refactor: Prune cached media using a worker (#529)

Previous code would prune any cached media every time `MainActivity` was
created, causing extra IO just as the user is trying to use the app.

Re-implement as a WorkManager worker so it can run when the device is
idle, without interfering with the responsiveness of the device.

app/src/main/java/app/pachli/MainActivity.kt

@@ -114,7 +114,6 @@ import app.pachli.pager.MainPagerAdapter
 import app.pachli.updatecheck.UpdateCheck
 import app.pachli.usecase.DeveloperToolsUseCase
 import app.pachli.usecase.LogoutUsecase
-import app.pachli.util.deleteStaleCachedMedia
 import app.pachli.util.getDimension
 import app.pachli.util.unsafeLazy
 import app.pachli.util.updateShortcut
@@ -383,11 +382,6 @@ class MainActivity : BottomSheetActivity(), ActionButtonActivity, MenuProvider {
             }
         }
 
-        externalScope.launch {
-            // Flush old media that was cached for sharing
-            deleteStaleCachedMedia(applicationContext.getExternalFilesDir("Pachli"))
-        }
-
         selectedEmojiPack = sharedPreferencesRepository.getString(EMOJI_PREFERENCE, "")
 
         onBackPressedDispatcher.addCallback(

app/src/main/java/app/pachli/PachliApplication.kt

@@ -37,6 +37,7 @@ import app.pachli.core.preferences.SharedPreferencesRepository
 import app.pachli.util.LocaleManager
 import app.pachli.util.setAppNightMode
 import app.pachli.worker.PruneCacheWorker
+import app.pachli.worker.PruneCachedMediaWorker
 import app.pachli.worker.PruneLogEntryEntityWorker
 import dagger.hilt.android.HiltAndroidApp
 import de.c1710.filemojicompat_defaults.DefaultEmojiPackList
@@ -133,6 +134,16 @@ class PachliApplication : Application() {
             ExistingPeriodicWorkPolicy.KEEP,
             pruneLogEntryEntityWorker,
         )
+
+        // Delete old cached media every ~ 12 hours when the device is idle
+        val pruneCachedMediaWorker = PeriodicWorkRequestBuilder<PruneCachedMediaWorker>(12, TimeUnit.HOURS)
+            .setConstraints(Constraints.Builder().setRequiresDeviceIdle(true).build())
+            .build()
+        workManager.enqueueUniquePeriodicWork(
+            PruneCachedMediaWorker.PERIODIC_WORK_TAG,
+            ExistingPeriodicWorkPolicy.KEEP,
+            pruneCachedMediaWorker,
+        )
     }
 
     private fun upgradeSharedPreferences(oldVersion: Int, newVersion: Int) {

app/src/main/java/app/pachli/util/MediaUtils.kt

@@ -24,22 +24,18 @@ import android.graphics.Matrix
 import android.net.Uri
 import android.provider.OpenableColumns
 import androidx.exifinterface.media.ExifInterface
-import java.io.File
 import java.io.FileNotFoundException
 import java.io.IOException
 import java.text.SimpleDateFormat
-import java.util.Calendar
 import java.util.Date
 import java.util.Locale
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.withContext
 import timber.log.Timber
 
 /**
  * Helper methods for obtaining and resizing media files
  */
 
-private const val MEDIA_TEMP_PREFIX = "Pachli_Share_Media"
+const val MEDIA_TEMP_PREFIX = "Pachli_Share_Media"
 const val MEDIA_SIZE_UNKNOWN = -1L
 
 /**
@@ -167,35 +163,6 @@ fun getImageOrientation(uri: Uri, contentResolver: ContentResolver): Int {
     }
 }
 
-suspend fun deleteStaleCachedMedia(mediaDirectory: File?) = withContext(Dispatchers.IO) {
-    if (mediaDirectory == null || !mediaDirectory.exists()) {
-        // Nothing to do
-        return@withContext
-    }
-
-    val twentyfourHoursAgo = Calendar.getInstance()
-    twentyfourHoursAgo.add(Calendar.HOUR, -24)
-    val unixTime = twentyfourHoursAgo.timeInMillis
-
-    val files = mediaDirectory.listFiles { file ->
-        unixTime > file.lastModified() && file.name.contains(
-            MEDIA_TEMP_PREFIX,
-        )
-    }
-    if (files == null || files.isEmpty()) {
-        // Nothing to do
-        return@withContext
-    }
-
-    for (file in files) {
-        try {
-            file.delete()
-        } catch (se: SecurityException) {
-            Timber.e("Error removing stale cached media")
-        }
-    }
-}
-
 fun getTemporaryMediaFilename(extension: String): String {
     return "${MEDIA_TEMP_PREFIX}_${SimpleDateFormat("yyyyMMdd_HHmmss", Locale.US).format(Date())}.$extension"
 }

app/src/main/java/app/pachli/worker/PruneCachedMediaWorker.kt

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2024 Pachli Association
+ *
+ * This file is a part of Pachli.
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * Pachli is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
+ * the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with Pachli; if not,
+ * see <http://www.gnu.org/licenses>.
+ */
+
+package app.pachli.worker
+
+import android.app.Notification
+import android.content.Context
+import androidx.hilt.work.HiltWorker
+import androidx.work.CoroutineWorker
+import androidx.work.ForegroundInfo
+import androidx.work.WorkerParameters
+import app.pachli.R
+import app.pachli.components.notifications.NOTIFICATION_ID_PRUNE_CACHE
+import app.pachli.components.notifications.createWorkerNotification
+import app.pachli.util.MEDIA_TEMP_PREFIX
+import dagger.assisted.Assisted
+import dagger.assisted.AssistedInject
+import java.time.Instant
+import kotlin.time.Duration.Companion.hours
+import timber.log.Timber
+
+/** Prune old media that was cached for sharing */
+@HiltWorker
+class PruneCachedMediaWorker @AssistedInject constructor(
+    @Assisted val appContext: Context,
+    @Assisted workerParams: WorkerParameters,
+) : CoroutineWorker(appContext, workerParams) {
+    val notification: Notification = createWorkerNotification(applicationContext, R.string.notification_prune_cache)
+
+    override suspend fun doWork(): Result {
+        val mediaDirectory = appContext.getExternalFilesDir("Pachli")
+        if (mediaDirectory == null) {
+            Timber.d("Skipping prune, shared storage is not available")
+            return Result.success()
+        }
+
+        if (!mediaDirectory.exists()) {
+            Timber.d("Skipping prune, media directory does not exist: %s", mediaDirectory.absolutePath)
+            return Result.success()
+        }
+
+        val now = Instant.now()
+        val cutoffInMs = now.minusSeconds(OLDEST_ENTRY.inWholeSeconds).toEpochMilli()
+
+        mediaDirectory.listFiles { file ->
+            file.lastModified() < cutoffInMs && file.name.startsWith(MEDIA_TEMP_PREFIX)
+        }?.forEach { file ->
+            try {
+                file.delete()
+            } catch (se: SecurityException) {
+                Timber.e(se, "Error removing %s", file.name)
+            }
+        }
+
+        return Result.success()
+    }
+
+    override suspend fun getForegroundInfo() = ForegroundInfo(NOTIFICATION_ID_PRUNE_CACHE, notification)
+
+    companion object {
+        private val OLDEST_ENTRY = 24.hours
+        const val PERIODIC_WORK_TAG = "PruneCachedMediaWorker"
+    }
+}