Get rid of deprecated AsyncTask

6 files changed, 586 insertions, 550 deletions

diff --git a/app/src/main/java/it/niedermann/owncloud/notes/main/MainViewModel.java b/app/src/main/java/it/niedermann/owncloud/notes/main/MainViewModel.java
index 4259d7f0..8c1b89f1 100644
--- a/app/src/main/java/it/niedermann/owncloud/notes/main/MainViewModel.java
+++ b/app/src/main/java/it/niedermann/owncloud/notes/main/MainViewModel.java
@@ -32,7 +32,7 @@ import it.niedermann.owncloud.notes.branding.BrandingUtil;
 import it.niedermann.owncloud.notes.main.navigation.NavigationAdapter;
 import it.niedermann.owncloud.notes.main.navigation.NavigationItem;
 import it.niedermann.owncloud.notes.persistence.CapabilitiesClient;
-import it.niedermann.owncloud.notes.persistence.NoteServerSyncHelper;
+import it.niedermann.owncloud.notes.persistence.NotesServerSyncHelper;
 import it.niedermann.owncloud.notes.persistence.NotesDatabase;
 import it.niedermann.owncloud.notes.persistence.entity.Account;
 import it.niedermann.owncloud.notes.persistence.entity.CategoryWithNotesCount;
@@ -375,7 +375,7 @@ public class MainViewModel extends AndroidViewModel {
                 return new MutableLiveData<>(false);
             } else {
                 Log.v(TAG, "[synchronize] - currentAccount: " + currentAccount.getAccountName());
-                NoteServerSyncHelper syncHelper = db.getNoteServerSyncHelper();
+                NotesServerSyncHelper syncHelper = db.getNoteServerSyncHelper();
                 if (!syncHelper.isSyncPossible()) {
                     syncHelper.updateNetworkStatus();
                 }
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NoteServerSyncHelper.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NoteServerSyncHelper.java
deleted file mode 100644
index a6aebf7a..00000000
--- a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NoteServerSyncHelper.java
+++ /dev/null
@@ -1,542 +0,0 @@
-package it.niedermann.owncloud.notes.persistence;
-
-import android.accounts.NetworkErrorException;
-import android.content.BroadcastReceiver;
-import android.content.Context;
-import android.content.Intent;
-import android.content.IntentFilter;
-import android.content.SharedPreferences;
-import android.net.ConnectivityManager;
-import android.net.NetworkInfo;
-import android.os.AsyncTask;
-import android.util.Log;
-
-import androidx.annotation.NonNull;
-import androidx.lifecycle.LiveData;
-import androidx.lifecycle.MutableLiveData;
-import androidx.preference.PreferenceManager;
-
-import com.nextcloud.android.sso.AccountImporter;
-import com.nextcloud.android.sso.exceptions.NextcloudFilesAppAccountNotFoundException;
-import com.nextcloud.android.sso.exceptions.NextcloudHttpRequestFailedException;
-import com.nextcloud.android.sso.exceptions.NoCurrentAccountSelectedException;
-import com.nextcloud.android.sso.exceptions.TokenMismatchException;
-import com.nextcloud.android.sso.helper.SingleAccountHelper;
-import com.nextcloud.android.sso.model.SingleSignOnAccount;
-
-import java.util.ArrayList;
-import java.util.Calendar;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.List;
-import java.util.Map;
-import java.util.Objects;
-import java.util.Set;
-
-import it.niedermann.owncloud.notes.R;
-import it.niedermann.owncloud.notes.persistence.entity.Account;
-import it.niedermann.owncloud.notes.persistence.entity.Note;
-import it.niedermann.owncloud.notes.shared.model.DBStatus;
-import it.niedermann.owncloud.notes.shared.model.ISyncCallback;
-import it.niedermann.owncloud.notes.shared.model.ServerResponse;
-import it.niedermann.owncloud.notes.shared.model.SyncResultStatus;
-import it.niedermann.owncloud.notes.shared.util.SSOUtil;
-
-import static androidx.lifecycle.Transformations.distinctUntilChanged;
-import static it.niedermann.owncloud.notes.shared.model.DBStatus.LOCAL_DELETED;
-import static it.niedermann.owncloud.notes.shared.util.NoteUtil.generateNoteExcerpt;
-import static java.net.HttpURLConnection.HTTP_NOT_FOUND;
-import static java.net.HttpURLConnection.HTTP_NOT_MODIFIED;
-
-/**
- * Helps to synchronize the Database to the Server.
- */
-public class NoteServerSyncHelper {
-
-    private static final String TAG = NoteServerSyncHelper.class.getSimpleName();
-
-    private static NoteServerSyncHelper instance;
-
-    private final NotesDatabase db;
-    private final Context context;
-
-    // Track network connection changes using a BroadcastReceiver
-    private boolean isSyncPossible = false;
-    private boolean networkConnected = false;
-    private String syncOnlyOnWifiKey;
-    private boolean syncOnlyOnWifi;
-    private final MutableLiveData<Boolean> syncStatus = new MutableLiveData<>(false);
-    private final MutableLiveData<ArrayList<Throwable>> syncErrors = new MutableLiveData<>();
-
-    /**
-     * @see <a href="https://stackoverflow.com/a/3104265">Do not make this a local variable.</a>
-     */
-    @SuppressWarnings("FieldCanBeLocal")
-    private final SharedPreferences.OnSharedPreferenceChangeListener onSharedPreferenceChangeListener = (SharedPreferences prefs, String key) -> {
-        if (syncOnlyOnWifiKey.equals(key)) {
-            syncOnlyOnWifi = prefs.getBoolean(syncOnlyOnWifiKey, false);
-            updateNetworkStatus();
-        }
-    };
-
-    private final BroadcastReceiver networkReceiver = new BroadcastReceiver() {
-        @Override
-        public void onReceive(Context context, Intent intent) {
-            updateNetworkStatus();
-            if (isSyncPossible() && SSOUtil.isConfigured(context)) {
-                new Thread(() -> {
-                    try {
-                        scheduleSync(db.getAccountDao().getAccountByName(SingleAccountHelper.getCurrentSingleSignOnAccount(context).name), false);
-                    } catch (NextcloudFilesAppAccountNotFoundException | NoCurrentAccountSelectedException e) {
-                        Log.v(TAG, "Can not select current SingleSignOn account after network changed, do not sync.");
-                    }
-                }).start();
-            }
-        }
-    };
-
-    // current state of the synchronization
-    private final Map<Long, Boolean> syncActive = new HashMap<>();
-    private final Map<Long, Boolean> syncScheduled = new HashMap<>();
-
-    // list of callbacks for both parts of synchronization
-    private final Map<Long, List<ISyncCallback>> callbacksPush = new HashMap<>();
-    private final Map<Long, List<ISyncCallback>> callbacksPull = new HashMap<>();
-
-    private NoteServerSyncHelper(NotesDatabase db) {
-        this.db = db;
-        this.context = db.getContext();
-        this.syncOnlyOnWifiKey = context.getApplicationContext().getResources().getString(R.string.pref_key_wifi_only);
-
-        // Registers BroadcastReceiver to track network connection changes.
-        context.getApplicationContext().registerReceiver(networkReceiver, new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION));
-
-        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.context.getApplicationContext());
-        prefs.registerOnSharedPreferenceChangeListener(onSharedPreferenceChangeListener);
-        syncOnlyOnWifi = prefs.getBoolean(syncOnlyOnWifiKey, false);
-
-        updateNetworkStatus();
-    }
-
-    /**
-     * Get (or create) instance from NoteServerSyncHelper.
-     * This has to be a singleton in order to realize correct registering and unregistering of
-     * the BroadcastReceiver, which listens on changes of network connectivity.
-     *
-     * @param db {@link NotesDatabase}
-     * @return NoteServerSyncHelper
-     */
-    public static synchronized NoteServerSyncHelper getInstance(NotesDatabase db) {
-        if (instance == null) {
-            instance = new NoteServerSyncHelper(db);
-        }
-        return instance;
-    }
-
-    @Override
-    protected void finalize() throws Throwable {
-        context.getApplicationContext().unregisterReceiver(networkReceiver);
-        super.finalize();
-    }
-
-    /**
-     * Synchronization is only possible, if there is an active network connection.
-     * <p>
-     * This method respects the user preference "Sync on Wi-Fi only".
-     * <p>
-     * NoteServerSyncHelper observes changes in the network connection.
-     * The current state can be retrieved with this method.
-     *
-     * @return true if sync is possible, otherwise false.
-     */
-    public boolean isSyncPossible() {
-        return isSyncPossible;
-    }
-
-    public boolean isNetworkConnected() {
-        return networkConnected;
-    }
-
-    public boolean isSyncOnlyOnWifi() {
-        return syncOnlyOnWifi;
-    }
-
-    /**
-     * Adds a callback method to the NoteServerSyncHelper for the synchronization part push local changes to the server.
-     * All callbacks will be executed once the synchronization operations are done.
-     * After execution the callback will be deleted, so it has to be added again if it shall be
-     * executed the next time all synchronize operations are finished.
-     *
-     * @param callback Implementation of ISyncCallback, contains one method that shall be executed.
-     */
-    public void addCallbackPush(Account account, ISyncCallback callback) {
-        if (account == null) {
-            Log.i(TAG, "ssoAccount is null. Is this a local account?");
-            callback.onScheduled();
-            callback.onFinish();
-        } else {
-            if (!callbacksPush.containsKey(account.getId())) {
-                callbacksPush.put(account.getId(), new ArrayList<>());
-            }
-            Objects.requireNonNull(callbacksPush.get(account.getId())).add(callback);
-        }
-    }
-
-    /**
-     * Adds a callback method to the NoteServerSyncHelper for the synchronization part pull remote changes from the server.
-     * All callbacks will be executed once the synchronization operations are done.
-     * After execution the callback will be deleted, so it has to be added again if it shall be
-     * executed the next time all synchronize operations are finished.
-     *
-     * @param callback Implementation of ISyncCallback, contains one method that shall be executed.
-     */
-    public void addCallbackPull(Account account, ISyncCallback callback) {
-        if (account == null) {
-            Log.i(TAG, "ssoAccount is null. Is this a local account?");
-            callback.onScheduled();
-            callback.onFinish();
-        } else {
-            if (!callbacksPull.containsKey(account.getId())) {
-                callbacksPull.put(account.getId(), new ArrayList<>());
-            }
-            Objects.requireNonNull(callbacksPull.get(account.getId())).add(callback);
-        }
-    }
-
-    /**
-     * Schedules a synchronization and start it directly, if the network is connected and no
-     * synchronization is currently running.
-     *
-     * @param onlyLocalChanges Whether to only push local changes to the server or to also load the whole list of notes from the server.
-     */
-    public void scheduleSync(Account account, boolean onlyLocalChanges) {
-        if (account == null) {
-            Log.i(TAG, SingleSignOnAccount.class.getSimpleName() + " is null. Is this a local account?");
-        } else {
-            if (syncActive.get(account.getId()) == null) {
-                syncActive.put(account.getId(), false);
-            }
-            Log.d(TAG, "Sync requested (" + (onlyLocalChanges ? "onlyLocalChanges" : "full") + "; " + (Boolean.TRUE.equals(syncActive.get(account.getId())) ? "sync active" : "sync NOT active") + ") ...");
-            if (isSyncPossible() && (!Boolean.TRUE.equals(syncActive.get(account.getId())) || onlyLocalChanges)) {
-                try {
-                    SingleSignOnAccount ssoAccount = AccountImporter.getSingleSignOnAccount(context, account.getAccountName());
-                    Log.d(TAG, "... starting now");
-                    final NotesClient notesClient = NotesClient.newInstance(account.getPreferredApiVersion(), context);
-                    final SyncTask syncTask = new SyncTask(notesClient, account, ssoAccount, onlyLocalChanges);
-                    syncTask.addCallbacks(account, callbacksPush.get(account.getId()));
-                    callbacksPush.put(account.getId(), new ArrayList<>());
-                    if (!onlyLocalChanges) {
-                        syncTask.addCallbacks(account, callbacksPull.get(account.getId()));
-                        callbacksPull.put(account.getId(), new ArrayList<>());
-                    }
-                    syncTask.execute();
-                } catch (NextcloudFilesAppAccountNotFoundException e) {
-                    Log.e(TAG, "... Could not find " + SingleSignOnAccount.class.getSimpleName() + " for account name " + account.getAccountName());
-                    e.printStackTrace();
-                }
-            } else if (!onlyLocalChanges) {
-                Log.d(TAG, "... scheduled");
-                syncScheduled.put(account.getId(), true);
-                if (callbacksPush.containsKey(account.getId()) && callbacksPush.get(account.getId()) != null) {
-                    final List<ISyncCallback> callbacks = callbacksPush.get(account.getId());
-                    if (callbacks != null) {
-                        for (ISyncCallback callback : callbacks) {
-                            callback.onScheduled();
-                        }
-                    } else {
-                        Log.w(TAG, "List of push-callbacks was set for account \"" + account.getAccountName() + "\" but it was null");
-                    }
-                }
-            } else {
-                Log.d(TAG, "... do nothing");
-                if (callbacksPush.containsKey(account.getId()) && callbacksPush.get(account.getId()) != null) {
-                    final List<ISyncCallback> callbacks = callbacksPush.get(account.getId());
-                    if (callbacks != null) {
-                        for (ISyncCallback callback : callbacks) {
-                            callback.onScheduled();
-                        }
-                    } else {
-                        Log.w(TAG, "List of push-callbacks was set for account \"" + account.getAccountName() + "\" but it was null");
-                    }
-                }
-            }
-        }
-    }
-
-    public void updateNetworkStatus() {
-        try {
-            final ConnectivityManager connMgr = (ConnectivityManager) context.getApplicationContext().getSystemService(Context.CONNECTIVITY_SERVICE);
-
-            if (connMgr == null) {
-                throw new NetworkErrorException("ConnectivityManager is null");
-            }
-
-            final NetworkInfo activeInfo = connMgr.getActiveNetworkInfo();
-
-            if (activeInfo == null) {
-                throw new NetworkErrorException("NetworkInfo is null");
-            }
-
-            if (activeInfo.isConnected()) {
-                networkConnected = true;
-
-                final NetworkInfo networkInfo = connMgr.getNetworkInfo((ConnectivityManager.TYPE_WIFI));
-
-                if (networkInfo == null) {
-                    throw new NetworkErrorException("connMgr.getNetworkInfo(ConnectivityManager.TYPE_WIFI) is null");
-                }
-
-                isSyncPossible = !syncOnlyOnWifi || networkInfo.isConnected();
-
-                if (isSyncPossible) {
-                    Log.d(TAG, "Network connection established.");
-                } else {
-                    Log.d(TAG, "Network connected, but not used because only synced on wifi.");
-                }
-            } else {
-                networkConnected = false;
-                isSyncPossible = false;
-                Log.d(TAG, "No network connection.");
-            }
-        } catch (NetworkErrorException e) {
-            e.printStackTrace();
-            networkConnected = false;
-            isSyncPossible = false;
-        }
-    }
-
-    /**
-     * SyncTask is an AsyncTask which performs the synchronization in a background thread.
-     * Synchronization consists of two parts: pushLocalChanges and pullRemoteChanges.
-     */
-    private class SyncTask extends AsyncTask<Void, Void, SyncResultStatus> {
-        @NonNull
-        private final NotesClient notesClient;
-        @NonNull
-        private final Account localAccount;
-        @NonNull
-        private final SingleSignOnAccount ssoAccount;
-        private final boolean onlyLocalChanges;
-        @NonNull
-        private final Map<Long, List<ISyncCallback>> callbacks = new HashMap<>();
-        @NonNull
-        private final ArrayList<Throwable> exceptions = new ArrayList<>();
-
-        SyncTask(@NonNull NotesClient notesClient, @NonNull Account localAccount, @NonNull SingleSignOnAccount ssoAccount, boolean onlyLocalChanges) {
-            this.notesClient = notesClient;
-            this.localAccount = localAccount;
-            this.ssoAccount = ssoAccount;
-            this.onlyLocalChanges = onlyLocalChanges;
-        }
-
-        private void addCallbacks(Account account, List<ISyncCallback> callbacks) {
-            this.callbacks.put(account.getId(), callbacks);
-        }
-
-        @Override
-        protected void onPreExecute() {
-            super.onPreExecute();
-            syncStatus.postValue(true);
-            if (!syncScheduled.containsKey(localAccount.getId()) || syncScheduled.get(localAccount.getId()) == null) {
-                syncScheduled.put(localAccount.getId(), false);
-            }
-            if (!onlyLocalChanges && Boolean.TRUE.equals(syncScheduled.get(localAccount.getId()))) {
-                syncScheduled.put(localAccount.getId(), false);
-            }
-            syncActive.put(localAccount.getId(), true);
-        }
-
-        @Override
-        protected SyncResultStatus doInBackground(Void... voids) {
-            Log.i(TAG, "STARTING SYNCHRONIZATION");
-            final SyncResultStatus status = new SyncResultStatus();
-            status.pushSuccessful = pushLocalChanges();
-            if (!onlyLocalChanges) {
-                status.pullSuccessful = pullRemoteChanges();
-            }
-            Log.i(TAG, "SYNCHRONIZATION FINISHED");
-            return status;
-        }
-
-        /**
-         * Push local changes: for each locally created/edited/deleted Note, use NotesClient in order to push the changed to the server.
-         */
-        private boolean pushLocalChanges() {
-            Log.d(TAG, "pushLocalChanges()");
-
-            boolean success = true;
-            List<Note> notes = db.getNoteDao().getLocalModifiedNotes(localAccount.getId());
-            for (Note note : notes) {
-                Log.d(TAG, "   Process Local Note: " + note);
-                try {
-                    Note remoteNote;
-                    switch (note.getStatus()) {
-                        case LOCAL_EDITED:
-                            Log.v(TAG, "   ...create/edit");
-                            if (note.getRemoteId() != null) {
-                                Log.v(TAG, "   ...Note has remoteId → try to edit");
-                                try {
-                                    remoteNote = notesClient.editNote(ssoAccount, note).getNote();
-                                } catch (NextcloudHttpRequestFailedException e) {
-                                    if (e.getStatusCode() == HTTP_NOT_FOUND) {
-                                        Log.v(TAG, "   ...Note does no longer exist on server → recreate");
-                                        remoteNote = notesClient.createNote(ssoAccount, note).getNote();
-                                    } else {
-                                        throw e;
-                                    }
-                                }
-                            } else {
-                                Log.v(TAG, "   ...Note does not have a remoteId yet → create");
-                                remoteNote = notesClient.createNote(ssoAccount, note).getNote();
-                                db.getNoteDao().updateRemoteId(note.getId(), remoteNote.getRemoteId());
-                            }
-                            // Please note, that db.updateNote() realized an optimistic conflict resolution, which is required for parallel changes of this Note from the UI.
-                            // TODO: check if the Rooms implementation does this correctly!
-                            db.getNoteDao().updateIfNotModifiedLocallyDuringSync(note.getId(), remoteNote.getModified().getTimeInMillis(), remoteNote.getTitle(), remoteNote.getFavorite(), remoteNote.getETag(), remoteNote.getContent(), generateNoteExcerpt(remoteNote.getContent(), remoteNote.getTitle()), note.getContent(), note.getCategory(), note.getFavorite());
-                            break;
-                        case LOCAL_DELETED:
-                            if (note.getRemoteId() == null) {
-                                Log.v(TAG, "   ...delete (only local, since it has never been synchronized)");
-                            } else {
-                                Log.v(TAG, "   ...delete (from server and local)");
-                                try {
-                                    notesClient.deleteNote(ssoAccount, note.getRemoteId());
-                                } catch (NextcloudHttpRequestFailedException e) {
-                                    if (e.getStatusCode() == HTTP_NOT_FOUND) {
-                                        Log.v(TAG, "   ...delete (note has already been deleted remotely)");
-                                    } else {
-                                        throw e;
-                                    }
-                                }
-                            }
-                            // Please note, that db.deleteNote() realizes an optimistic conflict resolution, which is required for parallel changes of this Note from the UI.
-                            db.getNoteDao().deleteByNoteId(note.getId(), LOCAL_DELETED);
-                            break;
-                        default:
-                            throw new IllegalStateException("Unknown State of Note " + note + ": " + note.getStatus());
-                    }
-                } catch (NextcloudHttpRequestFailedException e) {
-                    if (e.getStatusCode() == HTTP_NOT_MODIFIED) {
-                        Log.d(TAG, "Server returned HTTP Status Code 304 - Not Modified");
-                    } else {
-                        exceptions.add(e);
-                        success = false;
-                    }
-                } catch (Exception e) {
-                    if (e instanceof TokenMismatchException) {
-                        SSOClient.invalidateAPICache(ssoAccount);
-                    }
-                    exceptions.add(e);
-                    success = false;
-                }
-            }
-            return success;
-        }
-
-        /**
-         * Pull remote Changes: update or create each remote note (if local pendant has no changes) and remove remotely deleted notes.
-         */
-        private boolean pullRemoteChanges() {
-            Log.d(TAG, "pullRemoteChanges() for account " + localAccount.getAccountName());
-            try {
-                final Map<Long, Long> idMap = db.getIdMap(localAccount.getId());
-                final Calendar modified = localAccount.getModified();
-                final long modifiedForServer = modified == null ? 0 : modified.getTimeInMillis() / 1_000;
-                final ServerResponse.NotesResponse response = notesClient.getNotes(ssoAccount, modifiedForServer, localAccount.getETag());
-                List<Note> remoteNotes = response.getNotes();
-                Set<Long> remoteIDs = new HashSet<>();
-                // pull remote changes: update or create each remote note
-                for (Note remoteNote : remoteNotes) {
-                    Log.v(TAG, "   Process Remote Note: " + remoteNote);
-                    remoteIDs.add(remoteNote.getRemoteId());
-                    if (remoteNote.getModified() == null) {
-                        Log.v(TAG, "   ... unchanged");
-                    } else if (idMap.containsKey(remoteNote.getRemoteId())) {
-                        Log.v(TAG, "   ... found → Update");
-                        Long localId = idMap.get(remoteNote.getRemoteId());
-                        if (localId != null) {
-                            db.getNoteDao().updateIfNotModifiedLocallyAndAnyRemoteColumnHasChanged(
-                                    localId, remoteNote.getModified().getTimeInMillis(), remoteNote.getTitle(), remoteNote.getFavorite(), remoteNote.getCategory(), remoteNote.getETag(), remoteNote.getContent(), generateNoteExcerpt(remoteNote.getContent(), remoteNote.getTitle()));
-                        } else {
-                            Log.e(TAG, "Tried to update note from server, but local id of note is null. " + remoteNote);
-                        }
-                    } else {
-                        Log.v(TAG, "   ... create");
-                        db.addNote(localAccount.getId(), remoteNote);
-                    }
-                }
-                Log.d(TAG, "   Remove remotely deleted Notes (only those without local changes)");
-                // remove remotely deleted notes (only those without local changes)
-                for (Map.Entry<Long, Long> entry : idMap.entrySet()) {
-                    if (!remoteIDs.contains(entry.getKey())) {
-                        Log.v(TAG, "   ... remove " + entry.getValue());
-                        db.getNoteDao().deleteByNoteId(entry.getValue(), DBStatus.VOID);
-                    }
-                }
-
-                // update ETag and Last-Modified in order to reduce size of next response
-                localAccount.setETag(response.getETag());
-                Calendar calendar = Calendar.getInstance();
-                calendar.setTimeInMillis(response.getLastModified());
-                localAccount.setModified(calendar);
-                db.getAccountDao().updateETag(localAccount.getId(), localAccount.getETag());
-                db.getAccountDao().updateModified(localAccount.getId(), localAccount.getModified().getTimeInMillis());
-                try {
-                    if (db.updateApiVersion(localAccount.getId(), response.getSupportedApiVersions())) {
-                        localAccount.setApiVersion(response.getSupportedApiVersions());
-                    }
-                } catch (Exception e) {
-                    exceptions.add(e);
-                }
-                return true;
-            } catch (NextcloudHttpRequestFailedException e) {
-                Log.d(TAG, "Server returned HTTP Status Code " + e.getStatusCode() + " - " + e.getMessage());
-                if (e.getStatusCode() == HTTP_NOT_MODIFIED) {
-                    return true;
-                } else {
-                    exceptions.add(e);
-                    return false;
-                }
-            } catch (Exception e) {
-                if (e instanceof TokenMismatchException) {
-                    SSOClient.invalidateAPICache(ssoAccount);
-                }
-                exceptions.add(e);
-                return false;
-            }
-        }
-
-        @Override
-        protected void onPostExecute(SyncResultStatus status) {
-            super.onPostExecute(status);
-            for (Throwable e : exceptions) {
-                Log.e(TAG, e.getMessage(), e);
-            }
-            if (!status.pullSuccessful || !status.pushSuccessful) {
-                syncErrors.postValue(exceptions);
-            }
-            syncActive.put(localAccount.getId(), false);
-            // notify callbacks
-            if (callbacks.containsKey(localAccount.getId()) && callbacks.get(localAccount.getId()) != null) {
-                for (ISyncCallback callback : Objects.requireNonNull(callbacks.get(localAccount.getId()))) {
-                    callback.onFinish();
-                }
-            }
-            db.notifyWidgets();
-            db.updateDynamicShortcuts(localAccount.getId());
-            // start next sync if scheduled meanwhile
-            if (syncScheduled.containsKey(localAccount.getId()) && syncScheduled.get(localAccount.getId()) != null && Boolean.TRUE.equals(syncScheduled.get(localAccount.getId()))) {
-                scheduleSync(localAccount, false);
-            }
-            syncStatus.postValue(false);
-        }
-    }
-
-    public LiveData<Boolean> getSyncStatus() {
-        return distinctUntilChanged(this.syncStatus);
-    }
-
-    public LiveData<ArrayList<Throwable>> getSyncErrors() {
-        return this.syncErrors;
-    }
-}
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
index 334728c5..87ee7b20 100644
--- a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesDatabase.java
@@ -96,7 +96,7 @@ public abstract class NotesDatabase extends RoomDatabase {
     private static final String NOTES_DB_NAME = "OWNCLOUD_NOTES";
     private static NotesDatabase instance;
     private static Context context;
-    private static NoteServerSyncHelper serverSyncHelper;
+    private static NotesServerSyncHelper serverSyncHelper;
     private static String defaultNonEmptyTitle;
 
     private static NotesDatabase create(final Context context) {
@@ -149,12 +149,12 @@ public abstract class NotesDatabase extends RoomDatabase {
         if (instance == null) {
             instance = create(context.getApplicationContext());
             NotesDatabase.context = context.getApplicationContext();
-            NotesDatabase.serverSyncHelper = NoteServerSyncHelper.getInstance(instance);
+            NotesDatabase.serverSyncHelper = NotesServerSyncHelper.getInstance(instance);
         }
         return instance;
     }
 
-    public NoteServerSyncHelper getNoteServerSyncHelper() {
+    public NotesServerSyncHelper getNoteServerSyncHelper() {
         return NotesDatabase.serverSyncHelper;
     }
 
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncHelper.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncHelper.java
new file mode 100644
index 00000000..e9fab1b0
--- /dev/null
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncHelper.java
@@ -0,0 +1,346 @@
+package it.niedermann.owncloud.notes.persistence;
+
+import android.accounts.NetworkErrorException;
+import android.content.BroadcastReceiver;
+import android.content.Context;
+import android.content.Intent;
+import android.content.IntentFilter;
+import android.content.SharedPreferences;
+import android.net.ConnectivityManager;
+import android.net.NetworkInfo;
+import android.util.Log;
+
+import androidx.annotation.NonNull;
+import androidx.lifecycle.LiveData;
+import androidx.lifecycle.MutableLiveData;
+import androidx.preference.PreferenceManager;
+
+import com.nextcloud.android.sso.AccountImporter;
+import com.nextcloud.android.sso.exceptions.NextcloudFilesAppAccountNotFoundException;
+import com.nextcloud.android.sso.exceptions.NoCurrentAccountSelectedException;
+import com.nextcloud.android.sso.helper.SingleAccountHelper;
+import com.nextcloud.android.sso.model.SingleSignOnAccount;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import it.niedermann.owncloud.notes.R;
+import it.niedermann.owncloud.notes.persistence.entity.Account;
+import it.niedermann.owncloud.notes.shared.model.ISyncCallback;
+import it.niedermann.owncloud.notes.shared.model.SyncResultStatus;
+import it.niedermann.owncloud.notes.shared.util.SSOUtil;
+
+import static androidx.lifecycle.Transformations.distinctUntilChanged;
+
+/**
+ * Helps to synchronize the Database to the Server.
+ */
+public class NotesServerSyncHelper {
+
+    private static final String TAG = NotesServerSyncHelper.class.getSimpleName();
+
+    private final ExecutorService executor = Executors.newSingleThreadExecutor();
+
+    private static NotesServerSyncHelper instance;
+
+    private final NotesDatabase db;
+    private final Context context;
+
+    /**
+     * Track network connection changes using a {@link BroadcastReceiver}
+     */
+    private boolean isSyncPossible = false;
+    private boolean networkConnected = false;
+    private String syncOnlyOnWifiKey;
+    private boolean syncOnlyOnWifi;
+    private final MutableLiveData<Boolean> syncStatus = new MutableLiveData<>(false);
+    private final MutableLiveData<ArrayList<Throwable>> syncErrors = new MutableLiveData<>();
+
+    /**
+     * @see <a href="https://stackoverflow.com/a/3104265">Do not make this a local variable.</a>
+     */
+    @SuppressWarnings("FieldCanBeLocal")
+    private final SharedPreferences.OnSharedPreferenceChangeListener onSharedPreferenceChangeListener = (SharedPreferences prefs, String key) -> {
+        if (syncOnlyOnWifiKey.equals(key)) {
+            syncOnlyOnWifi = prefs.getBoolean(syncOnlyOnWifiKey, false);
+            updateNetworkStatus();
+        }
+    };
+
+    private final BroadcastReceiver networkReceiver = new BroadcastReceiver() {
+        @Override
+        public void onReceive(Context context, Intent intent) {
+            updateNetworkStatus();
+            if (isSyncPossible() && SSOUtil.isConfigured(context)) {
+                new Thread(() -> {
+                    try {
+                        scheduleSync(db.getAccountDao().getAccountByName(SingleAccountHelper.getCurrentSingleSignOnAccount(context).name), false);
+                    } catch (NextcloudFilesAppAccountNotFoundException | NoCurrentAccountSelectedException e) {
+                        Log.v(TAG, "Can not select current SingleSignOn account after network changed, do not sync.");
+                    }
+                }).start();
+            }
+        }
+    };
+
+    // current state of the synchronization
+    private final Map<Long, Boolean> syncActive = new HashMap<>();
+    private final Map<Long, Boolean> syncScheduled = new HashMap<>();
+
+    // list of callbacks for both parts of synchronization
+    private final Map<Long, List<ISyncCallback>> callbacksPush = new HashMap<>();
+    private final Map<Long, List<ISyncCallback>> callbacksPull = new HashMap<>();
+
+    private NotesServerSyncHelper(NotesDatabase db) {
+        this.db = db;
+        this.context = db.getContext();
+        this.syncOnlyOnWifiKey = context.getApplicationContext().getResources().getString(R.string.pref_key_wifi_only);
+
+        // Registers BroadcastReceiver to track network connection changes.
+        context.getApplicationContext().registerReceiver(networkReceiver, new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION));
+
+        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this.context.getApplicationContext());
+        prefs.registerOnSharedPreferenceChangeListener(onSharedPreferenceChangeListener);
+        syncOnlyOnWifi = prefs.getBoolean(syncOnlyOnWifiKey, false);
+
+        updateNetworkStatus();
+    }
+
+    /**
+     * Get (or create) instance from NoteServerSyncHelper.
+     * This has to be a singleton in order to realize correct registering and unregistering of
+     * the BroadcastReceiver, which listens on changes of network connectivity.
+     *
+     * @param db {@link NotesDatabase}
+     * @return NoteServerSyncHelper
+     */
+    public static synchronized NotesServerSyncHelper getInstance(NotesDatabase db) {
+        if (instance == null) {
+            instance = new NotesServerSyncHelper(db);
+        }
+        return instance;
+    }
+
+    @Override
+    protected void finalize() throws Throwable {
+        context.getApplicationContext().unregisterReceiver(networkReceiver);
+        super.finalize();
+    }
+
+    /**
+     * Synchronization is only possible, if there is an active network connection.
+     * <p>
+     * This method respects the user preference "Sync on Wi-Fi only".
+     * <p>
+     * NoteServerSyncHelper observes changes in the network connection.
+     * The current state can be retrieved with this method.
+     *
+     * @return true if sync is possible, otherwise false.
+     */
+    public boolean isSyncPossible() {
+        return isSyncPossible;
+    }
+
+    public boolean isNetworkConnected() {
+        return networkConnected;
+    }
+
+    public boolean isSyncOnlyOnWifi() {
+        return syncOnlyOnWifi;
+    }
+
+    /**
+     * Adds a callback method to the NoteServerSyncHelper for the synchronization part push local changes to the server.
+     * All callbacks will be executed once the synchronization operations are done.
+     * After execution the callback will be deleted, so it has to be added again if it shall be
+     * executed the next time all synchronize operations are finished.
+     *
+     * @param callback Implementation of ISyncCallback, contains one method that shall be executed.
+     */
+    public void addCallbackPush(Account account, ISyncCallback callback) {
+        if (account == null) {
+            Log.i(TAG, "ssoAccount is null. Is this a local account?");
+            callback.onScheduled();
+            callback.onFinish();
+        } else {
+            if (!callbacksPush.containsKey(account.getId())) {
+                callbacksPush.put(account.getId(), new ArrayList<>());
+            }
+            Objects.requireNonNull(callbacksPush.get(account.getId())).add(callback);
+        }
+    }
+
+    /**
+     * Adds a callback method to the NoteServerSyncHelper for the synchronization part pull remote changes from the server.
+     * All callbacks will be executed once the synchronization operations are done.
+     * After execution the callback will be deleted, so it has to be added again if it shall be
+     * executed the next time all synchronize operations are finished.
+     *
+     * @param callback Implementation of ISyncCallback, contains one method that shall be executed.
+     */
+    public void addCallbackPull(Account account, ISyncCallback callback) {
+        if (account == null) {
+            Log.i(TAG, "ssoAccount is null. Is this a local account?");
+            callback.onScheduled();
+            callback.onFinish();
+        } else {
+            if (!callbacksPull.containsKey(account.getId())) {
+                callbacksPull.put(account.getId(), new ArrayList<>());
+            }
+            Objects.requireNonNull(callbacksPull.get(account.getId())).add(callback);
+        }
+    }
+
+    /**
+     * Schedules a synchronization and start it directly, if the network is connected and no
+     * synchronization is currently running.
+     *
+     * @param onlyLocalChanges Whether to only push local changes to the server or to also load the whole list of notes from the server.
+     */
+    public void scheduleSync(Account account, boolean onlyLocalChanges) {
+        if (account == null) {
+            Log.i(TAG, SingleSignOnAccount.class.getSimpleName() + " is null. Is this a local account?");
+        } else {
+            if (syncActive.get(account.getId()) == null) {
+                syncActive.put(account.getId(), false);
+            }
+            Log.d(TAG, "Sync requested (" + (onlyLocalChanges ? "onlyLocalChanges" : "full") + "; " + (Boolean.TRUE.equals(syncActive.get(account.getId())) ? "sync active" : "sync NOT active") + ") ...");
+            if (isSyncPossible() && (!Boolean.TRUE.equals(syncActive.get(account.getId())) || onlyLocalChanges)) {
+                try {
+                    SingleSignOnAccount ssoAccount = AccountImporter.getSingleSignOnAccount(context, account.getAccountName());
+                    Log.d(TAG, "... starting now");
+                    final NotesClient notesClient = NotesClient.newInstance(account.getPreferredApiVersion(), context);
+                    final NotesServerSyncTask syncTask = new NotesServerSyncTask(notesClient, db, account, ssoAccount, onlyLocalChanges) {
+                        @Override
+                        void onPreExecute() {
+                            syncStatus.postValue(true);
+                            if (!syncScheduled.containsKey(localAccount.getId()) || syncScheduled.get(localAccount.getId()) == null) {
+                                syncScheduled.put(localAccount.getId(), false);
+                            }
+                            if (!onlyLocalChanges && Boolean.TRUE.equals(syncScheduled.get(localAccount.getId()))) {
+                                syncScheduled.put(localAccount.getId(), false);
+                            }
+                            syncActive.put(localAccount.getId(), true);
+                        }
+
+                        @Override
+                        void onPostExecute(SyncResultStatus status) {
+                            for (Throwable e : exceptions) {
+                                Log.e(TAG, e.getMessage(), e);
+                            }
+                            if (!status.pullSuccessful || !status.pushSuccessful) {
+                                syncErrors.postValue(exceptions);
+                            }
+                            syncActive.put(localAccount.getId(), false);
+                            // notify callbacks
+                            if (callbacks.containsKey(localAccount.getId()) && callbacks.get(localAccount.getId()) != null) {
+                                for (ISyncCallback callback : Objects.requireNonNull(callbacks.get(localAccount.getId()))) {
+                                    callback.onFinish();
+                                }
+                            }
+                            db.notifyWidgets();
+                            db.updateDynamicShortcuts(localAccount.getId());
+                            // start next sync if scheduled meanwhile
+                            if (syncScheduled.containsKey(localAccount.getId()) && syncScheduled.get(localAccount.getId()) != null && Boolean.TRUE.equals(syncScheduled.get(localAccount.getId()))) {
+                                scheduleSync(localAccount, false);
+                            }
+                            syncStatus.postValue(false);
+                        }
+                    };
+                    syncTask.addCallbacks(account, callbacksPush.get(account.getId()));
+                    callbacksPush.put(account.getId(), new ArrayList<>());
+                    if (!onlyLocalChanges) {
+                        syncTask.addCallbacks(account, callbacksPull.get(account.getId()));
+                        callbacksPull.put(account.getId(), new ArrayList<>());
+                    }
+                    executor.submit(syncTask);
+                } catch (NextcloudFilesAppAccountNotFoundException e) {
+                    Log.e(TAG, "... Could not find " + SingleSignOnAccount.class.getSimpleName() + " for account name " + account.getAccountName());
+                    e.printStackTrace();
+                }
+            } else if (!onlyLocalChanges) {
+                Log.d(TAG, "... scheduled");
+                syncScheduled.put(account.getId(), true);
+                if (callbacksPush.containsKey(account.getId()) && callbacksPush.get(account.getId()) != null) {
+                    final List<ISyncCallback> callbacks = callbacksPush.get(account.getId());
+                    if (callbacks != null) {
+                        for (ISyncCallback callback : callbacks) {
+                            callback.onScheduled();
+                        }
+                    } else {
+                        Log.w(TAG, "List of push-callbacks was set for account \"" + account.getAccountName() + "\" but it was null");
+                    }
+                }
+            } else {
+                Log.d(TAG, "... do nothing");
+                if (callbacksPush.containsKey(account.getId()) && callbacksPush.get(account.getId()) != null) {
+                    final List<ISyncCallback> callbacks = callbacksPush.get(account.getId());
+                    if (callbacks != null) {
+                        for (ISyncCallback callback : callbacks) {
+                            callback.onScheduled();
+                        }
+                    } else {
+                        Log.w(TAG, "List of push-callbacks was set for account \"" + account.getAccountName() + "\" but it was null");
+                    }
+                }
+            }
+        }
+    }
+
+    public void updateNetworkStatus() {
+        try {
+            final ConnectivityManager connMgr = (ConnectivityManager) context.getApplicationContext().getSystemService(Context.CONNECTIVITY_SERVICE);
+
+            if (connMgr == null) {
+                throw new NetworkErrorException("ConnectivityManager is null");
+            }
+
+            final NetworkInfo activeInfo = connMgr.getActiveNetworkInfo();
+
+            if (activeInfo == null) {
+                throw new NetworkErrorException("NetworkInfo is null");
+            }
+
+            if (activeInfo.isConnected()) {
+                networkConnected = true;
+
+                final NetworkInfo networkInfo = connMgr.getNetworkInfo((ConnectivityManager.TYPE_WIFI));
+
+                if (networkInfo == null) {
+                    throw new NetworkErrorException("connMgr.getNetworkInfo(ConnectivityManager.TYPE_WIFI) is null");
+                }
+
+                isSyncPossible = !syncOnlyOnWifi || networkInfo.isConnected();
+
+                if (isSyncPossible) {
+                    Log.d(TAG, "Network connection established.");
+                } else {
+                    Log.d(TAG, "Network connected, but not used because only synced on wifi.");
+                }
+            } else {
+                networkConnected = false;
+                isSyncPossible = false;
+                Log.d(TAG, "No network connection.");
+            }
+        } catch (NetworkErrorException e) {
+            e.printStackTrace();
+            networkConnected = false;
+            isSyncPossible = false;
+        }
+    }
+
+    @NonNull
+    public LiveData<Boolean> getSyncStatus() {
+        return distinctUntilChanged(this.syncStatus);
+    }
+
+    @NonNull
+    public LiveData<ArrayList<Throwable>> getSyncErrors() {
+        return this.syncErrors;
+    }
+}
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncTask.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncTask.java
new file mode 100644
index 00000000..054e6a63
--- /dev/null
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/NotesServerSyncTask.java
@@ -0,0 +1,232 @@
+package it.niedermann.owncloud.notes.persistence;
+
+import android.util.Log;
+
+import androidx.annotation.NonNull;
+
+import com.nextcloud.android.sso.exceptions.NextcloudHttpRequestFailedException;
+import com.nextcloud.android.sso.exceptions.TokenMismatchException;
+import com.nextcloud.android.sso.model.SingleSignOnAccount;
+
+import java.util.ArrayList;
+import java.util.Calendar;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import it.niedermann.owncloud.notes.persistence.entity.Account;
+import it.niedermann.owncloud.notes.persistence.entity.Note;
+import it.niedermann.owncloud.notes.shared.model.DBStatus;
+import it.niedermann.owncloud.notes.shared.model.ISyncCallback;
+import it.niedermann.owncloud.notes.shared.model.ServerResponse;
+import it.niedermann.owncloud.notes.shared.model.SyncResultStatus;
+
+import static it.niedermann.owncloud.notes.shared.model.DBStatus.LOCAL_DELETED;
+import static it.niedermann.owncloud.notes.shared.util.NoteUtil.generateNoteExcerpt;
+import static java.net.HttpURLConnection.HTTP_NOT_FOUND;
+import static java.net.HttpURLConnection.HTTP_NOT_MODIFIED;
+
+
+/**
+ * {@link NotesServerSyncTask} is a {@link Thread} which performs the synchronization in a background thread.
+ * Synchronization consists of two parts: {@link #pushLocalChanges()} and {@link #pullRemoteChanges}.
+ */
+abstract class NotesServerSyncTask extends Thread {
+
+    private static final String TAG = NotesServerSyncTask.class.getSimpleName();
+
+    @NonNull
+    private final NotesClient notesClient;
+    @NonNull
+    private final NotesDatabase db;
+    @NonNull
+    protected final Account localAccount;
+    @NonNull
+    private final SingleSignOnAccount ssoAccount;
+    private final boolean onlyLocalChanges;
+    @NonNull
+    protected final Map<Long, List<ISyncCallback>> callbacks = new HashMap<>();
+    @NonNull
+    protected final ArrayList<Throwable> exceptions = new ArrayList<>();
+
+    NotesServerSyncTask(@NonNull NotesClient notesClient, @NonNull NotesDatabase db, @NonNull Account localAccount, @NonNull SingleSignOnAccount ssoAccount, boolean onlyLocalChanges) {
+        super(TAG);
+        this.notesClient = notesClient;
+        this.db = db;
+        this.localAccount = localAccount;
+        this.ssoAccount = ssoAccount;
+        this.onlyLocalChanges = onlyLocalChanges;
+    }
+
+    void addCallbacks(Account account, List<ISyncCallback> callbacks) {
+        this.callbacks.put(account.getId(), callbacks);
+    }
+
+    @Override
+    public void run() {
+        onPreExecute();
+
+        Log.i(TAG, "STARTING SYNCHRONIZATION");
+        final SyncResultStatus status = new SyncResultStatus();
+        status.pushSuccessful = pushLocalChanges();
+        if (!onlyLocalChanges) {
+            status.pullSuccessful = pullRemoteChanges();
+        }
+        Log.i(TAG, "SYNCHRONIZATION FINISHED");
+
+        onPostExecute(status);
+    }
+
+    abstract void onPreExecute();
+
+    abstract void onPostExecute(SyncResultStatus status);
+
+    /**
+     * Push local changes: for each locally created/edited/deleted Note, use NotesClient in order to push the changed to the server.
+     */
+    private boolean pushLocalChanges() {
+        Log.d(TAG, "pushLocalChanges()");
+
+        boolean success = true;
+        final List<Note> notes = db.getNoteDao().getLocalModifiedNotes(localAccount.getId());
+        for (Note note : notes) {
+            Log.d(TAG, "   Process Local Note: " + note);
+            try {
+                Note remoteNote;
+                switch (note.getStatus()) {
+                    case LOCAL_EDITED:
+                        Log.v(TAG, "   ...create/edit");
+                        if (note.getRemoteId() != null) {
+                            Log.v(TAG, "   ...Note has remoteId → try to edit");
+                            try {
+                                remoteNote = notesClient.editNote(ssoAccount, note).getNote();
+                            } catch (NextcloudHttpRequestFailedException e) {
+                                if (e.getStatusCode() == HTTP_NOT_FOUND) {
+                                    Log.v(TAG, "   ...Note does no longer exist on server → recreate");
+                                    remoteNote = notesClient.createNote(ssoAccount, note).getNote();
+                                } else {
+                                    throw e;
+                                }
+                            }
+                        } else {
+                            Log.v(TAG, "   ...Note does not have a remoteId yet → create");
+                            remoteNote = notesClient.createNote(ssoAccount, note).getNote();
+                            db.getNoteDao().updateRemoteId(note.getId(), remoteNote.getRemoteId());
+                        }
+                        // Please note, that db.updateNote() realized an optimistic conflict resolution, which is required for parallel changes of this Note from the UI.
+                        db.getNoteDao().updateIfNotModifiedLocallyDuringSync(note.getId(), remoteNote.getModified().getTimeInMillis(), remoteNote.getTitle(), remoteNote.getFavorite(), remoteNote.getETag(), remoteNote.getContent(), generateNoteExcerpt(remoteNote.getContent(), remoteNote.getTitle()), note.getContent(), note.getCategory(), note.getFavorite());
+                        break;
+                    case LOCAL_DELETED:
+                        if (note.getRemoteId() == null) {
+                            Log.v(TAG, "   ...delete (only local, since it has never been synchronized)");
+                        } else {
+                            Log.v(TAG, "   ...delete (from server and local)");
+                            try {
+                                notesClient.deleteNote(ssoAccount, note.getRemoteId());
+                            } catch (NextcloudHttpRequestFailedException e) {
+                                if (e.getStatusCode() == HTTP_NOT_FOUND) {
+                                    Log.v(TAG, "   ...delete (note has already been deleted remotely)");
+                                } else {
+                                    throw e;
+                                }
+                            }
+                        }
+                        // Please note, that db.deleteNote() realizes an optimistic conflict resolution, which is required for parallel changes of this Note from the UI.
+                        db.getNoteDao().deleteByNoteId(note.getId(), LOCAL_DELETED);
+                        break;
+                    default:
+                        throw new IllegalStateException("Unknown State of Note " + note + ": " + note.getStatus());
+                }
+            } catch (NextcloudHttpRequestFailedException e) {
+                if (e.getStatusCode() == HTTP_NOT_MODIFIED) {
+                    Log.d(TAG, "Server returned HTTP Status Code 304 - Not Modified");
+                } else {
+                    exceptions.add(e);
+                    success = false;
+                }
+            } catch (Exception e) {
+                if (e instanceof TokenMismatchException) {
+                    SSOClient.invalidateAPICache(ssoAccount);
+                }
+                exceptions.add(e);
+                success = false;
+            }
+        }
+        return success;
+    }
+
+    /**
+     * Pull remote Changes: update or create each remote note (if local pendant has no changes) and remove remotely deleted notes.
+     */
+    private boolean pullRemoteChanges() {
+        Log.d(TAG, "pullRemoteChanges() for account " + localAccount.getAccountName());
+        try {
+            final Map<Long, Long> idMap = db.getIdMap(localAccount.getId());
+            final Calendar modified = localAccount.getModified();
+            final long modifiedForServer = modified == null ? 0 : modified.getTimeInMillis() / 1_000;
+            final ServerResponse.NotesResponse response = notesClient.getNotes(ssoAccount, modifiedForServer, localAccount.getETag());
+            List<Note> remoteNotes = response.getNotes();
+            Set<Long> remoteIDs = new HashSet<>();
+            // pull remote changes: update or create each remote note
+            for (Note remoteNote : remoteNotes) {
+                Log.v(TAG, "   Process Remote Note: " + remoteNote);
+                remoteIDs.add(remoteNote.getRemoteId());
+                if (remoteNote.getModified() == null) {
+                    Log.v(TAG, "   ... unchanged");
+                } else if (idMap.containsKey(remoteNote.getRemoteId())) {
+                    Log.v(TAG, "   ... found → Update");
+                    Long localId = idMap.get(remoteNote.getRemoteId());
+                    if (localId != null) {
+                        db.getNoteDao().updateIfNotModifiedLocallyAndAnyRemoteColumnHasChanged(
+                                localId, remoteNote.getModified().getTimeInMillis(), remoteNote.getTitle(), remoteNote.getFavorite(), remoteNote.getCategory(), remoteNote.getETag(), remoteNote.getContent(), generateNoteExcerpt(remoteNote.getContent(), remoteNote.getTitle()));
+                    } else {
+                        Log.e(TAG, "Tried to update note from server, but local id of note is null. " + remoteNote);
+                    }
+                } else {
+                    Log.v(TAG, "   ... create");
+                    db.addNote(localAccount.getId(), remoteNote);
+                }
+            }
+            Log.d(TAG, "   Remove remotely deleted Notes (only those without local changes)");
+            // remove remotely deleted notes (only those without local changes)
+            for (Map.Entry<Long, Long> entry : idMap.entrySet()) {
+                if (!remoteIDs.contains(entry.getKey())) {
+                    Log.v(TAG, "   ... remove " + entry.getValue());
+                    db.getNoteDao().deleteByNoteId(entry.getValue(), DBStatus.VOID);
+                }
+            }
+
+            // update ETag and Last-Modified in order to reduce size of next response
+            localAccount.setETag(response.getETag());
+            Calendar calendar = Calendar.getInstance();
+            calendar.setTimeInMillis(response.getLastModified());
+            localAccount.setModified(calendar);
+            db.getAccountDao().updateETag(localAccount.getId(), localAccount.getETag());
+            db.getAccountDao().updateModified(localAccount.getId(), localAccount.getModified().getTimeInMillis());
+            try {
+                if (db.updateApiVersion(localAccount.getId(), response.getSupportedApiVersions())) {
+                    localAccount.setApiVersion(response.getSupportedApiVersions());
+                }
+            } catch (Exception e) {
+                exceptions.add(e);
+            }
+            return true;
+        } catch (NextcloudHttpRequestFailedException e) {
+            Log.d(TAG, "Server returned HTTP Status Code " + e.getStatusCode() + " - " + e.getMessage());
+            if (e.getStatusCode() == HTTP_NOT_MODIFIED) {
+                return true;
+            } else {
+                exceptions.add(e);
+                return false;
+            }
+        } catch (Exception e) {
+            if (e instanceof TokenMismatchException) {
+                SSOClient.invalidateAPICache(ssoAccount);
+            }
+            exceptions.add(e);
+            return false;
+        }
+    }
+}
diff --git a/app/src/main/java/it/niedermann/owncloud/notes/persistence/dao/NoteDao.java b/app/src/main/java/it/niedermann/owncloud/notes/persistence/dao/NoteDao.java
index 561ce8a4..2dfad616 100644
--- a/app/src/main/java/it/niedermann/owncloud/notes/persistence/dao/NoteDao.java
+++ b/app/src/main/java/it/niedermann/owncloud/notes/persistence/dao/NoteDao.java
@@ -10,7 +10,7 @@ import androidx.room.Update;
 import java.util.List;
 import java.util.Set;
 
-import it.niedermann.owncloud.notes.persistence.NoteServerSyncHelper;
+import it.niedermann.owncloud.notes.persistence.NotesServerSyncHelper;
 import it.niedermann.owncloud.notes.persistence.entity.Account;
 import it.niedermann.owncloud.notes.persistence.entity.CategoryWithNotesCount;
 import it.niedermann.owncloud.notes.persistence.entity.Note;
@@ -169,7 +169,7 @@ public interface NoteDao {
     void updateRemoteId(long id, Long remoteId);
 
     /**
-     * used by: {@link NoteServerSyncHelper.SyncTask#pushLocalChanges()} update only, if not modified locally during the synchronization
+     * used by: {@link NotesServerSyncHelper.SyncTask#pushLocalChanges()} update only, if not modified locally during the synchronization
      * (i.e. all (!) user changeable columns (content, favorite, category) must still have the same value), uses reference value gathered at start of synchronization
      */
     @Query("UPDATE NOTE SET title = :targetTitle, modified = :targetModified, favorite = :targetFavorite, etag = :targetETag, content = :targetContent, status = '', excerpt = :targetExcerpt " +
@@ -177,7 +177,7 @@ public interface NoteDao {
     int updateIfNotModifiedLocallyDuringSync(long noteId, Long targetModified, String targetTitle, boolean targetFavorite, String targetETag, String targetContent, String targetExcerpt, String contentBeforeSyncStart, String categoryBeforeSyncStart, boolean favoriteBeforeSyncStart);
 
     /**
-     * used by: {@link NoteServerSyncHelper.SyncTask#pullRemoteChanges()} update only, if not modified locally (i.e. STATUS="") and if modified remotely (i.e. any (!) column has changed)
+     * used by: {@link NotesServerSyncHelper.SyncTask#pullRemoteChanges()} update only, if not modified locally (i.e. STATUS="") and if modified remotely (i.e. any (!) column has changed)
      */
     @Query("UPDATE NOTE SET title = :title, modified = :modified, favorite = :favorite, etag = :eTag, content = :content, status = '', excerpt = :excerpt " +
             "WHERE id = :id AND status = '' AND (title != :title OR modified != :modified OR favorite != :favorite OR category != :category OR (eTag IS NULL OR eTag != :eTag) OR content != :content)")