Add "Allow sync on metered connections" preference

If disabled, auto-sync isn't triggered on metered connections and the user is asked if they want to continue if they trigger a manual sync

If enabled, sync occurs normally

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

@@ -112,6 +112,7 @@ import com.ichi2.libanki.utils.TimeManager
 import com.ichi2.themes.StyledProgressDialog
 import com.ichi2.ui.BadgeDrawableBuilder
 import com.ichi2.utils.*
+import com.ichi2.utils.NetworkUtils.isActiveNetworkMetered
 import com.ichi2.utils.Permissions.hasStorageAccessPermission
 import com.ichi2.widget.WidgetStatus
 import kotlinx.coroutines.Job
@@ -923,14 +924,15 @@ open class DeckPicker :
     private fun automaticSync() {
         val preferences = AnkiDroidApp.getSharedPrefs(baseContext)
 
-        // Check whether the option is selected, the user is signed in and last sync was AUTOMATIC_SYNC_TIME ago
-        // (currently 10 minutes)
+        // Check whether the option is selected, the user is signed in, last sync was AUTOMATIC_SYNC_TIME ago
+        // (currently 10 minutes), and is not under a metered connection (if not allowed by preference)
         val isLoggedIn = preferences.getString("hkey", "")!!.isNotEmpty()
         val lastSyncTime = preferences.getLong("lastSyncTime", 0)
         val autoSyncIsEnabled = preferences.getBoolean("automaticSyncMode", false)
         val syncIntervalPassed = TimeManager.time.intTimeMS() - lastSyncTime > AUTOMATIC_SYNC_MIN_INTERVAL
+        val isNotBlockedByMeteredConnection = preferences.getBoolean(getString(R.string.metered_sync_key), false) || !isActiveNetworkMetered()
 
-        if (isLoggedIn && autoSyncIsEnabled && Connection.isOnline && syncIntervalPassed) {
+        if (isLoggedIn && autoSyncIsEnabled && Connection.isOnline && syncIntervalPassed && isNotBlockedByMeteredConnection) {
             Timber.i("Triggering Automatic Sync")
             sync()
         }
@@ -1563,25 +1565,30 @@ open class DeckPicker :
             Timber.w("User not logged in")
             mPullToSyncWrapper.isRefreshing = false
             showSyncErrorDialog(SyncErrorDialog.DIALOG_USER_NOT_LOGGED_IN_SYNC)
-        } else {
+            return
+        }
+        /** Nested function that makes the connection to
+         * the sync server and starts syncing the data */
+        fun doSync() {
             val syncMedia = preferences.getBoolean("syncFetchesMedia", true)
-
             if (!BackendFactory.defaultLegacySchema) {
                 handleNewSync(conflict, syncMedia)
             } else {
-                Connection.sync(
-                    mSyncListener,
-                    Connection.Payload(
-                        arrayOf(
-                            hkey,
-                            syncMedia,
-                            conflict,
-                            HostNumFactory.getInstance(baseContext)
-                        )
-                    )
-                )
+                val data = arrayOf(hkey, syncMedia, conflict, HostNumFactory.getInstance(baseContext))
+                Connection.sync(mSyncListener, Connection.Payload(data))
             }
         }
+        // Warn the user in case the connection is metered
+        val meteredSyncIsAllowed = preferences.getBoolean(getString(R.string.metered_sync_key), false)
+        if (!meteredSyncIsAllowed && isActiveNetworkMetered()) {
+            MaterialDialog(this).show {
+                message(R.string.metered_sync_warning)
+                positiveButton(R.string.dialog_continue) { doSync() }
+                negativeButton(R.string.dialog_cancel)
+            }
+        } else {
+            doSync()
+        }
     }
 
     private val mSyncListener: Connection.TaskListener = object : CancellableTaskListener {

AnkiDroid/src/main/res/values/03-dialogs.xml

@@ -257,6 +257,7 @@
         <item quantity="one">An automatic sync may be triggered in %d second</item>
         <item quantity="other">An automatic sync may be triggered in %d seconds</item>
     </plurals>
+    <string name="metered_sync_warning">Your connection is metered. Data transfer may cost money</string>
     <string name="deck_picker_new">Number of new cards to see today in this deck.</string>
     <string name="deck_picker_rev">Number of cards due today in this deck.</string>
     <string name="deck_picker_lrn">Number of cards in learning in this deck.</string> <!-- This description is valid fos

AnkiDroid/src/main/res/values/10-preferences.xml

@@ -100,6 +100,7 @@
     <string name="swipe_sensitivity" maxLength="41">Swipe sensitivity</string>
     <string name="tts" maxLength="41">Text to speech</string>
     <string name="tts_summ">Reads out question and answer if no sound file is included</string>
+    <!-- Sync -->
     <string name="sync_fetch_missing_media" maxLength="41">Fetch media on sync</string>
     <string name="sync_fetch_missing_media_summ">Automatically fetch missing media when syncing</string>
     <string name="sync_account" maxLength="41">AnkiWeb account</string>
@@ -109,6 +110,8 @@
         minutes ago.</string>
     <string name="sync_status_badge" maxLength="41">Display synchronization status</string>
     <string name="sync_status_badge_summ">Change the sync icon when changes can be uploaded</string>
+    <string name="metered_sync_title" maxLength="41">Allow sync on metered connections</string>
+    <string name="metered_sync_summary">If disabled, you will be warned if you try to sync on a metered connection</string>
     <string name="app_theme" maxLength="41">Theme</string>
     <string name="day_theme" maxLength="41">Day theme</string>
     <string name="night_theme" maxLength="41">Night theme</string>

AnkiDroid/src/main/res/values/preferences.xml

@@ -27,6 +27,7 @@
     <string name="automatic_sync_choice_key">automaticSyncMode</string>
     <string name="force_full_sync_key">force_full_sync</string>
     <string name="sync_status_badge_key">showSyncStatusBadge</string>
+    <string name="metered_sync_key">allowMetered</string>
     <string name="custom_sync_server_key">custom_sync_server_link</string>
     <!-- Appearance -->
     <string name="pref_appearance_screen_key">appearance_preference_group</string>

AnkiDroid/src/main/res/xml/preferences_sync.xml

@@ -42,6 +42,11 @@
         android:key="@string/sync_status_badge_key"
         android:summary="@string/sync_status_badge_summ"
         android:title="@string/sync_status_badge" />
+    <SwitchPreference
+        android:defaultValue="false"
+        android:key="@string/metered_sync_key"
+        android:title="@string/metered_sync_title"
+        android:summary="@string/metered_sync_summary" />
     <PreferenceCategory android:title="@string/pref_cat_advanced">
         <Preference
             android:key="@string/force_full_sync_key"