Add draft Nextcloud sync service

lib/sync/nextcloud/nextcloud_sync.dart

@@ -0,0 +1,72 @@
+import 'dart:io';
+
+import 'package:flutter/widgets.dart';
+import 'package:http/http.dart';
+import 'package:nextcloud/nextcloud.dart';
+import 'package:nextcloud/nextcloud.dart' as nc;
+import 'package:txt/sync/file.dart';
+import 'package:txt/sync/sync.dart';
+
+class NextcloudSyncService implements SyncService {
+  NextCloudClient client;
+
+  @override
+  Future<void> authorize(BuildContext context) async {
+    // TODO Show login GUI.
+    client = NextCloudClient(
+        "cloud.reimer.family", "test", "jx@Wf*FfLknD+F=,MpUXHi&~veKtKq:=");
+    // TODO Test connection e.g. via user backend.
+  }
+
+  @override
+  Future<String> getAccountName() async {
+    nc.MetaData metadata = await client.users.getMetaData(client.username);
+    return "${metadata.fullName} (${client.username})";
+  }
+
+  @override
+  Future<List<RemoteFile>> listFolder(String path) async {
+    List<WebDavFile> files = await client.webDav.ls(path);
+    return files
+        .map((file) => NextcloudRemoteFile(file))
+        .toList(growable: false);
+  }
+
+  @override
+  Future<void> download(String remotePath, String localPath) async {
+    ByteStream downloadStream = await client.webDav.downloadStream(remotePath);
+    File localFile = File(localPath);
+    if (localFile.existsSync()) localFile.deleteSync();
+    IOSink localSink = localFile.openWrite();
+    await localSink.addStream(downloadStream);
+    await localSink.close();
+  }
+
+  @override
+  Future<void> upload(String localPath, String remotePath) async {
+    File localFile = File(localPath);
+    var localData = await localFile.readAsBytes();
+    await client.webDav.upload(localData, remotePath);
+  }
+}
+
+class NextcloudRemoteFile extends RemoteFile {
+  final WebDavFile file;
+
+  NextcloudRemoteFile(this.file);
+
+  @override
+  DateTime get createdDate => file.createdDate;
+
+  @override
+  bool get isDirectory => file.isDirectory;
+
+  @override
+  DateTime get lastModified => file.lastModified;
+
+  @override
+  String get path => file.path;
+
+  @override
+  int get size => file.size;
+}