autocomplete [nfc]: Factor out generic AutocompleteView

The mechanism of query and results, is tricky. It's also generic to
the idea of autocomplete in general, is not specific to autocomplete
on @-mentions vs. topics vs. anything else.

So this commit factors it out into the generic class AutocompleteView
which MentionAutocompleteView uses.

Author: Khader-1

lib/model/autocomplete.dart

@@ -156,45 +156,22 @@ class AutocompleteViewManager {
   }
 }
 
-/// A view-model for a mention-autocomplete interaction.
+/// A generic view-model for an autocomplete interaction.
 ///
 /// The owner of one of these objects must call [dispose] when the object
 /// will no longer be used, in order to free resources on the [PerAccountStore].
 ///
 /// Lifecycle:
-///  * Create with [init].
+///  * Instantiate.
 ///  * Add listeners with [addListener].
 ///  * Use the [query] setter to start a search for a query.
 ///  * On reassemble, call [reassemble].
 ///  * When the object will no longer be used, call [dispose] to free
 ///    resources on the [PerAccountStore].
-class MentionAutocompleteView extends ChangeNotifier {
-  MentionAutocompleteView._({required this.store, required this.narrow});
-
-  factory MentionAutocompleteView.init({
-    required PerAccountStore store,
-    required Narrow narrow,
-  }) {
-    final view = MentionAutocompleteView._(store: store, narrow: narrow);
-    store.autocompleteViewManager.registerMentionAutocomplete(view);
-    return view;
-  }
-
-  @override
-  void dispose() {
-    store.autocompleteViewManager.unregisterMentionAutocomplete(this);
-    // We cancel in-progress computations by checking [hasListeners] between tasks.
-    // After [super.dispose] is called, [hasListeners] returns false.
-    // TODO test that logic (may involve detecting an unhandled Future rejection; how?)
-    super.dispose();
-  }
-
-  final PerAccountStore store;
-  final Narrow narrow;
-
-  MentionAutocompleteQuery? get query => _query;
-  MentionAutocompleteQuery? _query;
-  set query(MentionAutocompleteQuery? query) {
+abstract class AutocompleteView<Q, R> extends ChangeNotifier {
+  Q? get query => _query;
+  Q? _query;
+  set query(Q? query) {
     _query = query;
     if (query != null) {
       _startSearch(query);
@@ -219,11 +196,11 @@ class MentionAutocompleteView extends ChangeNotifier {
     }
   }
 
-  Iterable<MentionAutocompleteResult> get results => _results;
-  List<MentionAutocompleteResult> _results = [];
+  Iterable<R> get results => _results;
+  List<R> _results = [];
 
-  _startSearch(MentionAutocompleteQuery query) async {
-    List<MentionAutocompleteResult>? newResults;
+  _startSearch(Q query) async {
+    List<R>? newResults;
 
     while (true) {
       try {
@@ -244,6 +221,40 @@ class MentionAutocompleteView extends ChangeNotifier {
     notifyListeners();
   }
 
+  Future<List<R>?> _computeResults(Q query);
+}
+
+/// A view-model for a mention-autocomplete interaction.
+///
+/// The owner of one of these objects must call [dispose] when the object
+/// will no longer be used, in order to free resources on the [PerAccountStore].
+///
+///    resources on the [PerAccountStore].
+class MentionAutocompleteView extends AutocompleteView<MentionAutocompleteQuery, MentionAutocompleteResult> {
+  MentionAutocompleteView._({required this.store, required this.narrow});
+
+  factory MentionAutocompleteView.init({
+    required PerAccountStore store,
+    required Narrow narrow,
+  }) {
+    final view = MentionAutocompleteView._(store: store, narrow: narrow);
+    store.autocompleteViewManager.registerMentionAutocomplete(view);
+    return view;
+  }
+
+  @override
+  void dispose() {
+    store.autocompleteViewManager.unregisterMentionAutocomplete(this);
+    // We cancel in-progress computations by checking [hasListeners] between tasks.
+    // After [super.dispose] is called, [hasListeners] returns false.
+    // TODO test that logic (may involve detecting an unhandled Future rejection; how?)
+    super.dispose();
+  }
+
+  final PerAccountStore store;
+  final Narrow narrow;
+
+  @override
   Future<List<MentionAutocompleteResult>?> _computeResults(MentionAutocompleteQuery query) async {
     final List<MentionAutocompleteResult> results = [];
     final Iterable<User> users = store.users.values;