Integrated ChangeSubscriber for DeckPickerWidget updates on study …

…queue changes

 - Added `ChangeSubscriber` object to subscribe to `ChangeManager` and monitor changes in the study queue (`OpChanges.studyQueues`).
 - Implemented `opExecuted` in `ChangeSubscriber` to trigger `updateDeckPickerWidgets()` when relevant changes are detected.

AnkiDroid/src/main/java/com/ichi2/anki/AnkiDroidApp.kt

@@ -19,6 +19,8 @@ package com.ichi2.anki
 
 import android.app.Activity
 import android.app.Application
+import android.appwidget.AppWidgetManager
+import android.content.ComponentName
 import android.content.Context
 import android.content.Intent
 import android.content.SharedPreferences
@@ -34,6 +36,7 @@ import androidx.core.content.edit
 import androidx.fragment.app.FragmentActivity
 import androidx.lifecycle.MutableLiveData
 import androidx.work.Configuration
+import anki.collection.OpChanges
 import com.ichi2.anki.CrashReportService.sendExceptionReport
 import com.ichi2.anki.analytics.UsageAnalytics
 import com.ichi2.anki.browser.SharedPreferencesLastDeckIdRepository
@@ -53,11 +56,14 @@ import com.ichi2.anki.services.NotificationService
 import com.ichi2.anki.ui.dialogs.ActivityAgnosticDialogs
 import com.ichi2.annotations.NeedsTest
 import com.ichi2.compat.CompatHelper
+import com.ichi2.libanki.ChangeManager
 import com.ichi2.utils.AdaptionUtil
 import com.ichi2.utils.ExceptionUtil
 import com.ichi2.utils.KotlinCleanup
 import com.ichi2.utils.LanguageUtil
 import com.ichi2.utils.Permissions
+import com.ichi2.widget.DeckPickerWidget
+import com.ichi2.widget.WidgetPreferences
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.SupervisorJob
@@ -117,6 +123,8 @@ open class AnkiDroidApp : Application(), Configuration.Provider {
         // Get preferences
         val preferences = this.sharedPrefs()
 
+        ChangeSubscriber.initialize(this)
+
         CrashReportService.initialize(this)
         val logType = LogType.value
         when (logType) {
@@ -280,6 +288,81 @@ open class AnkiDroidApp : Application(), Configuration.Provider {
         }
     }
 
+    /**
+     * `ChangeSubscriber` is a singleton object that subscribes to changes in the `ChangeManager`
+     * and updates the Deck Picker Widgets when relevant changes occur.
+     *
+     * This object is designed to be initialized with an `Application` context, allowing it
+     * to manage widget updates efficiently across the application lifecycle.
+     */
+    object ChangeSubscriber : ChangeManager.Subscriber {
+
+        /**
+         * Application context for accessing resources and system services.
+         */
+        private lateinit var application: Application
+
+        /**
+         * Initializes the `ChangeSubscriber` with the provided [app] context and subscribes to
+         * the `ChangeManager`. This method must be called before any changes are detected.
+         *
+         * @param app The application context used to manage widget updates.
+         */
+        fun initialize(app: Application) {
+            Timber.d("Initializing ChangeSubscriber")
+            application = app
+            ChangeManager.subscribe(this)
+        }
+
+        /**
+         * Callback method invoked when operations that affect the app state are executed.
+         * If relevant changes related to the study queues are detected, the Deck Picker Widgets
+         * are updated accordingly.
+         *
+         * @param changes The set of changes that occurred.
+         * @param handler An optional handler that can be used for custom processing (unused here).
+         */
+        override fun opExecuted(changes: OpChanges, handler: Any?) {
+            Timber.d("ChangeSubscriber - opExecuted called with changes: $changes")
+            if (changes.studyQueues) {
+                updateDeckPickerWidgets()
+            } else {
+                Timber.d("No relevant changes to update the widget")
+            }
+        }
+
+        /**
+         * Updates the Deck Picker Widgets based on the current state of the application.
+         * It fetches the App Widget IDs and updates each widget with the associated deck IDs.
+         */
+        private fun updateDeckPickerWidgets() {
+            val context = application.applicationContext
+            val appWidgetManager = AppWidgetManager.getInstance(context)
+
+            val provider = ComponentName(context, DeckPickerWidget::class.java)
+            Timber.d("Fetching appWidgetIds for provider: $provider")
+
+            val appWidgetIds = appWidgetManager.getAppWidgetIds(provider)
+            Timber.d("AppWidgetIds to update: ${appWidgetIds.joinToString(", ")}")
+
+            for (appWidgetId in appWidgetIds) {
+                val deckIds = fetchDeckIdsForWidget(appWidgetId)
+                DeckPickerWidget.updateWidget(context, appWidgetManager, appWidgetId, deckIds)
+            }
+        }
+
+        /**
+         * Fetches the selected deck IDs for the specified [appWidgetId] from the widget preferences.
+         *
+         * @param appWidgetId The ID of the widget for which to fetch the deck IDs.
+         * @return A `LongArray` of deck IDs associated with the widget.
+         */
+        private fun fetchDeckIdsForWidget(appWidgetId: Int): LongArray {
+            val widgetPreferences = WidgetPreferences(application.applicationContext)
+            return widgetPreferences.getSelectedDeckIdsFromPreferencesDeckPickerWidget(appWidgetId)
+        }
+    }
+
     companion object {
 
         /**