Show the backlinks of a Note

This is the initial proof of concept and doesn't look very pretty. It's
also not very efficient.

Related to #141

lib/core/note.dart

@@ -437,6 +437,11 @@ class Link {
       other is Note &&
           runtimeType == other.runtimeType &&
           filePath == other.filePath;
+
+  @override
+  String toString() {
+    return 'Link{term: $term, filePath: $filePath}';
+  }
 }
 
 class LinkExtractor implements md.NodeVisitor {

lib/core/notes_folder_fs.dart

@@ -458,8 +458,33 @@ class NotesFolderFS with NotesFolderNotifier implements NotesFolder {
   Set<String> getNoteTagsRecursively() {
     return _fetchTags(this, {});
   }
+
+  Future<List<Note>> matchNotes(NoteMatcherAsync pred) async {
+    var matchedNotes = <Note>[];
+    await _matchNotes(matchedNotes, pred);
+    return matchedNotes;
+  }
+
+  Future<List<Note>> _matchNotes(
+    List<Note> matchedNotes,
+    NoteMatcherAsync pred,
+  ) async {
+    for (var note in _notes) {
+      var matches = await pred(note);
+      if (matches) {
+        matchedNotes.add(note);
+      }
+    }
+
+    for (var folder in _folders) {
+      await folder._matchNotes(matchedNotes, pred);
+    }
+    return matchedNotes;
+  }
 }
 
+typedef NoteMatcherAsync = Future<bool> Function(Note n);
+
 Set<String> _fetchTags(NotesFolder folder, Set<String> tags) {
   for (var note in folder.notes) {
     tags.addAll(note.tags);

lib/widgets/note_viewer.dart

@@ -47,6 +47,7 @@ class NoteViewer extends StatelessWidget {
       ),
     );
 
+    final rootFolder = Provider.of<NotesFolderFS>(context);
     var view = EditorScrollView(
       child: Column(
         children: <Widget>[
@@ -58,7 +59,6 @@ class NoteViewer extends StatelessWidget {
               styleSheet: markdownStyleSheet,
               onTapLink: (String link) {
                 if (link.startsWith('./')) {
-                  final rootFolder = Provider.of<NotesFolderFS>(context);
                   var spec = link.substring(2);
                   var note = rootFolder.getNoteWithSpec(spec);
                   if (note != null) {
@@ -75,6 +75,7 @@ class NoteViewer extends StatelessWidget {
                   url, note.parent.folderPath + p.separator, null, null),
             ),
           ),
+          NoteBacklinkRenderer(note: note, rootFolder: rootFolder),
           // _buildFooter(context),
         ],
         crossAxisAlignment: CrossAxisAlignment.start,
@@ -181,3 +182,86 @@ Widget _handleDataSchemeUri(Uri uri, final double width, final double height) {
   }
   return const SizedBox();
 }
+
+class NoteBacklinkRenderer extends StatefulWidget {
+  final Note note;
+  final NotesFolderFS rootFolder;
+
+  NoteBacklinkRenderer({
+    @required this.note,
+    @required this.rootFolder,
+  });
+
+  @override
+  _NoteBacklinkRendererState createState() => _NoteBacklinkRendererState();
+}
+
+class _NoteBacklinkRendererState extends State<NoteBacklinkRenderer> {
+  List<Note> linkedNotes = [];
+
+  @override
+  void initState() {
+    super.initState();
+
+    _initStateAsync();
+  }
+
+  Future<void> _initStateAsync() async {
+    var predicate = (Note n) async {
+      var links = await n.fetchLinks();
+      var matchedLink = links.firstWhere(
+        (l) => l.filePath == widget.note.filePath,
+        orElse: () => null,
+      );
+      return matchedLink != null;
+    };
+
+    var l = await widget.rootFolder.matchNotes(predicate);
+    if (!mounted) return;
+    setState(() {
+      linkedNotes = l;
+      print("linkedNote $linkedNotes");
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (linkedNotes.isEmpty) {
+      return Container();
+    }
+
+    var textTheme = Theme.of(context).textTheme;
+    var c = Column(
+      children: <Widget>[
+        Text('BackLinks', style: textTheme.headline5),
+        const SizedBox(height: 8.0),
+        for (var n in linkedNotes) _buildNoteLink(n),
+      ],
+      mainAxisSize: MainAxisSize.min,
+      crossAxisAlignment: CrossAxisAlignment.start,
+    );
+
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(0.0, 16.0, 0.0, 16.0),
+      child: c,
+    );
+  }
+
+  Widget _buildNoteLink(Note note) {
+    var textTheme = Theme.of(context).textTheme;
+    var title = note.title;
+    if (title.isEmpty) {
+      title = note.fileName;
+    }
+
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(0.0, 8.0, 0, 8.0),
+      child: GestureDetector(
+        onTap: () {
+          openNoteEditor(context, note);
+        },
+        child: Text('-   $title', style: textTheme.bodyText1),
+      ),
+    );
+  }
+}