Add synchronization with gPodder Nextcloud server app (#5243)

18 files changed, 621 insertions, 455 deletions

diff --git a/core/src/main/java/de/danoeh/antennapod/core/preferences/GpodnetPreferences.java b/core/src/main/java/de/danoeh/antennapod/core/preferences/GpodnetPreferences.java
deleted file mode 100644
index e338e0d01..000000000
--- a/core/src/main/java/de/danoeh/antennapod/core/preferences/GpodnetPreferences.java
+++ /dev/null
@@ -1,115 +0,0 @@
-package de.danoeh.antennapod.core.preferences;
-
-import android.content.Context;
-import android.content.SharedPreferences;
-import android.util.Log;
-import de.danoeh.antennapod.core.BuildConfig;
-import de.danoeh.antennapod.core.ClientConfig;
-import de.danoeh.antennapod.core.sync.SyncService;
-import de.danoeh.antennapod.net.sync.gpoddernet.GpodnetService;
-
-/**
- * Manages preferences for accessing gpodder.net service
- */
-public class GpodnetPreferences {
-
-    private GpodnetPreferences(){}
-
-    private static final String TAG = "GpodnetPreferences";
-
-    private static final String PREF_NAME = "gpodder.net";
-    private static final String PREF_GPODNET_USERNAME = "de.danoeh.antennapod.preferences.gpoddernet.username";
-    private static final String PREF_GPODNET_PASSWORD = "de.danoeh.antennapod.preferences.gpoddernet.password";
-    private static final String PREF_GPODNET_DEVICEID = "de.danoeh.antennapod.preferences.gpoddernet.deviceID";
-    private static final String PREF_GPODNET_HOSTNAME = "prefGpodnetHostname";
-
-    private static String username;
-    private static String password;
-    private static String deviceID;
-    private static String hosturl;
-
-    private static boolean preferencesLoaded = false;
-
-    private static SharedPreferences getPreferences() {
-        return ClientConfig.applicationCallbacks.getApplicationInstance().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-    }
-
-    private static synchronized void ensurePreferencesLoaded() {
-        if (!preferencesLoaded) {
-            SharedPreferences prefs = getPreferences();
-            username = prefs.getString(PREF_GPODNET_USERNAME, null);
-            password = prefs.getString(PREF_GPODNET_PASSWORD, null);
-            deviceID = prefs.getString(PREF_GPODNET_DEVICEID, null);
-            hosturl = prefs.getString(PREF_GPODNET_HOSTNAME, GpodnetService.DEFAULT_BASE_HOST);
-
-            preferencesLoaded = true;
-        }
-    }
-
-    private static void writePreference(String key, String value) {
-        SharedPreferences.Editor editor = getPreferences().edit();
-        editor.putString(key, value);
-        editor.apply();
-    }
-
-    public static String getUsername() {
-        ensurePreferencesLoaded();
-        return username;
-    }
-
-    public static void setUsername(String username) {
-        GpodnetPreferences.username = username;
-        writePreference(PREF_GPODNET_USERNAME, username);
-    }
-
-    public static String getPassword() {
-        ensurePreferencesLoaded();
-        return password;
-    }
-
-    public static void setPassword(String password) {
-        GpodnetPreferences.password = password;
-        writePreference(PREF_GPODNET_PASSWORD, password);
-    }
-
-    public static String getDeviceID() {
-        ensurePreferencesLoaded();
-        return deviceID;
-    }
-
-    public static void setDeviceID(String deviceID) {
-        GpodnetPreferences.deviceID = deviceID;
-        writePreference(PREF_GPODNET_DEVICEID, deviceID);
-    }
-
-    public static String getHosturl() {
-        ensurePreferencesLoaded();
-        return hosturl;
-    }
-
-    public static void setHosturl(String value) {
-        if (!value.equals(hosturl)) {
-            logout();
-            writePreference(PREF_GPODNET_HOSTNAME, value);
-            hosturl = value;
-        }
-    }
-
-    /**
-     * Returns true if device ID, username and password have a non-null value
-     */
-    public static boolean loggedIn() {
-        ensurePreferencesLoaded();
-        return deviceID != null && username != null && password != null;
-    }
-
-    public static synchronized void logout() {
-        if (BuildConfig.DEBUG) Log.d(TAG, "Logout: Clearing preferences");
-        setUsername(null);
-        setPassword(null);
-        setDeviceID(null);
-        SyncService.clearQueue(ClientConfig.applicationCallbacks.getApplicationInstance());
-        UserPreferences.setGpodnetNotificationsEnabled();
-    }
-
-}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/service/download/handler/MediaDownloadedHandler.java b/core/src/main/java/de/danoeh/antennapod/core/service/download/handler/MediaDownloadedHandler.java
index 8c9035621..6bbd704e2 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/service/download/handler/MediaDownloadedHandler.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/service/download/handler/MediaDownloadedHandler.java
@@ -6,21 +6,22 @@ import android.util.Log;
 
 import androidx.annotation.NonNull;
 
+import org.greenrobot.eventbus.EventBus;
+
 import java.io.File;
 import java.util.concurrent.ExecutionException;
 
 import de.danoeh.antennapod.core.event.UnreadItemsUpdateEvent;
-import de.danoeh.antennapod.model.feed.FeedItem;
-import de.danoeh.antennapod.model.feed.FeedMedia;
 import de.danoeh.antennapod.core.service.download.DownloadRequest;
 import de.danoeh.antennapod.core.service.download.DownloadStatus;
 import de.danoeh.antennapod.core.storage.DBReader;
 import de.danoeh.antennapod.core.storage.DBWriter;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueSink;
 import de.danoeh.antennapod.core.util.ChapterUtils;
 import de.danoeh.antennapod.core.util.DownloadError;
-import de.danoeh.antennapod.core.sync.SyncService;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import de.danoeh.antennapod.model.feed.FeedMedia;
 import de.danoeh.antennapod.net.sync.model.EpisodeAction;
-import org.greenrobot.eventbus.EventBus;
 
 /**
  * Handles a completed media download.
@@ -103,7 +104,7 @@ public class MediaDownloadedHandler implements Runnable {
             EpisodeAction action = new EpisodeAction.Builder(item, EpisodeAction.DOWNLOAD)
                     .currentTimestamp()
                     .build();
-            SyncService.enqueueEpisodeAction(context, action);
+            SynchronizationQueueSink.enqueueEpisodeActionIfSynchronizationIsActive(context, action);
         }
     }
 
diff --git a/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackService.java b/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackService.java
index f503c16f4..848ea7cfc 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackService.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/service/playback/PlaybackService.java
@@ -1,5 +1,7 @@
 package de.danoeh.antennapod.core.service.playback;
 
+import static de.danoeh.antennapod.model.feed.FeedPreferences.SPEED_USE_GLOBAL;
+
 import android.app.NotificationManager;
 import android.app.PendingIntent;
 import android.app.Service;
@@ -21,13 +23,7 @@ import android.os.Build;
 import android.os.Bundle;
 import android.os.IBinder;
 import android.os.Vibrator;
-import androidx.preference.PreferenceManager;
-import androidx.annotation.NonNull;
-import androidx.annotation.StringRes;
-import androidx.core.app.NotificationCompat;
-import androidx.core.app.NotificationManagerCompat;
 import android.support.v4.media.MediaBrowserCompat;
-import androidx.media.MediaBrowserServiceCompat;
 import android.support.v4.media.MediaDescriptionCompat;
 import android.support.v4.media.MediaMetadataCompat;
 import android.support.v4.media.session.MediaSessionCompat;
@@ -40,6 +36,17 @@ import android.view.SurfaceHolder;
 import android.webkit.URLUtil;
 import android.widget.Toast;
 
+import androidx.annotation.NonNull;
+import androidx.annotation.StringRes;
+import androidx.core.app.NotificationCompat;
+import androidx.core.app.NotificationManagerCompat;
+import androidx.media.MediaBrowserServiceCompat;
+import androidx.preference.PreferenceManager;
+
+import org.greenrobot.eventbus.EventBus;
+import org.greenrobot.eventbus.Subscribe;
+import org.greenrobot.eventbus.ThreadMode;
+
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -52,12 +59,6 @@ import de.danoeh.antennapod.core.event.ServiceEvent;
 import de.danoeh.antennapod.core.event.settings.SkipIntroEndingChangedEvent;
 import de.danoeh.antennapod.core.event.settings.SpeedPresetChangedEvent;
 import de.danoeh.antennapod.core.event.settings.VolumeAdaptionChangedEvent;
-import de.danoeh.antennapod.model.feed.Chapter;
-import de.danoeh.antennapod.model.feed.Feed;
-import de.danoeh.antennapod.model.feed.FeedItem;
-import de.danoeh.antennapod.model.feed.FeedMedia;
-import de.danoeh.antennapod.model.feed.FeedPreferences;
-import de.danoeh.antennapod.model.playback.MediaType;
 import de.danoeh.antennapod.core.preferences.PlaybackPreferences;
 import de.danoeh.antennapod.core.preferences.SleepTimerPreferences;
 import de.danoeh.antennapod.core.preferences.UserPreferences;
@@ -66,15 +67,21 @@ import de.danoeh.antennapod.core.storage.DBReader;
 import de.danoeh.antennapod.core.storage.DBTasks;
 import de.danoeh.antennapod.core.storage.DBWriter;
 import de.danoeh.antennapod.core.storage.FeedSearcher;
-import de.danoeh.antennapod.core.sync.SyncService;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueSink;
 import de.danoeh.antennapod.core.util.FeedItemUtil;
 import de.danoeh.antennapod.core.util.IntentUtils;
 import de.danoeh.antennapod.core.util.NetworkUtils;
 import de.danoeh.antennapod.core.util.gui.NotificationUtils;
-import de.danoeh.antennapod.model.playback.Playable;
 import de.danoeh.antennapod.core.util.playback.PlayableUtils;
 import de.danoeh.antennapod.core.util.playback.PlaybackServiceStarter;
 import de.danoeh.antennapod.core.widget.WidgetUpdater;
+import de.danoeh.antennapod.model.feed.Chapter;
+import de.danoeh.antennapod.model.feed.Feed;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import de.danoeh.antennapod.model.feed.FeedMedia;
+import de.danoeh.antennapod.model.feed.FeedPreferences;
+import de.danoeh.antennapod.model.playback.MediaType;
+import de.danoeh.antennapod.model.playback.Playable;
 import de.danoeh.antennapod.ui.appstartintent.MainActivityStarter;
 import de.danoeh.antennapod.ui.appstartintent.VideoPlayerActivityStarter;
 import io.reactivex.Completable;
@@ -83,11 +90,6 @@ import io.reactivex.Single;
 import io.reactivex.android.schedulers.AndroidSchedulers;
 import io.reactivex.disposables.Disposable;
 import io.reactivex.schedulers.Schedulers;
-import org.greenrobot.eventbus.EventBus;
-import org.greenrobot.eventbus.Subscribe;
-import org.greenrobot.eventbus.ThreadMode;
-
-import static de.danoeh.antennapod.model.feed.FeedPreferences.SPEED_USE_GLOBAL;
 
 /**
  * Controls the MediaPlayer that plays a FeedMedia-file
@@ -966,7 +968,8 @@ public class PlaybackService extends MediaBrowserServiceCompat {
             taskManager.cancelWidgetUpdater();
             if (playable != null) {
                 if (playable instanceof FeedMedia) {
-                    SyncService.enqueueEpisodePlayed(getApplicationContext(), (FeedMedia) playable, false);
+                    SynchronizationQueueSink.enqueueEpisodePlayedIfSynchronizationIsActive(getApplicationContext(),
+                            (FeedMedia) playable, false);
                 }
                 playable.onPlaybackPause(getApplicationContext());
             }
@@ -1110,10 +1113,12 @@ public class PlaybackService extends MediaBrowserServiceCompat {
         }
 
         if (ended || smartMarkAsPlayed) {
-            SyncService.enqueueEpisodePlayed(getApplicationContext(), media, true);
+            SynchronizationQueueSink.enqueueEpisodePlayedIfSynchronizationIsActive(
+                    getApplicationContext(), media, true);
             media.onPlaybackCompleted(getApplicationContext());
         } else {
-            SyncService.enqueueEpisodePlayed(getApplicationContext(), media, false);
+            SynchronizationQueueSink.enqueueEpisodePlayedIfSynchronizationIsActive(
+                    getApplicationContext(), media, false);
             media.onPlaybackPause(getApplicationContext());
         }
 
diff --git a/core/src/main/java/de/danoeh/antennapod/core/storage/DBReader.java b/core/src/main/java/de/danoeh/antennapod/core/storage/DBReader.java
index 185d85e7a..f776fe111 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/storage/DBReader.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/storage/DBReader.java
@@ -575,7 +575,6 @@ public final class DBReader {
     @Nullable
     private static FeedItem getFeedItemByGuidOrEpisodeUrl(final String guid, final String episodeUrl,
             PodDBAdapter adapter) {
-        Log.d(TAG, "Loading feeditem with guid " + guid + " or episode url " + episodeUrl);
         try (Cursor cursor = adapter.getFeedItemCursor(guid, episodeUrl)) {
             if (!cursor.moveToNext()) {
                 return null;
@@ -633,8 +632,6 @@ public final class DBReader {
      *          Does NOT load additional attributes like feed or queue state.
      */
     public static FeedItem getFeedItemByGuidOrEpisodeUrl(final String guid, final String episodeUrl) {
-        Log.d(TAG, "getFeedItem() called with: " + "guid = [" + guid + "], episodeUrl = [" + episodeUrl + "]");
-
         PodDBAdapter adapter = PodDBAdapter.getInstance();
         adapter.open();
         try {
diff --git a/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java b/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
index 9dd979dc7..377202c4b 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/storage/DBTasks.java
@@ -1,5 +1,7 @@
 package de.danoeh.antennapod.core.storage;
 
+import static android.content.Context.MODE_PRIVATE;
+
 import android.content.Context;
 import android.content.SharedPreferences;
 import android.database.Cursor;
@@ -9,22 +11,6 @@ import android.util.Log;
 
 import androidx.annotation.VisibleForTesting;
 
-import de.danoeh.antennapod.core.R;
-import de.danoeh.antennapod.core.event.FeedItemEvent;
-import de.danoeh.antennapod.core.event.FeedListUpdateEvent;
-import de.danoeh.antennapod.core.event.MessageEvent;
-import de.danoeh.antennapod.model.feed.Feed;
-import de.danoeh.antennapod.model.feed.FeedItem;
-import de.danoeh.antennapod.model.feed.FeedMedia;
-import de.danoeh.antennapod.core.feed.LocalFeedUpdater;
-import de.danoeh.antennapod.core.preferences.UserPreferences;
-import de.danoeh.antennapod.core.service.download.DownloadStatus;
-import de.danoeh.antennapod.core.storage.mapper.FeedCursorMapper;
-import de.danoeh.antennapod.core.sync.SyncService;
-import de.danoeh.antennapod.core.util.DownloadError;
-import de.danoeh.antennapod.core.util.LongList;
-import de.danoeh.antennapod.core.util.comparator.FeedItemPubdateComparator;
-import de.danoeh.antennapod.net.sync.model.EpisodeAction;
 import org.greenrobot.eventbus.EventBus;
 
 import java.util.ArrayList;
@@ -41,7 +27,23 @@ import java.util.concurrent.Future;
 import java.util.concurrent.FutureTask;
 import java.util.concurrent.atomic.AtomicBoolean;
 
-import static android.content.Context.MODE_PRIVATE;
+import de.danoeh.antennapod.core.R;
+import de.danoeh.antennapod.core.event.FeedItemEvent;
+import de.danoeh.antennapod.core.event.FeedListUpdateEvent;
+import de.danoeh.antennapod.core.event.MessageEvent;
+import de.danoeh.antennapod.core.feed.LocalFeedUpdater;
+import de.danoeh.antennapod.core.preferences.UserPreferences;
+import de.danoeh.antennapod.core.service.download.DownloadStatus;
+import de.danoeh.antennapod.core.storage.mapper.FeedCursorMapper;
+import de.danoeh.antennapod.core.sync.SyncService;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueSink;
+import de.danoeh.antennapod.core.util.DownloadError;
+import de.danoeh.antennapod.core.util.LongList;
+import de.danoeh.antennapod.core.util.comparator.FeedItemPubdateComparator;
+import de.danoeh.antennapod.model.feed.Feed;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import de.danoeh.antennapod.model.feed.FeedMedia;
+import de.danoeh.antennapod.net.sync.model.EpisodeAction;
 
 /**
  * Provides methods for doing common tasks that use DBReader and DBWriter.
@@ -482,7 +484,7 @@ public final class DBTasks {
                                     .position(oldItem.getMedia().getDuration() / 1000)
                                     .total(oldItem.getMedia().getDuration() / 1000)
                                     .build();
-                            SyncService.enqueueEpisodeAction(context, action);
+                            SynchronizationQueueSink.enqueueEpisodeActionIfSynchronizationIsActive(context, action);
                         }
                     }
                 }
diff --git a/core/src/main/java/de/danoeh/antennapod/core/storage/DBWriter.java b/core/src/main/java/de/danoeh/antennapod/core/storage/DBWriter.java
index 34ea5e207..479a7763c 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/storage/DBWriter.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/storage/DBWriter.java
@@ -7,8 +7,6 @@ import android.util.Log;
 import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 
-import de.danoeh.antennapod.core.sync.SyncService;
-import de.danoeh.antennapod.net.sync.model.EpisodeAction;
 import org.greenrobot.eventbus.EventBus;
 
 import java.io.File;
@@ -32,23 +30,24 @@ import de.danoeh.antennapod.core.event.MessageEvent;
 import de.danoeh.antennapod.core.event.PlaybackHistoryEvent;
 import de.danoeh.antennapod.core.event.QueueEvent;
 import de.danoeh.antennapod.core.event.UnreadItemsUpdateEvent;
-import de.danoeh.antennapod.model.feed.Feed;
 import de.danoeh.antennapod.core.feed.FeedEvent;
-import de.danoeh.antennapod.model.feed.FeedItem;
-import de.danoeh.antennapod.model.feed.FeedMedia;
-import de.danoeh.antennapod.model.feed.FeedPreferences;
-import de.danoeh.antennapod.core.preferences.GpodnetPreferences;
 import de.danoeh.antennapod.core.preferences.PlaybackPreferences;
 import de.danoeh.antennapod.core.preferences.UserPreferences;
 import de.danoeh.antennapod.core.service.download.DownloadStatus;
 import de.danoeh.antennapod.core.service.playback.PlaybackService;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueSink;
 import de.danoeh.antennapod.core.util.FeedItemPermutors;
 import de.danoeh.antennapod.core.util.IntentUtils;
 import de.danoeh.antennapod.core.util.LongList;
 import de.danoeh.antennapod.core.util.Permutor;
+import de.danoeh.antennapod.core.util.playback.PlayableUtils;
+import de.danoeh.antennapod.model.feed.Feed;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import de.danoeh.antennapod.model.feed.FeedMedia;
+import de.danoeh.antennapod.model.feed.FeedPreferences;
 import de.danoeh.antennapod.model.feed.SortOrder;
 import de.danoeh.antennapod.model.playback.Playable;
-import de.danoeh.antennapod.core.util.playback.PlayableUtils;
+import de.danoeh.antennapod.net.sync.model.EpisodeAction;
 
 /**
  * Provides methods for writing data to AntennaPod's database.
@@ -132,13 +131,11 @@ public class DBWriter {
             }
 
             // Gpodder: queue delete action for synchronization
-            if (GpodnetPreferences.loggedIn()) {
-                FeedItem item = media.getItem();
-                EpisodeAction action = new EpisodeAction.Builder(item, EpisodeAction.DELETE)
-                        .currentTimestamp()
-                        .build();
-                SyncService.enqueueEpisodeAction(context, action);
-            }
+            FeedItem item = media.getItem();
+            EpisodeAction action = new EpisodeAction.Builder(item, EpisodeAction.DELETE)
+                    .currentTimestamp()
+                    .build();
+            SynchronizationQueueSink.enqueueEpisodeActionIfSynchronizationIsActive(context, action);
         }
         EventBus.getDefault().post(FeedItemEvent.deletedMedia(Collections.singletonList(media.getItem())));
         return true;
@@ -170,7 +167,7 @@ public class DBWriter {
             adapter.removeFeed(feed);
             adapter.close();
 
-            SyncService.enqueueFeedRemoved(context, feed.getDownload_url());
+            SynchronizationQueueSink.enqueueFeedRemovedIfSynchronizationIsActive(context, feed.getDownload_url());
             EventBus.getDefault().post(new FeedListUpdateEvent(feed));
         });
     }
@@ -782,7 +779,7 @@ public class DBWriter {
             adapter.close();
 
             for (Feed feed : feeds) {
-                SyncService.enqueueFeedAdded(context, feed.getDownload_url());
+                SynchronizationQueueSink.enqueueFeedAddedIfSynchronizationIsActive(context, feed.getDownload_url());
             }
 
             BackupManager backupManager = new BackupManager(context);
diff --git a/core/src/main/java/de/danoeh/antennapod/core/storage/PodDBAdapter.java b/core/src/main/java/de/danoeh/antennapod/core/storage/PodDBAdapter.java
index 85ce2dc99..55cfafbbb 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/storage/PodDBAdapter.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/storage/PodDBAdapter.java
@@ -1123,7 +1123,6 @@ public class PodDBAdapter {
                 + " INNER JOIN " + TABLE_NAME_FEEDS
                 + " ON " + TABLE_NAME_FEED_ITEMS + "." + KEY_FEED + "=" + TABLE_NAME_FEEDS + "." + KEY_ID
                 + " WHERE " + whereClauseCondition;
-        Log.d(TAG, "SQL: " + query);
         return db.rawQuery(query, null);
     }
 
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/GuidValidator.java b/core/src/main/java/de/danoeh/antennapod/core/sync/GuidValidator.java
index 6d80a6457..74e5d5cdf 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/sync/GuidValidator.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/GuidValidator.java
@@ -4,7 +4,8 @@ public class GuidValidator {
 
     public static boolean isValidGuid(String guid) {
         return guid != null
-                && !guid.trim().isEmpty();
+                && !guid.trim().isEmpty()
+                && !guid.equals("null");
     }
 }
 
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/LockingAsyncExecutor.java b/core/src/main/java/de/danoeh/antennapod/core/sync/LockingAsyncExecutor.java
new file mode 100644
index 000000000..e7dbbbd3c
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/LockingAsyncExecutor.java
@@ -0,0 +1,35 @@
+package de.danoeh.antennapod.core.sync;
+
+import java.util.concurrent.locks.ReentrantLock;
+
+import io.reactivex.Completable;
+import io.reactivex.schedulers.Schedulers;
+
+public class LockingAsyncExecutor {
+
+    static final ReentrantLock lock = new ReentrantLock();
+
+    /**
+     * Take the lock and execute runnable (to prevent changes to preferences being lost when enqueueing while sync is
+     * in progress). If the lock is free, the runnable is directly executed in the calling thread to prevent overhead.
+     */
+    public static void executeLockedAsync(Runnable runnable) {
+        if (lock.tryLock()) {
+            try {
+                runnable.run();
+            } finally {
+                lock.unlock();
+            }
+        } else {
+            Completable.fromRunnable(() -> {
+                lock.lock();
+                try {
+                    runnable.run();
+                } finally {
+                    lock.unlock();
+                }
+            }).subscribeOn(Schedulers.io())
+                    .subscribe();
+        }
+    }
+}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/SyncService.java b/core/src/main/java/de/danoeh/antennapod/core/sync/SyncService.java
index 9803a29db..8edc37ac4 100644
--- a/core/src/main/java/de/danoeh/antennapod/core/sync/SyncService.java
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/SyncService.java
@@ -5,7 +5,6 @@ import android.app.NotificationManager;
 import android.app.PendingIntent;
 import android.content.Context;
 import android.content.Intent;
-import android.content.SharedPreferences;
 import android.util.Log;
 
 import androidx.annotation.NonNull;
@@ -20,12 +19,16 @@ import androidx.work.WorkManager;
 import androidx.work.Worker;
 import androidx.work.WorkerParameters;
 
+import org.apache.commons.lang3.StringUtils;
+import org.greenrobot.eventbus.EventBus;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.TimeUnit;
+
 import de.danoeh.antennapod.core.R;
 import de.danoeh.antennapod.core.event.SyncServiceEvent;
-import de.danoeh.antennapod.model.feed.Feed;
-import de.danoeh.antennapod.model.feed.FeedItem;
-import de.danoeh.antennapod.model.feed.FeedMedia;
-import de.danoeh.antennapod.core.preferences.GpodnetPreferences;
 import de.danoeh.antennapod.core.preferences.UserPreferences;
 import de.danoeh.antennapod.core.service.download.AntennapodHttpClient;
 import de.danoeh.antennapod.core.storage.DBReader;
@@ -33,10 +36,14 @@ import de.danoeh.antennapod.core.storage.DBTasks;
 import de.danoeh.antennapod.core.storage.DBWriter;
 import de.danoeh.antennapod.core.storage.DownloadRequestException;
 import de.danoeh.antennapod.core.storage.DownloadRequester;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueStorage;
 import de.danoeh.antennapod.core.util.FeedItemUtil;
 import de.danoeh.antennapod.core.util.LongList;
 import de.danoeh.antennapod.core.util.URLChecker;
 import de.danoeh.antennapod.core.util.gui.NotificationUtils;
+import de.danoeh.antennapod.model.feed.Feed;
+import de.danoeh.antennapod.model.feed.FeedItem;
+import de.danoeh.antennapod.model.feed.FeedMedia;
 import de.danoeh.antennapod.net.sync.gpoddernet.GpodnetService;
 import de.danoeh.antennapod.net.sync.model.EpisodeAction;
 import de.danoeh.antennapod.net.sync.model.EpisodeActionChanges;
@@ -44,156 +51,54 @@ import de.danoeh.antennapod.net.sync.model.ISyncService;
 import de.danoeh.antennapod.net.sync.model.SubscriptionChanges;
 import de.danoeh.antennapod.net.sync.model.SyncServiceException;
 import de.danoeh.antennapod.net.sync.model.UploadChangesResponse;
-import io.reactivex.Completable;
-import io.reactivex.schedulers.Schedulers;
-
-import org.apache.commons.lang3.StringUtils;
-import org.greenrobot.eventbus.EventBus;
-import org.json.JSONArray;
-import org.json.JSONException;
-
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Map;
-import java.util.concurrent.TimeUnit;
-import java.util.concurrent.locks.ReentrantLock;
+import de.danoeh.antennapod.net.sync.nextcloud.NextcloudSyncService;
 
 public class SyncService extends Worker {
-    private static final String PREF_NAME = "SyncService";
-    private static final String PREF_LAST_SUBSCRIPTION_SYNC_TIMESTAMP = "last_sync_timestamp";
-    private static final String PREF_LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP = "last_episode_actions_sync_timestamp";
-    private static final String PREF_QUEUED_FEEDS_ADDED = "sync_added";
-    private static final String PREF_QUEUED_FEEDS_REMOVED = "sync_removed";
-    private static final String PREF_QUEUED_EPISODE_ACTIONS = "sync_queued_episode_actions";
-    private static final String PREF_LAST_SYNC_ATTEMPT_TIMESTAMP = "last_sync_attempt_timestamp";
-    private static final String PREF_LAST_SYNC_ATTEMPT_SUCCESS = "last_sync_attempt_success";
-    private static final String TAG = "SyncService";
-    private static final String WORK_ID_SYNC = "SyncServiceWorkId";
-    private static final ReentrantLock lock = new ReentrantLock();
+    public static final String TAG = "SyncService";
 
-    private ISyncService syncServiceImpl;
+    private static final String WORK_ID_SYNC = "SyncServiceWorkId";
+    private final SynchronizationQueueStorage synchronizationQueueStorage;
 
     public SyncService(@NonNull Context context, @NonNull WorkerParameters params) {
         super(context, params);
+        synchronizationQueueStorage = new SynchronizationQueueStorage(context);
     }
 
     @Override
     @NonNull
     public Result doWork() {
-        if (!GpodnetPreferences.loggedIn()) {
+        ISyncService activeSyncProvider = getActiveSyncProvider();
+        if (activeSyncProvider == null) {
             return Result.success();
         }
-        syncServiceImpl = new GpodnetService(AntennapodHttpClient.getHttpClient(),
-                GpodnetPreferences.getHosturl(), GpodnetPreferences.getDeviceID(),
-                GpodnetPreferences.getUsername(), GpodnetPreferences.getPassword());
-        SharedPreferences.Editor prefs = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE)
-                .edit();
-        prefs.putLong(PREF_LAST_SYNC_ATTEMPT_TIMESTAMP, System.currentTimeMillis()).apply();
+
+        SynchronizationSettings.updateLastSynchronizationAttempt();
         try {
-            syncServiceImpl.login();
+            activeSyncProvider.login();
             EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_subscriptions));
-            syncSubscriptions();
-            syncEpisodeActions();
-            syncServiceImpl.logout();
+            syncSubscriptions(activeSyncProvider);
+            syncEpisodeActions(activeSyncProvider);
+            activeSyncProvider.logout();
             clearErrorNotifications();
             EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_success));
-            prefs.putBoolean(PREF_LAST_SYNC_ATTEMPT_SUCCESS, true).apply();
+            SynchronizationSettings.setLastSynchronizationAttemptSuccess(true);
             return Result.success();
-        } catch (SyncServiceException e) {
+        } catch (Exception e) {
             EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_error));
-            prefs.putBoolean(PREF_LAST_SYNC_ATTEMPT_SUCCESS, false).apply();
+            SynchronizationSettings.setLastSynchronizationAttemptSuccess(false);
             Log.e(TAG, Log.getStackTraceString(e));
-            if (getRunAttemptCount() % 3 == 2) {
-                // Do not spam users with notification and retry before notifying
-                updateErrorNotification(e);
-            }
-            return Result.retry();
-        }
-    }
-
-    public static void clearQueue(Context context) {
-        executeLockedAsync(() ->
-                context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                        .putLong(PREF_LAST_SUBSCRIPTION_SYNC_TIMESTAMP, 0)
-                        .putLong(PREF_LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, 0)
-                        .putLong(PREF_LAST_SYNC_ATTEMPT_TIMESTAMP, 0)
-                        .putString(PREF_QUEUED_EPISODE_ACTIONS, "[]")
-                        .putString(PREF_QUEUED_FEEDS_ADDED, "[]")
-                        .putString(PREF_QUEUED_FEEDS_REMOVED, "[]")
-                        .apply());
-    }
 
-    public static void enqueueFeedAdded(Context context, String downloadUrl) {
-        if (!GpodnetPreferences.loggedIn()) {
-            return;
-        }
-        executeLockedAsync(() -> {
-            try {
-                SharedPreferences prefs = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-                String json = prefs.getString(PREF_QUEUED_FEEDS_ADDED, "[]");
-                JSONArray queue = new JSONArray(json);
-                queue.put(downloadUrl);
-                prefs.edit().putString(PREF_QUEUED_FEEDS_ADDED, queue.toString()).apply();
-            } catch (JSONException e) {
-                e.printStackTrace();
-            }
-            sync(context);
-        });
-    }
-
-    public static void enqueueFeedRemoved(Context context, String downloadUrl) {
-        if (!GpodnetPreferences.loggedIn()) {
-            return;
-        }
-        executeLockedAsync(() -> {
-            try {
-                SharedPreferences prefs = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-                String json = prefs.getString(PREF_QUEUED_FEEDS_REMOVED, "[]");
-                JSONArray queue = new JSONArray(json);
-                queue.put(downloadUrl);
-                prefs.edit().putString(PREF_QUEUED_FEEDS_REMOVED, queue.toString()).apply();
-            } catch (JSONException e) {
-                e.printStackTrace();
-            }
-            sync(context);
-        });
-    }
-
-    public static void enqueueEpisodeAction(Context context, EpisodeAction action) {
-        if (!GpodnetPreferences.loggedIn()) {
-            return;
-        }
-        executeLockedAsync(() -> {
-            try {
-                SharedPreferences prefs = context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-                String json = prefs.getString(PREF_QUEUED_EPISODE_ACTIONS, "[]");
-                JSONArray queue = new JSONArray(json);
-                queue.put(action.writeToJsonObject());
-                prefs.edit().putString(PREF_QUEUED_EPISODE_ACTIONS, queue.toString()).apply();
-            } catch (JSONException e) {
-                e.printStackTrace();
+            if (e instanceof SyncServiceException) {
+                if (getRunAttemptCount() % 3 == 2) {
+                    // Do not spam users with notification and retry before notifying
+                    updateErrorNotification(e);
+                }
+                return Result.retry();
+            } else {
+                updateErrorNotification(e);
+                return Result.failure();
             }
-            sync(context);
-        });
-    }
-
-    public static void enqueueEpisodePlayed(Context context, FeedMedia media, boolean completed) {
-        if (!GpodnetPreferences.loggedIn()) {
-            return;
         }
-        if (media.getItem() == null) {
-            return;
-        }
-        if (media.getStartPosition() < 0 || (!completed && media.getStartPosition() >= media.getPosition())) {
-            return;
-        }
-        EpisodeAction action = new EpisodeAction.Builder(media.getItem(), EpisodeAction.PLAY)
-                .currentTimestamp()
-                .started(media.getStartPosition() / 1000)
-                .position((completed ? media.getDuration() : media.getPosition()) / 1000)
-                .total(media.getDuration() / 1000)
-                .build();
-        SyncService.enqueueEpisodeAction(context, action);
     }
 
     public static void sync(Context context) {
@@ -211,13 +116,8 @@ public class SyncService extends Worker {
     }
 
     public static void fullSync(Context context) {
-        executeLockedAsync(() -> {
-            context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                    .putLong(PREF_LAST_SUBSCRIPTION_SYNC_TIMESTAMP, 0)
-                    .putLong(PREF_LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, 0)
-                    .putLong(PREF_LAST_SYNC_ATTEMPT_TIMESTAMP, 0)
-                    .apply();
-
+        LockingAsyncExecutor.executeLockedAsync(() -> {
+            SynchronizationSettings.resetTimestamps();
             OneTimeWorkRequest workRequest = getWorkRequest()
                     .setInitialDelay(0L, TimeUnit.SECONDS)
                     .build();
@@ -226,108 +126,14 @@ public class SyncService extends Worker {
         });
     }
 
-    private static OneTimeWorkRequest.Builder getWorkRequest() {
-        Constraints.Builder constraints = new Constraints.Builder();
-        if (UserPreferences.isAllowMobileFeedRefresh()) {
-            constraints.setRequiredNetworkType(NetworkType.CONNECTED);
-        } else {
-            constraints.setRequiredNetworkType(NetworkType.UNMETERED);
-        }
-
-        return new OneTimeWorkRequest.Builder(SyncService.class)
-                .setConstraints(constraints.build())
-                .setBackoffCriteria(BackoffPolicy.EXPONENTIAL, 10, TimeUnit.MINUTES)
-                .setInitialDelay(5L, TimeUnit.SECONDS); // Give it some time, so other actions can be queued
-    }
-
-    /**
-     * Take the lock and execute runnable (to prevent changes to preferences being lost when enqueueing while sync is
-     * in progress). If the lock is free, the runnable is directly executed in the calling thread to prevent overhead.
-     */
-    private static void executeLockedAsync(Runnable runnable) {
-        if (lock.tryLock()) {
-            try {
-                runnable.run();
-            } finally {
-                lock.unlock();
-            }
-        } else {
-            Completable.fromRunnable(() -> {
-                lock.lock();
-                try {
-                    runnable.run();
-                } finally {
-                    lock.unlock();
-                }
-            }).subscribeOn(Schedulers.io())
-                    .subscribe();
-        }
-    }
-
-    public static boolean isLastSyncSuccessful(Context context) {
-        return context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE)
-                .getBoolean(PREF_LAST_SYNC_ATTEMPT_SUCCESS, false);
-    }
-
-    public static long getLastSyncAttempt(Context context) {
-        return context.getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE)
-                .getLong(PREF_LAST_SYNC_ATTEMPT_TIMESTAMP, 0);
-    }
-
-    private List<EpisodeAction> getQueuedEpisodeActions() {
-        ArrayList<EpisodeAction> actions = new ArrayList<>();
-        try {
-            SharedPreferences prefs = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-            String json = prefs.getString(PREF_QUEUED_EPISODE_ACTIONS, "[]");
-            JSONArray queue = new JSONArray(json);
-            for (int i = 0; i < queue.length(); i++) {
-                actions.add(EpisodeAction.readFromJsonObject(queue.getJSONObject(i)));
-            }
-        } catch (JSONException e) {
-            e.printStackTrace();
-        }
-        return actions;
-    }
-
-    private List<String> getQueuedRemovedFeeds() {
-        ArrayList<String> actions = new ArrayList<>();
-        try {
-            SharedPreferences prefs = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-            String json = prefs.getString(PREF_QUEUED_FEEDS_REMOVED, "[]");
-            JSONArray queue = new JSONArray(json);
-            for (int i = 0; i < queue.length(); i++) {
-                actions.add(queue.getString(i));
-            }
-        } catch (JSONException e) {
-            e.printStackTrace();
-        }
-        return actions;
-    }
-
-    private List<String> getQueuedAddedFeeds() {
-        ArrayList<String> actions = new ArrayList<>();
-        try {
-            SharedPreferences prefs = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
-            String json = prefs.getString(PREF_QUEUED_FEEDS_ADDED, "[]");
-            JSONArray queue = new JSONArray(json);
-            for (int i = 0; i < queue.length(); i++) {
-                actions.add(queue.getString(i));
-            }
-        } catch (JSONException e) {
-            e.printStackTrace();
-        }
-        return actions;
-    }
-
-    private void syncSubscriptions() throws SyncServiceException {
-        final long lastSync = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE)
-                .getLong(PREF_LAST_SUBSCRIPTION_SYNC_TIMESTAMP, 0);
+    private void syncSubscriptions(ISyncService syncServiceImpl) throws SyncServiceException {
+        final long lastSync = SynchronizationSettings.getLastSubscriptionSynchronizationTimestamp();
         final List<String> localSubscriptions = DBReader.getFeedListDownloadUrls();
         SubscriptionChanges subscriptionChanges = syncServiceImpl.getSubscriptionChanges(lastSync);
         long newTimeStamp = subscriptionChanges.getTimestamp();
 
-        List<String> queuedRemovedFeeds = getQueuedRemovedFeeds();
-        List<String> queuedAddedFeeds = getQueuedAddedFeeds();
+        List<String> queuedRemovedFeeds = synchronizationQueueStorage.getQueuedRemovedFeeds();
+        List<String> queuedAddedFeeds = synchronizationQueueStorage.getQueuedAddedFeeds();
 
         Log.d(TAG, "Downloaded subscription changes: " + subscriptionChanges);
         for (String downloadUrl : subscriptionChanges.getAdded()) {
@@ -359,26 +165,21 @@ public class SyncService extends Worker {
             Log.d(TAG, "Added: " + StringUtils.join(queuedAddedFeeds, ", "));
             Log.d(TAG, "Removed: " + StringUtils.join(queuedRemovedFeeds, ", "));
 
-            lock.lock();
+            LockingAsyncExecutor.lock.lock();
             try {
                 UploadChangesResponse uploadResponse = syncServiceImpl
                         .uploadSubscriptionChanges(queuedAddedFeeds, queuedRemovedFeeds);
-                getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                        .putString(PREF_QUEUED_FEEDS_ADDED, "[]").apply();
-                getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                        .putString(PREF_QUEUED_FEEDS_REMOVED, "[]").apply();
+                synchronizationQueueStorage.clearFeedQueues();
                 newTimeStamp = uploadResponse.timestamp;
             } finally {
-                lock.unlock();
+                LockingAsyncExecutor.lock.unlock();
             }
         }
-        getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                .putLong(PREF_LAST_SUBSCRIPTION_SYNC_TIMESTAMP, newTimeStamp).apply();
+        SynchronizationSettings.setLastSubscriptionSynchronizationAttemptTimestamp(newTimeStamp);
     }
 
-    private void syncEpisodeActions() throws SyncServiceException {
-        final long lastSync = getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE)
-                .getLong(PREF_LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, 0);
+    private void syncEpisodeActions(ISyncService syncServiceImpl) throws SyncServiceException {
+        final long lastSync = SynchronizationSettings.getLastEpisodeActionSynchronizationTimestamp();
         EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_episodes_download));
         EpisodeActionChanges getResponse = syncServiceImpl.getEpisodeActionChanges(lastSync);
         long newTimeStamp = getResponse.getTimestamp();
@@ -387,7 +188,7 @@ public class SyncService extends Worker {
 
         // upload local actions
         EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_episodes_upload));
-        List<EpisodeAction> queuedEpisodeActions = getQueuedEpisodeActions();
+        List<EpisodeAction> queuedEpisodeActions = synchronizationQueueStorage.getQueuedEpisodeActions();
         if (lastSync == 0) {
             EventBus.getDefault().postSticky(new SyncServiceEvent(R.string.sync_status_upload_played));
             List<FeedItem> readItems = DBReader.getPlayedItems();
@@ -407,24 +208,21 @@ public class SyncService extends Worker {
             }
         }
         if (queuedEpisodeActions.size() > 0) {
-            lock.lock();
+            LockingAsyncExecutor.lock.lock();
             try {
                 Log.d(TAG, "Uploading " + queuedEpisodeActions.size() + " actions: "
                         + StringUtils.join(queuedEpisodeActions, ", "));
                 UploadChangesResponse postResponse = syncServiceImpl.uploadEpisodeActions(queuedEpisodeActions);
                 newTimeStamp = postResponse.timestamp;
                 Log.d(TAG, "Upload episode response: " + postResponse);
-                getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                        .putString(PREF_QUEUED_EPISODE_ACTIONS, "[]").apply();
+                synchronizationQueueStorage.clearEpisodeActionQueue();
             } finally {
-                lock.unlock();
+                LockingAsyncExecutor.lock.unlock();
             }
         }
-        getApplicationContext().getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE).edit()
-                .putLong(PREF_LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, newTimeStamp).apply();
+        SynchronizationSettings.setLastEpisodeActionSynchronizationAttemptTimestamp(newTimeStamp);
     }
 
-
     private synchronized void processEpisodeActions(List<EpisodeAction> remoteActions) {
         Log.d(TAG, "Processing " + remoteActions.size() + " actions");
         if (remoteActions.size() == 0) {
@@ -432,7 +230,8 @@ public class SyncService extends Worker {
         }
 
         Map<Pair<String, String>, EpisodeAction> playActionsToUpdate = EpisodeActionFilter
-                .getRemoteActionsOverridingLocalActions(remoteActions, getQueuedEpisodeActions());
+                .getRemoteActionsOverridingLocalActions(remoteActions,
+                        synchronizationQueueStorage.getQueuedEpisodeActions());
         LongList queueToBeRemoved = new LongList();
         List<FeedItem> updatedItems = new ArrayList<>();
         for (EpisodeAction action : playActionsToUpdate.values()) {
@@ -442,20 +241,24 @@ public class SyncService extends Worker {
                 Log.i(TAG, "Unknown feed item: " + action);
                 continue;
             }
+            if (feedItem.getMedia() == null) {
+                Log.i(TAG, "Feed item has no media: " + action);
+                continue;
+            }
             if (action.getAction() == EpisodeAction.NEW) {
                 DBWriter.markItemPlayed(feedItem, FeedItem.UNPLAYED, true);
                 continue;
             }
-            Log.d(TAG, "Most recent play action: " + action.toString());
-            FeedMedia media = feedItem.getMedia();
-            media.setPosition(action.getPosition() * 1000);
+            feedItem.getMedia().setPosition(action.getPosition() * 1000);
             if (FeedItemUtil.hasAlmostEnded(feedItem.getMedia())) {
-                Log.d(TAG, "Marking as played");
+                Log.d(TAG, "Marking as played: " + action);
                 feedItem.setPlayed(true);
+                feedItem.getMedia().setPosition(0);
                 queueToBeRemoved.add(feedItem.getId());
+            } else {
+                Log.d(TAG, "Setting position: " + action);
             }
             updatedItems.add(feedItem);
-
         }
         DBWriter.removeQueueItem(getApplicationContext(), false, queueToBeRemoved.toArray());
         DBReader.loadAdditionalFeedItemListData(updatedItems);
@@ -469,7 +272,7 @@ public class SyncService extends Worker {
         nm.cancel(R.id.notification_gpodnet_sync_autherror);
     }
 
-    private void updateErrorNotification(SyncServiceException exception) {
+    private void updateErrorNotification(Exception exception) {
         if (!UserPreferences.gpodnetNotificationsEnabled()) {
             Log.d(TAG, "Skipping sync error notification because of user setting");
             return;
@@ -486,6 +289,7 @@ public class SyncService extends Worker {
                 NotificationUtils.CHANNEL_ID_SYNC_ERROR)
                 .setContentTitle(getApplicationContext().getString(R.string.gpodnetsync_error_title))
                 .setContentText(description)
+                .setStyle(new NotificationCompat.BigTextStyle().bigText(description))
                 .setContentIntent(pendingIntent)
                 .setSmallIcon(R.drawable.ic_notification_sync_error)
                 .setAutoCancel(true)
@@ -495,4 +299,36 @@ public class SyncService extends Worker {
                 .getSystemService(Context.NOTIFICATION_SERVICE);
         nm.notify(R.id.notification_gpodnet_sync_error, notification);
     }
+
+    private static OneTimeWorkRequest.Builder getWorkRequest() {
+        Constraints.Builder constraints = new Constraints.Builder();
+        if (UserPreferences.isAllowMobileFeedRefresh()) {
+            constraints.setRequiredNetworkType(NetworkType.CONNECTED);
+        } else {
+            constraints.setRequiredNetworkType(NetworkType.UNMETERED);
+        }
+
+        return new OneTimeWorkRequest.Builder(SyncService.class)
+                .setConstraints(constraints.build())
+                .setBackoffCriteria(BackoffPolicy.EXPONENTIAL, 10, TimeUnit.MINUTES)
+                .setInitialDelay(5L, TimeUnit.SECONDS); // Give it some time, so other actions can be queued
+    }
+
+    private ISyncService getActiveSyncProvider() {
+        String selectedSyncProviderKey = SynchronizationSettings.getSelectedSyncProviderKey();
+        SynchronizationProviderViewData selectedService = SynchronizationProviderViewData
+                .valueOf(selectedSyncProviderKey);
+        switch (selectedService) {
+            case GPODDER_NET:
+                return new GpodnetService(AntennapodHttpClient.getHttpClient(),
+                        SynchronizationCredentials.getHosturl(), SynchronizationCredentials.getDeviceID(),
+                        SynchronizationCredentials.getUsername(), SynchronizationCredentials.getPassword());
+            case NEXTCLOUD_GPODDER:
+                return new NextcloudSyncService(AntennapodHttpClient.getHttpClient(),
+                        SynchronizationCredentials.getHosturl(), SynchronizationCredentials.getUsername(),
+                        SynchronizationCredentials.getPassword());
+            default:
+                return null;
+        }
+    }
 }
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationCredentials.java b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationCredentials.java
new file mode 100644
index 000000000..e08bc66ad
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationCredentials.java
@@ -0,0 +1,67 @@
+package de.danoeh.antennapod.core.sync;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+import de.danoeh.antennapod.core.ClientConfig;
+import de.danoeh.antennapod.core.preferences.UserPreferences;
+import de.danoeh.antennapod.core.sync.queue.SynchronizationQueueSink;
+
+/**
+ * Manages preferences for accessing gpodder.net service and other sync providers
+ */
+public class SynchronizationCredentials {
+
+    private SynchronizationCredentials() {
+    }
+
+    private static final String PREF_NAME = "gpodder.net";
+    private static final String PREF_USERNAME = "de.danoeh.antennapod.preferences.gpoddernet.username";
+    private static final String PREF_PASSWORD = "de.danoeh.antennapod.preferences.gpoddernet.password";
+    private static final String PREF_DEVICEID = "de.danoeh.antennapod.preferences.gpoddernet.deviceID";
+    private static final String PREF_HOSTNAME = "prefGpodnetHostname";
+
+    private static SharedPreferences getPreferences() {
+        return ClientConfig.applicationCallbacks.getApplicationInstance()
+                .getSharedPreferences(PREF_NAME, Context.MODE_PRIVATE);
+    }
+
+    public static String getUsername() {
+        return getPreferences().getString(PREF_USERNAME, null);
+    }
+
+    public static void setUsername(String username) {
+        getPreferences().edit().putString(PREF_USERNAME, username).apply();
+    }
+
+    public static String getPassword() {
+        return getPreferences().getString(PREF_PASSWORD, null);
+    }
+
+    public static void setPassword(String password) {
+        getPreferences().edit().putString(PREF_PASSWORD, password).apply();
+    }
+
+    public static String getDeviceID() {
+        return getPreferences().getString(PREF_DEVICEID, null);
+    }
+
+    public static void setDeviceID(String deviceID) {
+        getPreferences().edit().putString(PREF_DEVICEID, deviceID).apply();
+    }
+
+    public static String getHosturl() {
+        return getPreferences().getString(PREF_HOSTNAME, null);
+    }
+
+    public static void setHosturl(String value) {
+        getPreferences().edit().putString(PREF_HOSTNAME, value).apply();
+    }
+
+    public static synchronized void clear(Context context) {
+        setUsername(null);
+        setPassword(null);
+        setDeviceID(null);
+        SynchronizationQueueSink.clearQueue(context);
+        UserPreferences.setGpodnetNotificationsEnabled();
+    }
+}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationProviderViewData.java b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationProviderViewData.java
new file mode 100644
index 000000000..cba713f60
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationProviderViewData.java
@@ -0,0 +1,47 @@
+package de.danoeh.antennapod.core.sync;
+
+import de.danoeh.antennapod.core.R;
+
+public enum SynchronizationProviderViewData {
+    GPODDER_NET(
+            "GPODDER_NET",
+            R.string.gpodnet_description,
+            R.drawable.gpodder_icon
+    ),
+    NEXTCLOUD_GPODDER(
+            "NEXTCLOUD_GPODDER",
+            R.string.synchronization_summary_nextcloud,
+            R.drawable.nextcloud_logo
+    );
+
+    public static SynchronizationProviderViewData fromIdentifier(String provider) {
+        for (SynchronizationProviderViewData synchronizationProvider : SynchronizationProviderViewData.values()) {
+            if (synchronizationProvider.getIdentifier().equals(provider)) {
+                return synchronizationProvider;
+            }
+        }
+        return null;
+    }
+
+    private final String identifier;
+    private final int iconResource;
+    private final int summaryResource;
+
+    SynchronizationProviderViewData(String identifier, int summaryResource, int iconResource) {
+        this.identifier = identifier;
+        this.iconResource = iconResource;
+        this.summaryResource = summaryResource;
+    }
+
+    public String getIdentifier() {
+        return identifier;
+    }
+
+    public int getIconResource() {
+        return iconResource;
+    }
+
+    public int getSummaryResource() {
+        return summaryResource;
+    }
+}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationSettings.java b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationSettings.java
new file mode 100644
index 000000000..1a53ac0fb
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/SynchronizationSettings.java
@@ -0,0 +1,83 @@
+package de.danoeh.antennapod.core.sync;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+
+import de.danoeh.antennapod.core.ClientConfig;
+
+public class SynchronizationSettings {
+
+    public static final String LAST_SYNC_ATTEMPT_TIMESTAMP = "last_sync_attempt_timestamp";
+    private static final String NAME = "synchronization";
+    private static final String SELECTED_SYNC_PROVIDER = "selected_sync_provider";
+    private static final String LAST_SYNC_ATTEMPT_SUCCESS = "last_sync_attempt_success";
+    private static final String LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP = "last_episode_actions_sync_timestamp";
+    private static final String LAST_SUBSCRIPTION_SYNC_TIMESTAMP = "last_sync_timestamp";
+
+    public static boolean isProviderConnected() {
+        return getSelectedSyncProviderKey() != null;
+    }
+
+    public static void resetTimestamps() {
+        getSharedPreferences().edit()
+                .putLong(LAST_SUBSCRIPTION_SYNC_TIMESTAMP, 0)
+                .putLong(LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, 0)
+                .putLong(LAST_SYNC_ATTEMPT_TIMESTAMP, 0)
+                .apply();
+    }
+
+    public static boolean isLastSyncSuccessful() {
+        return getSharedPreferences().getBoolean(LAST_SYNC_ATTEMPT_SUCCESS, false);
+    }
+
+    public static long getLastSyncAttempt() {
+        return getSharedPreferences().getLong(LAST_SYNC_ATTEMPT_TIMESTAMP, 0);
+    }
+
+    public static void setSelectedSyncProvider(SynchronizationProviderViewData provider) {
+        getSharedPreferences()
+                .edit()
+                .putString(SELECTED_SYNC_PROVIDER, provider == null ? null : provider.getIdentifier())
+                .apply();
+    }
+
+    public static String getSelectedSyncProviderKey() {
+        return getSharedPreferences().getString(SELECTED_SYNC_PROVIDER, null);
+    }
+
+    public static void updateLastSynchronizationAttempt() {
+        getSharedPreferences().edit()
+                .putLong(LAST_SYNC_ATTEMPT_TIMESTAMP, System.currentTimeMillis())
+                .apply();
+    }
+
+    public static void setLastSynchronizationAttemptSuccess(boolean isSuccess) {
+        getSharedPreferences().edit()
+                .putBoolean(LAST_SYNC_ATTEMPT_SUCCESS, isSuccess)
+                .apply();
+    }
+
+    public static long getLastSubscriptionSynchronizationTimestamp() {
+        return getSharedPreferences().getLong(LAST_SUBSCRIPTION_SYNC_TIMESTAMP, 0);
+    }
+
+    public static void setLastSubscriptionSynchronizationAttemptTimestamp(long newTimeStamp) {
+        getSharedPreferences().edit()
+                .putLong(LAST_SUBSCRIPTION_SYNC_TIMESTAMP, newTimeStamp).apply();
+    }
+
+    public static long getLastEpisodeActionSynchronizationTimestamp() {
+        return getSharedPreferences()
+                .getLong(LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, 0);
+    }
+
+    public static void setLastEpisodeActionSynchronizationAttemptTimestamp(long timestamp) {
+        getSharedPreferences().edit()
+                .putLong(LAST_EPISODE_ACTIONS_SYNC_TIMESTAMP, timestamp).apply();
+    }
+
+    private static SharedPreferences getSharedPreferences() {
+        return ClientConfig.applicationCallbacks.getApplicationInstance()
+                .getSharedPreferences(NAME, Context.MODE_PRIVATE);
+    }
+}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueSink.java b/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueSink.java
new file mode 100644
index 000000000..445faf60f
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueSink.java
@@ -0,0 +1,67 @@
+package de.danoeh.antennapod.core.sync.queue;
+
+import android.content.Context;
+
+import de.danoeh.antennapod.core.sync.LockingAsyncExecutor;
+import de.danoeh.antennapod.core.sync.SyncService;
+import de.danoeh.antennapod.core.sync.SynchronizationSettings;
+import de.danoeh.antennapod.model.feed.FeedMedia;
+import de.danoeh.antennapod.net.sync.model.EpisodeAction;
+
+public class SynchronizationQueueSink {
+
+    public static void clearQueue(Context context) {
+        LockingAsyncExecutor.executeLockedAsync(new SynchronizationQueueStorage(context)::clearQueue);
+    }
+
+    public static void enqueueFeedAddedIfSynchronizationIsActive(Context context, String downloadUrl) {
+        if (!SynchronizationSettings.isProviderConnected()) {
+            return;
+        }
+        LockingAsyncExecutor.executeLockedAsync(() -> {
+            new SynchronizationQueueStorage(context).enqueueFeedAdded(downloadUrl);
+            SyncService.sync(context);
+        });
+    }
+
+    public static void enqueueFeedRemovedIfSynchronizationIsActive(Context context, String downloadUrl) {
+        if (!SynchronizationSettings.isProviderConnected()) {
+            return;
+        }
+        LockingAsyncExecutor.executeLockedAsync(() -> {
+            new SynchronizationQueueStorage(context).enqueueFeedRemoved(downloadUrl);
+            SyncService.sync(context);
+        });
+    }
+
+    public static void enqueueEpisodeActionIfSynchronizationIsActive(Context context, EpisodeAction action) {
+        if (!SynchronizationSettings.isProviderConnected()) {
+            return;
+        }
+        LockingAsyncExecutor.executeLockedAsync(() -> {
+            new SynchronizationQueueStorage(context).enqueueEpisodeAction(action);
+            SyncService.sync(context);
+        });
+    }
+
+    public static void enqueueEpisodePlayedIfSynchronizationIsActive(Context context, FeedMedia media,
+                                                                     boolean completed) {
+        if (!SynchronizationSettings.isProviderConnected()) {
+            return;
+        }
+        if (media.getItem() == null) {
+            return;
+        }
+        if (media.getStartPosition() < 0 || (!completed && media.getStartPosition() >= media.getPosition())) {
+            return;
+        }
+        EpisodeAction action = new EpisodeAction.Builder(media.getItem(), EpisodeAction.PLAY)
+                .currentTimestamp()
+                .started(media.getStartPosition() / 1000)
+                .position((completed ? media.getDuration() : media.getPosition()) / 1000)
+                .total(media.getDuration() / 1000)
+                .build();
+        enqueueEpisodeActionIfSynchronizationIsActive(context, action);
+    }
+
+}
diff --git a/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueStorage.java b/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueStorage.java
new file mode 100644
index 000000000..5c6d58fe3
--- /dev/null
+++ b/core/src/main/java/de/danoeh/antennapod/core/sync/queue/SynchronizationQueueStorage.java
@@ -0,0 +1,140 @@
+package de.danoeh.antennapod.core.sync.queue;
+
+import android.content.Context;
+import android.content.SharedPreferences;
+
+import org.json.JSONArray;
+import org.json.JSONException;
+
+import java.util.ArrayList;
+
+import de.danoeh.antennapod.core.sync.SynchronizationSettings;
+import de.danoeh.antennapod.net.sync.model.EpisodeAction;
+
+public class SynchronizationQueueStorage {
+
+    private static final String NAME = "synchronization";
+    private static final String QUEUED_EPISODE_ACTIONS = "sync_queued_episode_actions";
+    private static final String QUEUED_FEEDS_REMOVED = "sync_removed";
+    private static final String QUEUED_FEEDS_ADDED = "sync_added";
+    private final SharedPreferences sharedPreferences;
+
+    public SynchronizationQueueStorage(Context context) {
+        this.sharedPreferences = context.getSharedPreferences(NAME, Context.MODE_PRIVATE);
+    }
+
+    public ArrayList<EpisodeAction> getQueuedEpisodeActions() {
+        ArrayList<EpisodeAction> actions = new ArrayList<>();
+        try {
+            String json = getSharedPreferences()
+                    .getString(QUEUED_EPISODE_ACTIONS, "[]");
+            JSONArray queue = new JSONArray(json);
+            for (int i = 0; i < queue.length(); i++) {
+                actions.add(EpisodeAction.readFromJsonObject(queue.getJSONObject(i)));
+            }
+        } catch (JSONException e) {
+            e.printStackTrace();
+        }
+        return actions;
+    }
+
+    public ArrayList<String> getQueuedRemovedFeeds() {
+        ArrayList<String> removedFeedUrls = new ArrayList<>();
+        try {
+            String json = getSharedPreferences()
+                    .getString(QUEUED_FEEDS_REMOVED, "[]");
+            JSONArray queue = new JSONArray(json);
+            for (int i = 0; i < queue.length(); i++) {
+                removedFeedUrls.add(queue.getString(i));
+            }
+        } catch (JSONException e) {
+            e.printStackTrace();
+        }
+        return removedFeedUrls;
+
+    }
+
+    public ArrayList<String> getQueuedAddedFeeds() {
+        ArrayList<String> addedFeedUrls = new ArrayList<>();
+        try {
+            String json = getSharedPreferences()
+                    .getString(QUEUED_FEEDS_ADDED, "[]");
+            JSONArray queue = new JSONArray(json);
+            for (int i = 0; i < queue.length(); i++) {
+                addedFeedUrls.add(queue.getString(i));
+            }
+        } catch (JSONException e) {
+            e.printStackTrace();
+        }
+        return addedFeedUrls;
+    }
+
+    public void clearEpisodeActionQueue() {
+        getSharedPreferences().edit()
+                .putString(QUEUED_EPISODE_ACTIONS, "[]").apply();
+
+    }
+
+    public void clearFeedQueues() {
+        getSharedPreferences().edit()
+                .putString(QUEUED_FEEDS_ADDED, "[]")
+                .putString(QUEUED_FEEDS_REMOVED, "[]")
+                .apply();
+    }
+
+    protected void clearQueue() {
+        SynchronizationSettings.resetTimestamps();
+        getSharedPreferences().edit()
+                .putString(QUEUED_EPISODE_ACTIONS, "[]")
+                .putString(QUEUED_FEEDS_ADDED, "[]")
+                .putString(QUEUED_FEEDS_REMOVED, "[]")
+                .apply();
+
+    }
+
+    protected void enqueueFeedAdded(String downloadUrl) {
+        SharedPreferences sharedPreferences = getSharedPreferences();
+        String json = sharedPreferences
+                .getString(QUEUED_FEEDS_ADDED, "[]");
+        try {
+            JSONArray queue = new JSONArray(json);
+            queue.put(downloadUrl);
+            sharedPreferences
+                    .edit().putString(QUEUED_FEEDS_ADDED, queue.toString()).apply();
+
+        } catch (JSONException jsonException) {
+            jsonException.printStackTrace();
+        }
+    }
+
+    protected void enqueueFeedRemoved(String downloadUrl) {
+        SharedPreferences sharedPreferences = getSharedPreferences();
+        String json = sharedPreferences.getString(QUEUED_FEEDS_REMOVED, "[]");
+        try {
+            JSONArray queue = new JSONArray(json);
+            queue.put(downloadUrl);
+            sharedPreferences.edit().putString(QUEUED_FEEDS_REMOVED, queue.toString())
+                    .apply();
+        } catch (JSONException jsonException) {
+            jsonException.printStackTrace();
+        }
+    }
+
+    protected void enqueueEpisodeAction(EpisodeAction action) {
+        SharedPreferences sharedPreferences = getSharedPreferences();
+        String json = sharedPreferences.getString(QUEUED_EPISODE_ACTIONS, "[]");
+        try {
+            JSONArray queue = new JSONArray(json);
+            queue.put(action.writeToJsonObject());
+            sharedPreferences.edit().putString(
+                    QUEUED_EPISODE_ACTIONS, queue.toString()
+            ).apply();
+        } catch (JSONException jsonException) {
+            jsonException.printStackTrace();
+        }
+    }
+
+    private SharedPreferences getSharedPreferences() {
+        return sharedPreferences;
+    }
+}
diff --git a/core/src/main/res/drawable-nodpi/nextcloud_logo.png b/core/src/main/res/drawable-nodpi/nextcloud_logo.png
new file mode 100644
index 000000000..2164e37fb
--- /dev/null
+++ b/core/src/main/res/drawable-nodpi/nextcloud_logo.png
diff --git a/core/src/main/res/values/strings.xml b/core/src/main/res/values/strings.xml
index 5f509a9b6..7aa32abe3 100644
--- a/core/src/main/res/values/strings.xml
+++ b/core/src/main/res/values/strings.xml
@@ -356,7 +356,7 @@
     <string name="storage_sum">Episode auto delete, Import, Export</string>
     <string name="project_pref">Project</string>
     <string name="synchronization_pref">Synchronization</string>
-    <string name="synchronization_sum">Synchronize with other devices using gpodder.net</string>
+    <string name="synchronization_sum">Synchronize with other devices</string>
     <string name="automation">Automation</string>
     <string name="download_pref_details">Details</string>
     <string name="import_export_pref">Import/Export</string>
@@ -447,17 +447,20 @@
     <string name="pref_theme_title_dark">Dark</string>
     <string name="pref_theme_title_trueblack">Black (AMOLED ready)</string>
     <string name="pref_episode_cache_unlimited">Unlimited</string>
-    <string name="pref_gpodnet_authenticate_title">Login</string>
-    <string name="pref_gpodnet_authenticate_sum">Login with your gpodder.net account in order to sync your subscriptions.</string>
-    <string name="pref_gpodnet_logout_title">Logout</string>
-    <string name="pref_gpodnet_logout_toast">Logout was successful</string>
+    <string name="synchronization_logout">Logout</string>
+    <string name="pref_synchronization_logout_toast">Logout was successful</string>
     <string name="pref_gpodnet_setlogin_information_title">Change login information</string>
     <string name="pref_gpodnet_setlogin_information_sum">Change the login information for your gpodder.net account.</string>
-    <string name="pref_gpodnet_sync_changes_title">Synchronize now</string>
-    <string name="pref_gpodnet_sync_changes_sum">Sync subscription and episode state changes with gpodder.net.</string>
-    <string name="pref_gpodnet_full_sync_title">Force full synchronization</string>
-    <string name="pref_gpodnet_full_sync_sum">Sync all subscriptions and episode states with gpodder.net.</string>
-    <string name="pref_gpodnet_login_status"><![CDATA[Logged in as <i>%1$s</i> with device <i>%2$s</i>]]></string>
+    <string name="synchronization_sync_changes_title">Synchronize now</string>
+    <string name="synchronization_full_sync_title">Force full synchronization</string>
+    <string name="synchronization_login_status"><![CDATA[Logged in as <i>%1$s</i> on <i>%2$s</i>. <br/><br/>You can choose your synchronization provider again once you have logged out]]></string>
+    <string name="synchronization_summary_unchoosen">You can choose from multiple providers to synchronize your subscriptions and episode play state with</string>
+    <string name="synchronization_summary_nextcloud">Gpoddersync is an open-source Nextcloud app that you can easily install on your own server. The app is independent of the AntennaPod project.</string>
+    <string name="synchronization_nextcloud_authenticate_browser">Grant access using the opened web browser and come back to AntennaPod.</string>
+    <string name="synchronization_choose_title">Choose synchronization provider</string>
+    <string name="synchronization_force_sync_summary">Re-synchronize all subscriptions and episode states</string>
+    <string name="synchronization_sync_summary">Synchronize subscription and episode state changes</string>
+    <string name="dialog_choose_sync_service_title">Choose synchronization provider</string>
     <string name="pref_playback_speed_sum">Customize the speeds available for variable speed playback</string>
     <string name="pref_feed_playback_speed_sum">The speed to use when starting audio playback for episodes in this podcast</string>
     <string name="pref_feed_skip">Auto Skip</string>
diff --git a/core/src/test/java/de/danoeh/antennapod/core/sync/GuidValidatorTest.java b/core/src/test/java/de/danoeh/antennapod/core/sync/GuidValidatorTest.java
index 356a7f77e..552f7d70a 100644
--- a/core/src/test/java/de/danoeh/antennapod/core/sync/GuidValidatorTest.java
+++ b/core/src/test/java/de/danoeh/antennapod/core/sync/GuidValidatorTest.java
@@ -14,5 +14,6 @@ public class GuidValidatorTest extends TestCase {
         assertFalse(GuidValidator.isValidGuid("\n"));
         assertFalse(GuidValidator.isValidGuid(" \n"));
         assertFalse(GuidValidator.isValidGuid(null));
+        assertFalse(GuidValidator.isValidGuid("null"));
     }
 }
\ No newline at end of file