let the user allow downloads on mobile data

Downloads asked the platform for an unmetered network and nothing more, so
there was no way to run them on mobile. A setting under Downloads now decides,
defaulting to Wi-Fi only, and it also governs roaming: the platform permits
roaming by default, which would have contradicted the Wi-Fi-only choice.

The flag rides the enqueue request like the download quality does, cached in the
source repository so the poller does not read the preference per row.

app/src/main/kotlin/com/subtracks/data/download/DownloadEngine.kt

@@ -4,6 +4,7 @@ data class EngineRequest(
     val uri: String,
     val path: String,
     val title: String,
+    val allowMetered: Boolean = false,
 )
 
 enum class EngineStatus { Pending, Running, Completed, Failed }

app/src/main/kotlin/com/subtracks/data/download/SystemDownloadEngine.kt

@@ -15,8 +15,10 @@ class SystemDownloadEngine(
                 .setTitle(request.title)
                 .setDestinationInExternalFilesDir(context, Environment.DIRECTORY_MUSIC, request.path)
                 .setNotificationVisibility(DownloadManager.Request.VISIBILITY_HIDDEN)
-                // Downloads wait for an unmetered network until the preference for them exists.
+                // Wi-Fi only unless the preference allows a metered network, and roaming counts as
-                .setAllowedOverMetered(false)
+                // one of those; the platform otherwise permits it by default.
+                .setAllowedOverMetered(request.allowMetered)
+                .setAllowedOverRoaming(request.allowMetered)
         return manager().enqueue(download)
     }
 

app/src/main/kotlin/com/subtracks/data/prefs/UserPreferences.kt

@@ -3,6 +3,7 @@ package com.subtracks.data.prefs
 import android.content.Context
 import androidx.datastore.core.DataStore
 import androidx.datastore.preferences.core.Preferences
+import androidx.datastore.preferences.core.booleanPreferencesKey
 import androidx.datastore.preferences.core.edit
 import androidx.datastore.preferences.core.intPreferencesKey
 import androidx.datastore.preferences.core.stringPreferencesKey
@@ -88,6 +89,14 @@ class UserPreferences(
 
     private val downloadQualityKey = stringPreferencesKey("download_quality")
 
+    fun downloadOverMetered(): Flow<Boolean> = store.data.map { prefs -> prefs[downloadOverMeteredKey] ?: false }
+
+    suspend fun setDownloadOverMetered(allowed: Boolean) {
+        store.edit { prefs -> prefs[downloadOverMeteredKey] = allowed }
+    }
+
+    private val downloadOverMeteredKey = booleanPreferencesKey("download_over_metered")
+
     fun syncConcurrency(): Flow<Int> = store.data.map { prefs -> (prefs[syncConcurrencyKey] ?: DEFAULT_FETCH_CONCURRENCY).coerceAtLeast(1) }
 
     suspend fun setSyncConcurrency(value: Int) {

app/src/main/kotlin/com/subtracks/data/repo/DownloadRepository.kt

@@ -163,7 +163,12 @@ class DownloadRepository(
                 val engineId =
                     try {
                         engine.enqueue(
-                            EngineRequest(uri = url, path = enginePath(row.sourceId, row.songId), title = title),
+                            EngineRequest(
+                                uri = url,
+                                path = enginePath(row.sourceId, row.songId),
+                                title = title,
+                                allowMetered = sourceRepository.downloadsAllowedOverMetered(),
+                            ),
                         )
                     } catch (e: CancellationException) {
                         throw e

app/src/main/kotlin/com/subtracks/data/repo/SourceRepository.kt

@@ -52,10 +52,18 @@ class SourceRepository(
     @Volatile
     private var downloadQuality = StreamQuality()
 
+    @Volatile
+    private var allowMeteredDownloads = false
+
+    fun downloadsAllowedOverMetered(): Boolean = allowMeteredDownloads
+
     init {
         scope.launch {
             prefs.downloadQuality().collect { downloadQuality = it }
         }
+        scope.launch {
+            prefs.downloadOverMetered().collect { allowMeteredDownloads = it }
+        }
         scope.launch {
             combine(
                 db.sourcesDao().activeSubsonicConfig(),

app/src/main/kotlin/com/subtracks/ui/settings/SettingsScreen.kt

@@ -31,6 +31,7 @@ import androidx.compose.material3.ListItemDefaults
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.RadioButton
 import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Switch
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.material3.TopAppBar
@@ -70,6 +71,7 @@ fun SettingsRoute(
     val mobileQuality by viewModel.mobileQuality.collectAsStateWithLifecycle()
     val syncConcurrency by viewModel.syncConcurrency.collectAsStateWithLifecycle()
     val downloadQuality by viewModel.downloadQuality.collectAsStateWithLifecycle()
+    val downloadOverMetered by viewModel.downloadOverMetered.collectAsStateWithLifecycle()
     SettingsScreen(
         sources = sources,
         activeSourceId = activeSourceId,
@@ -77,12 +79,14 @@ fun SettingsRoute(
         mobileQuality = mobileQuality,
         syncConcurrency = syncConcurrency,
         downloadQuality = downloadQuality,
+        downloadOverMetered = downloadOverMetered,
         onSelectSource = viewModel::selectSource,
         onEditServer = onEditServer,
         onWifiQualityChange = viewModel::setWifiQuality,
         onMobileQualityChange = viewModel::setMobileQuality,
         onSyncConcurrencyChange = viewModel::setSyncConcurrency,
         onDownloadQualityChange = viewModel::setDownloadQuality,
+        onDownloadOverMeteredChange = viewModel::setDownloadOverMetered,
         onAddServer = onAddServer,
         onOpenDownloads = onOpenDownloads,
         onBack = onBack,
@@ -98,12 +102,14 @@ fun SettingsScreen(
     mobileQuality: StreamQuality,
     syncConcurrency: Int,
     downloadQuality: StreamQuality,
+    downloadOverMetered: Boolean,
     onSelectSource: (Long) -> Unit,
     onEditServer: (Long) -> Unit,
     onWifiQualityChange: (StreamQuality) -> Unit,
     onMobileQualityChange: (StreamQuality) -> Unit,
     onSyncConcurrencyChange: (Int) -> Unit,
     onDownloadQualityChange: (StreamQuality) -> Unit,
+    onDownloadOverMeteredChange: (Boolean) -> Unit,
     onAddServer: () -> Unit,
     onOpenDownloads: () -> Unit,
     onBack: () -> Unit,
@@ -199,6 +205,17 @@ fun SettingsScreen(
                     colors = ListItemDefaults.colors(containerColor = Color.Transparent),
                 )
             }
+            item {
+                ListItem(
+                    headlineContent = { Text("Download over mobile data") },
+                    supportingContent = { Text(if (downloadOverMetered) "Wi-Fi and mobile" else "Wi-Fi only") },
+                    trailingContent = {
+                        Switch(checked = downloadOverMetered, onCheckedChange = onDownloadOverMeteredChange)
+                    },
+                    modifier = Modifier.clickable { onDownloadOverMeteredChange(!downloadOverMetered) },
+                    colors = ListItemDefaults.colors(containerColor = Color.Transparent),
+                )
+            }
             item {
                 ListItem(
                     headlineContent = { Text("Manage downloads") },

app/src/main/kotlin/com/subtracks/ui/settings/SettingsViewModel.kt

@@ -38,6 +38,11 @@ class SettingsViewModel(
             .downloadQuality()
             .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), StreamQuality())
 
+    val downloadOverMetered: StateFlow<Boolean> =
+        userPreferences
+            .downloadOverMetered()
+            .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), false)
+
     val syncConcurrency: StateFlow<Int> =
         userPreferences
             .syncConcurrency()
@@ -59,6 +64,10 @@ class SettingsViewModel(
         viewModelScope.launch { userPreferences.setDownloadQuality(quality) }
     }
 
+    fun setDownloadOverMetered(allowed: Boolean) {
+        viewModelScope.launch { userPreferences.setDownloadOverMetered(allowed) }
+    }
+
     fun setSyncConcurrency(value: Int) {
         viewModelScope.launch { userPreferences.setSyncConcurrency(value) }
     }

app/src/test/kotlin/com/subtracks/data/repo/DownloadRepositoryTest.kt

@@ -52,6 +52,7 @@ class DownloadRepositoryTest {
     private lateinit var repository: DownloadRepository
     private lateinit var artwork: ArtworkStore
     private lateinit var dir: File
+    private val prefs = fakeUserPreferences()
     private val messages = CopyOnWriteArrayList<String>()
     private val requestedArt = CopyOnWriteArrayList<String>()
     private val artFetchedAfterTheEngineRequest = CopyOnWriteArrayList<Boolean>()
@@ -73,7 +74,7 @@ class DownloadRepositoryTest {
                 .setDriver(BundledSQLiteDriver())
                 .build()
         artwork = ArtworkStore(File(context.cacheDir, "art-${System.nanoTime()}"))
-        sources = SourceRepository(db, OkHttpClient(), fakeUserPreferences(), artwork)
+        sources = SourceRepository(db, OkHttpClient(), prefs, artwork)
         engine = FakeDownloadEngine()
         dir = File(context.cacheDir, "downloads-${System.nanoTime()}")
         repository =
@@ -956,6 +957,29 @@ class DownloadRepositoryTest {
             assertNull(repository.localUri("s1"))
         }
 
+    @Test
+    fun downloadsAreWifiOnlyUntilThePreferenceAllowsMetered() =
+        runTest {
+            seedLibrary()
+            runBlocking { repository.download(1, "s1") }
+            assertFalse(
+                engine.requests
+                    .single()
+                    .second.allowMetered,
+            )
+
+            prefs.setDownloadOverMetered(true)
+            await { sources.downloadsAllowedOverMetered() }
+
+            runBlocking { repository.download(1, "s2") }
+
+            assertTrue(
+                engine.requests
+                    .last()
+                    .second.allowMetered,
+            )
+        }
+
     private fun requestedSongIds(): List<String> = engine.requests.map { it.second.path.substringAfterLast('/') }
 
     private fun seedSongs(tracks: IntRange) {

app/src/test/kotlin/com/subtracks/ui/AppScreenshotTest.kt

@@ -344,12 +344,14 @@ class AppScreenshotTest {
                     mobileQuality = StreamQuality(192, "mp3"),
                     syncConcurrency = 4,
                     downloadQuality = StreamQuality(128, "opus"),
+                    downloadOverMetered = false,
                     onSelectSource = {},
                     onEditServer = {},
                     onWifiQualityChange = {},
                     onMobileQualityChange = {},
                     onSyncConcurrencyChange = {},
                     onDownloadQualityChange = {},
+                    onDownloadOverMeteredChange = {},
                     onAddServer = {},
                     onOpenDownloads = {},
                     onBack = {},