding, FragmentNotesBinding, ...
    }
}

dependencies {
    implementation(libs.androidx.core.ktx)
    implementation(libs.androidx.appcompat)
    implementation(libs.material)
    implementation(libs.androidx.constraintlayout)
    implementation(libs.androidx.recyclerview)
    implementation(libs.androidx.viewpager2)
    implementation(libs.androidx.activity.ktx)
    implementation(libs.androidx.fragment.ktx)
    implementation(libs.androidx.lifecycle.viewmodel.ktx)
    implementation(libs.androidx.lifecycle.livedata.ktx)
    implementation(libs.androidx.lifecycle.runtime.ktx)
    implementation(libs.androidx.navigation.fragment.ktx)
    implementation(libs.androidx.navigation.ui.ktx)
    implementation(libs.androidx.room.runtime)
    implementation(libs.androidx.room.ktx)
    ksp(libs.androidx.room.compiler)
    implementation(libs.androidx.work.runtime.ktx)
    implementation(libs.kotlinx.coroutines.android)
    implementation(libs.retrofit)
    implementation(libs.retrofit.converter.gson)
    implementation(libs.glide)
    testImplementation(libs.junit)
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android">

    <!-- Needed for the Help screen (WebView) and the "Tip of the day" (Retrofit) -->
    <uses-permission android:name="android.permission.INTERNET" />
    <!-- Android 13+ asks the user before showing notifications -->
    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />

    <application
        android:name=".StudentDiaryApp"
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/Theme.StudentDiary">

        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:windowSoftInputMode="adjustResize">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

package com.codingeasily.studentdiary

import android.app.Application
import com.codingeasily.studentdiary.data.SettingsStore
import com.codingeasily.studentdiary.reminder.NotificationHelper

/**
 * Runs once when the app process starts (declared in AndroidManifest.xml).
 * We apply the saved light/dark theme and create the notification channel here.
 */
class StudentDiaryApp : Application() {
    override fun onCreate() {
        super.onCreate()
        SettingsStore(this).applyTheme()
        NotificationHelper.createChannel(this)
    }
}

package com.codingeasily.studentdiary.data

import androidx.room.Entity
import androidx.room.PrimaryKey

/** One row of the "notes" table. */
@Entity(tableName = "notes")
data class Note(
    @PrimaryKey(autoGenerate = true) val id: Long = 0,
    val title: String,
    val content: String = "",
    val subject: String = "General",
    val priority: Int = PRIORITY_MEDIUM,      // 0 = low, 1 = medium, 2 = high
    val dueAt: Long? = null,                  // reminder time in millis, null = no reminder
    val isDone: Boolean = false,
    val isFavorite: Boolean = false,
    val imagePath: String? = null,            // photo copied into app storage
    val createdAt: Long = System.currentTimeMillis()
) {
    companion object {
        const val PRIORITY_LOW = 0
        const val PRIORITY_MEDIUM = 1
        const val PRIORITY_HIGH = 2
    }
}

/** Result of a GROUP BY query: how many notes each subject has. */
data class SubjectCount(val subject: String, val total: Int)

package com.codingeasily.studentdiary.data

import androidx.room.Dao
import androidx.room.Delete
import androidx.room.Insert
import androidx.room.Query
import androidx.room.Update
import kotlinx.coroutines.flow.Flow

/** All SQL for the notes table lives here. Room generates the implementation. */
@Dao
interface NoteDao {

    // Flow = the UI is updated automatically whenever the table changes
    @Query("SELECT * FROM notes ORDER BY isDone ASC, priority DESC, createdAt DESC")
    fun getAll(): Flow<List<Note>>

    @Query("SELECT * FROM notes WHERE isDone = :done ORDER BY priority DESC, createdAt DESC")
    fun getByDone(done: Boolean): Flow<List<Note>>

    @Query("SELECT * FROM notes WHERE isFavorite = 1 ORDER BY createdAt DESC")
    fun getFavorites(): Flow<List<Note>>

    @Query("SELECT * FROM notes WHERE subject = :subject ORDER BY createdAt DESC")
    fun getBySubject(subject: String): Flow<List<Note>>

    @Query("SELECT * FROM notes WHERE title LIKE '%' || :q || '%' OR content LIKE '%' || :q || '%' ORDER BY createdAt DESC")
    fun search(q: String): Flow<List<Note>>

    @Query("SELECT subject, COUNT(*) AS total FROM notes GROUP BY subject ORDER BY total DESC")
    fun subjectCounts(): Flow<List<SubjectCount>>

    @Query("SELECT COUNT(*) FROM notes")
    fun countAll(): Flow<Int>

    @Query("SELECT COUNT(*) FROM notes WHERE isDone = 1")
    fun countDone(): Flow<Int>

    @Query("SELECT * FROM notes WHERE id = :id")
    suspend fun getById(id: Long): Note?

    @Insert
    suspend fun insert(note: Note): Long

    @Update
    suspend fun update(note: Note)

    @Delete
    suspend fun delete(note: Note)

    @Query("DELETE FROM notes WHERE isDone = 1")
    suspend fun deleteDone(): Int
}

package com.codingeasily.studentdiary.data

import android.content.Context
import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase

@Database(entities = [Note::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {

    abstract fun noteDao(): NoteDao

    companion object {
        @Volatile private var INSTANCE: AppDatabase? = null

        /** Singleton: the whole app shares one database connection. */
        fun get(context: Context): AppDatabase =
            INSTANCE ?: synchronized(this) {
                INSTANCE ?: Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "student_diary.db"
                ).build().also { INSTANCE = it }
            }
    }
}

package com.codingeasily.studentdiary.data

import kotlinx.coroutines.flow.Flow

/**
 * The single source of truth for notes. ViewModels talk to the repository,
 * never directly to the DAO. Later you could add a network source here.
 */
class NoteRepository(private val dao: NoteDao) {
    val allNotes: Flow<List<Note>> = dao.getAll()
    val pendingNotes: Flow<List<Note>> = dao.getByDone(false)
    val doneNotes: Flow<List<Note>> = dao.getByDone(true)
    val favorites: Flow<List<Note>> = dao.getFavorites()
    val subjectCounts: Flow<List<SubjectCount>> = dao.subjectCounts()
    val countAll: Flow<Int> = dao.countAll()
    val countDone: Flow<Int> = dao.countDone()

    fun bySubject(subject: String) = dao.getBySubject(subject)
    fun search(q: String) = dao.search(q)

    suspend fun get(id: Long) = dao.getById(id)
    suspend fun save(note: Note): Long =
        if (note.id == 0L) dao.insert(note) else { dao.update(note); note.id }
    suspend fun delete(note: Note) = dao.delete(note)
    suspend fun deleteDone() = dao.deleteDone()
}

package com.codingeasily.studentdiary.util

import java.text.SimpleDateFormat
import java.util.Date
import java.util.Locale

object DateUtils {
    private val full = SimpleDateFormat("dd MMM yyyy, hh:mm a", Locale.getDefault())
    private val short = SimpleDateFormat("dd MMM", Locale.getDefault())

    fun format(millis: Long): String = full.format(Date(millis))
    fun formatShort(millis: Long): String = short.format(Date(millis))
}

package com.codingeasily.studentdiary.ui

import android.app.Application
import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.asLiveData
import androidx.lifecycle.viewModelScope
import com.codingeasily.studentdiary.data.AppDatabase
import com.codingeasily.studentdiary.data.Note
import com.codingeasily.studentdiary.data.NoteRepository
import com.codingeasily.studentdiary.data.SubjectCount
import com.codingeasily.studentdiary.network.Tip
import com.codingeasily.studentdiary.network.TipsApi
import com.codingeasily.studentdiary.reminder.ReminderScheduler
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.flatMapLatest
import kotlinx.coroutines.flow.map
import kotlinx.coroutines.launch

/** Which notes a list screen shows. */
enum class NoteFilter { ALL, PENDING, DONE, FAVORITES, SUBJECT }

/** State of the "Tip of the day" card. */
sealed class TipState {
    data object Loading : TipState()
    data class Success(val tip: Tip) : TipState()
    data class Error(val message: String) : TipState()
}

/**
 * Shared by all screens (activityViewModels). Survives screen rotation.
 * The UI observes LiveData; all database work runs in viewModelScope.
 */
class NoteViewModel(application: Application) : AndroidViewModel(application) {

    private val repo = NoteRepository(AppDatabase.get(application).noteDao())

    // ---------- lists ----------
    private val searchQuery = MutableStateFlow("")

    fun setSearchQuery(q: String) { searchQuery.value = q.trim() }

    @OptIn(ExperimentalCoroutinesApi::class)
    fun notes(filter: NoteFilter, subject: String? = null): LiveData<List<Note>> {
        val source: Flow<List<Note>> = when (filter) {
            NoteFilter.ALL -> repo.allNotes
            NoteFilter.PENDING -> repo.pendingNotes
            NoteFilter.DONE -> repo.doneNotes
            NoteFilter.FAVORITES -> repo.favorites
            NoteFilter.SUBJECT -> repo.bySubject(subject ?: "")
        }
        return searchQuery.flatMapLatest { q ->
            if (q.isEmpty()) source
            else source.map { list ->
                list.filter { it.title.contains(q, ignoreCase = true) || it.content.contains(q, ignoreCase = true) }
            }
        }.asLiveData()
    }

    val subjectCounts: LiveData<List<SubjectCount>> = repo.subjectCounts.asLiveData()
    val countAll: LiveData<Int> = repo.countAll.asLiveData()
    val countDone: LiveData<Int> = repo.countDone.asLiveData()

    // ---------- single note ----------
    suspend fun getNote(id: Long): Note? = repo.get(id)

    fun save(note: Note, onSaved: (Long) -> Unit) {
        viewModelScope.launch {
            val id = repo.save(note)
            val due = note.dueAt
            if (due != null && !note.isDone) ReminderScheduler.schedule(getApplication(), id, due)
            else ReminderScheduler.cancel(getApplication(), id)
            onSaved(id)
        }
    }

    fun toggleDone(note: Note) = viewModelScope.launch {
        repo.save(note.copy(isDone = !note.isDone))
        if (!note.isDone) ReminderScheduler.cancel(getApplication(), note.id)
    }

    fun toggleFavorite(note: Note) = viewModelScope.launch {
        repo.save(note.copy(isFavorite = !note.isFavorite))
    }

    fun delete(note: Note) = viewModelScope.launch {
        repo.delete(note)
        ReminderScheduler.cancel(getApplication(), note.id)
    }

    /** Used by the "Undo" button of the Snackbar. */
    fun restore(note: Note) = viewModelScope.launch {
        repo.save(note.copy(id = 0))
    }

    fun deleteDone(onDone: (Int) -> Unit) = viewModelScope.launch {
        onDone(repo.deleteDone())
    }

    // ---------- tip of the day (Retrofit) ----------
    private val _tip = MutableLiveData<TipState>()
    val tip: LiveData<TipState> = _tip

    fun loadTip(force: Boolean = false) {
        if (!force && _tip.value is TipState.Success) return
        _tip.value = TipState.Loading
        viewModelScope.launch {
            _tip.value = try {
                val tips = TipsApi.service.getTips().tips
                if (tips.isEmpty()) TipState.Error("No tips yet")
                else TipState.Success(tips[(System.currentTimeMillis() / 86_400_000L % tips.size).toInt()])
            } catch (e: Exception) {
                TipState.Error(e.message ?: "Network error")
            }
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipToPadding="false"
        android:paddingTop="8dp"
        android:paddingBottom="88dp"
        tools:listitem="@layout/item_note" />

    <LinearLayout
        android:id="@+id/emptyView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:gravity="center"
        android:orientation="vertical"
        android:visibility="gone">

        <ImageView
            android:layout_width="96dp"
            android:layout_height="96dp"
            android:alpha="0.4"
            android:contentDescription="@null"
            android:src="@drawable/ic_note" />

        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="12dp"
            android:text="@string/empty_notes"
            android:textAppearance="?attr/textAppearanceBodyLarge" />
    </LinearLayout>
</FrameLayout>

<?xml version="1.0" encoding="utf-8"?>
<com.google.android.material.card.MaterialCardView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginHorizontal="12dp"
    android:layout_marginVertical="6dp"
    app:cardCornerRadius="14dp"
    app:cardElevation="2dp">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:minHeight="76dp">

        <!-- coloured strip shows priority -->
        <View
            android:id="@+id/priorityStrip"
            android:layout_width="6dp"
            android:layout_height="0dp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <com.google.android.material.checkbox.MaterialCheckBox
            android:id="@+id/cbDone"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="8dp"
            android:contentDescription="@string/mark_done"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintStart_toEndOf="@id/priorityStrip"
            app:layout_constraintTop_toTopOf="parent" />

        <TextView
            android:id="@+id/tvTitle"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginTop="12dp"
            android:ellipsize="end"
            android:maxLines="1"
            android:textAppearance="?attr/textAppearanceTitleMedium"
            app:layout_constraintEnd_toStartOf="@id/ivThumb"
            app:layout_constraintStart_toEndOf="@id/cbDone"
            app:layout_constraintTop_toTopOf="parent" />

        <TextView
            android:id="@+id/tvContent"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:ellipsize="end"
            android:maxLines="2"
            android:textAppearance="?attr/textAppearanceBodyMedium"
            app:layout_constraintEnd_toEndOf="@id/tvTitle"
            app:layout_constraintStart_toStartOf="@id/tvTitle"
            app:layout_constraintTop_toBottomOf="@id/tvTitle" />

        <TextView
            android:id="@+id/tvSubject"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="6dp"
            android:layout_marginBottom="12dp"
            android:background="@drawable/bg_chip"
            android:paddingHorizontal="10dp"
            android:paddingVertical="2dp"
            android:textAppearance="?attr/textAppearanceLabelMedium"
            android:textColor="@color/brand"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintStart_toStartOf="@id/tvTitle"
            app:layout_constraintTop_toBottomOf="@id/tvContent" />

        <TextView
            android:id="@+id/tvDue"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginStart="8dp"
            android:drawablePadding="4dp"
            android:gravity="center_vertical"
            android:textAppearance="?attr/textAppearanceLabelMedium"
            app:drawableStartCompat="@drawable/ic_alarm"
            app:layout_constraintBaseline_toBaselineOf="@id/tvSubject"
            app:layout_constraintStart_toEndOf="@id/tvSubject" />

        <ImageView
            android:id="@+id/ivThumb"
            android:layout_width="52dp"
            android:layout_height="52dp"
            android:layout_marginEnd="4dp"
            android:contentDescription="@string/photo"
            android:scaleType="centerCrop"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toStartOf="@id/btnFavorite"
            app:layout_constraintTop_toTopOf="parent" />

        <ImageButton
            android:id="@+id/btnFavorite"
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:background="?attr/selectableItemBackgroundBorderless"
            android:contentDescription="@string/favourite"
            android:src="@drawable/ic_star_outline"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:tint="@color/star" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</com.google.android.material.card.MaterialCardView>

package com.codingeasily.studentdiary.ui.notes

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.core.os.bundleOf
import androidx.core.view.isVisible
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.navigation.fragment.findNavController
import androidx.recyclerview.widget.ItemTouchHelper
import androidx.recyclerview.widget.LinearLayoutManager
import androidx.recyclerview.widget.RecyclerView
import com.codingeasily.studentdiary.R
import com.codingeasily.studentdiary.databinding.FragmentNoteListBinding
import com.codingeasily.studentdiary.ui.NoteFilter
import com.codingeasily.studentdiary.ui.NoteViewModel
import com.google.android.material.snackbar.Snackbar

/**
 * A reusable list screen. It is used:
 *  - inside the 3 tabs of NotesFragment (ALL / PENDING / DONE)
 *  - as the Favourites screen (drawer)
 *  - as the notes of one subject (from SubjectsFragment)
 */
class NoteListFragment : Fragment() {

    private var _binding: FragmentNoteListBinding? = null
    private val binding get() = _binding!!          // valid only between onCreateView and onDestroyView

    private val viewModel: NoteViewModel by activityViewModels()

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
        _binding = FragmentNoteListBinding.inflate(inflater, container, false)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val filter = NoteFilter.valueOf(requireArguments().getString(ARG_FILTER, NoteFilter.ALL.name))
        val subject = arguments?.getString(ARG_SUBJECT)

        val adapter = NoteAdapter(
            onClick = { note ->
                findNavController().navigate(R.id.action_global_editor, bundleOf("noteId" to note.id))
            },
            onToggleDone = { viewModel.toggleDone(it) },
            onToggleFavorite = { viewModel.toggleFavorite(it) }
        )
        binding.recyclerView.layoutManager = LinearLayoutManager(requireContext())
        binding.recyclerView.adapter = adapter

        viewModel.notes(filter, subject).observe(viewLifecycleOwner) { list ->
            adapter.submitList(list)
            binding.emptyView.isVisible = list.isEmpty()
        }

        // Swipe left or right to delete, with UNDO
        val swipe = object : ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT or ItemTouchHelper.RIGHT) {
            override fun onMove(rv: RecyclerView, vh: RecyclerView.ViewHolder, target: RecyclerView.ViewHolder) = false
            override fun onSwiped(vh: RecyclerView.ViewHolder, direction: Int) {
                val position = vh.bindingAdapterPosition
                if (position == RecyclerView.NO_POSITION) return
                val note = adapter.currentList[position]
                viewModel.delete(note)
                Snackbar.make(binding.root, getString(R.string.note_deleted, note.title), Snackbar.LENGTH_LONG)
                    .setAction(R.string.undo) { viewModel.restore(note) }
                    .show()
            }
        }
        ItemTouchHelper(swipe).attachToRecyclerView(binding.recyclerView)
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null       // avoid memory leaks: the view is gone, the fragment may live on
    }

    companion object {
        const val ARG_FILTER = "filter"
        const val ARG_SUBJECT = "subject"

        fun newInstance(filter: NoteFilter) = NoteListFragment().apply {
            arguments = bundleOf(ARG_FILTER to filter.name)
        }
    }
}