Files selector: Return parent document ID

lib/editors/tracks.dart

@@ -55,12 +55,16 @@ class _TracksEditorState extends State<TracksEditor> {
               trailing: IconButton(
                 icon: const Icon(Icons.add),
                 onPressed: () async {
-                  final Set<String> paths = await Navigator.push(
+                  final FilesSelectorResult result = await Navigator.push(
                     context,
                     MaterialPageRoute(
                       builder: (context) => FilesSelector(),
                     ),
                   );
+
+                  if (result != null) {
+                    // TODO: Add tracks.
+                  }
                 },
               ),
             ),

lib/selectors/files.dart

@@ -3,6 +3,21 @@ import 'package:flutter/material.dart';
 import '../backend.dart';
 import '../platform.dart';
 
+/// Result of the user's interaction with the files selector.
+/// 
+/// This will be given back when popping the navigator.
+class FilesSelectorResult {
+  /// Document ID of the parent directory of the selected files.
+  /// 
+  /// This will be null, if they are in the toplevel directory.
+  final String parentId;
+
+  /// Document IDs of the selected files.
+  final List<String> trackIds;
+
+  FilesSelectorResult(this.parentId, this.trackIds);
+}
+
 class FilesSelector extends StatefulWidget {
   @override
   _FilesSelectorState createState() => _FilesSelectorState();
@@ -38,7 +53,13 @@ class _FilesSelectorState extends State<FilesSelector> {
             FlatButton(
               child: Text('DONE'),
               onPressed: () {
-                Navigator.pop(context, selectedIds);
+                Navigator.pop(
+                  context,
+                  FilesSelectorResult(
+                    history.isNotEmpty ? history.last.id : null,
+                    selectedIds.toList(),
+                  ),
+                );
               },
             ),
           ],