Added filter callback on dropdown menu (flutter#143939)

DropdownMenu can now customize its filter using the new parameter DropdownMenu.filterCallback, similar to DropdownMenu.searchCallback.

Author: dacianf

packages/flutter/lib/src/material/dropdown_menu.dart

@@ -25,6 +25,12 @@ import 'theme_data.dart';
 // late BuildContext context;
 // late FocusNode myFocusNode;
 
+/// A callback function that returns the list of the items that matches the
+/// current applied filter.
+///
+/// Used by [DropdownMenu.filterCallback].
+typedef FilterCallback<T> = List<DropdownMenuEntry<T>> Function(List<DropdownMenuEntry<T>> entries, String filter);
+
 /// A callback function that returns the index of the item that matches the
 /// current contents of a text field.
 ///
@@ -163,10 +169,11 @@ class DropdownMenu<T> extends StatefulWidget {
     this.focusNode,
     this.requestFocusOnTap,
     this.expandedInsets,
+    this.filterCallback,
     this.searchCallback,
     required this.dropdownMenuEntries,
     this.inputFormatters,
-  });
+  }) : assert(filterCallback == null || enableFilter);
 
   /// Determine if the [DropdownMenu] is enabled.
   ///
@@ -382,6 +389,41 @@ class DropdownMenu<T> extends StatefulWidget {
   /// Defaults to null.
   final EdgeInsets? expandedInsets;
 
+  /// When [DropdownMenu.enableFilter] is true, this callback is used to
+  /// compute the list of filtered items.
+  ///
+  /// {@tool snippet}
+  ///
+  /// In this example the `filterCallback` returns the items that contains the
+  /// trimmed query.
+  ///
+  /// ```dart
+  /// DropdownMenu<Text>(
+  ///   enableFilter: true,
+  ///   filterCallback: (List<DropdownMenuEntry<Text>> entries, String filter) {
+  ///     final String trimmedFilter = filter.trim().toLowerCase();
+  ///       if (trimmedFilter.isEmpty) {
+  ///         return entries;
+  ///       }
+  ///
+  ///       return entries
+  ///         .where((DropdownMenuEntry<Text> entry) =>
+  ///           entry.label.toLowerCase().contains(trimmedFilter),
+  ///         )
+  ///         .toList();
+  ///   },
+  ///   dropdownMenuEntries: const <DropdownMenuEntry<Text>>[],
+  /// )
+  /// ```
+  /// {@end-tool}
+  ///
+  /// Defaults to null. If this parameter is null and the
+  /// [DropdownMenu.enableFilter] property is set to true, the default behavior
+  /// will return a filtered list. The filtered list will contain items
+  /// that match the text provided by the input field, with a case-insensitive
+  /// comparison. When this is not null, `enableFilter` must be set to true.
+  final FilterCallback<T>? filterCallback;
+
   /// When [DropdownMenu.enableSearch] is true, this callback is used to compute
   /// the index of the search result to be highlighted.
   ///
@@ -691,7 +733,8 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     final DropdownMenuThemeData defaults = _DropdownMenuDefaultsM3(context);
 
     if (_enableFilter) {
-      filteredEntries = filter(widget.dropdownMenuEntries, _localTextEditingController!);
+      filteredEntries = widget.filterCallback?.call(filteredEntries, _localTextEditingController!.text)
+        ?? filter(widget.dropdownMenuEntries, _localTextEditingController!);
     }
 
     if (widget.enableSearch) {

packages/flutter/test/material/dropdown_menu_test.dart

@@ -1130,6 +1130,70 @@ void main() {
     }
   });
 
+  testWidgets('Enable filtering with custom filter callback that filter text case sensitive', (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData();
+    final TextEditingController controller = TextEditingController();
+    addTearDown(controller.dispose);
+
+    await tester.pumpWidget(MaterialApp(
+      theme: themeData,
+      home: Scaffold(
+        body: DropdownMenu<TestMenu>(
+          requestFocusOnTap: true,
+          enableFilter: true,
+          filterCallback: (List<DropdownMenuEntry<TestMenu>> entries, String filter) {
+            return entries.where((DropdownMenuEntry<TestMenu> element) => element.label.contains(filter)).toList();
+          },
+          dropdownMenuEntries: menuChildren,
+          controller: controller,
+        ),
+      ),
+    ));
+
+    // Open the menu.
+    await tester.tap(find.byType(DropdownMenu<TestMenu>));
+    await tester.pump();
+
+    await tester.enterText(find.byType(TextField).first, 'item');
+    expect(controller.text, 'item');
+    await tester.pumpAndSettle();
+    for (final TestMenu menu in TestMenu.values) {
+      expect(find.widgetWithText(MenuItemButton, menu.label).hitTestable(), findsNothing);
+    }
+
+    await tester.enterText(find.byType(TextField).first, 'Item');
+    expect(controller.text, 'Item');
+    await tester.pumpAndSettle();
+    expect(find.widgetWithText(MenuItemButton, 'Item 0').hitTestable(), findsOneWidget);
+    expect(find.widgetWithText(MenuItemButton, 'Menu 1').hitTestable(), findsNothing);
+    expect(find.widgetWithText(MenuItemButton, 'Item 2').hitTestable(), findsOneWidget);
+    expect(find.widgetWithText(MenuItemButton, 'Item 3').hitTestable(), findsOneWidget);
+    expect(find.widgetWithText(MenuItemButton, 'Item 4').hitTestable(), findsOneWidget);
+    expect(find.widgetWithText(MenuItemButton, 'Item 5').hitTestable(), findsOneWidget);
+  });
+
+  testWidgets('Throw assertion error when enable filtering with custom filter callback and enableFilter set on False', (WidgetTester tester) async {
+    final ThemeData themeData = ThemeData();
+    final TextEditingController controller = TextEditingController();
+    addTearDown(controller.dispose);
+
+    expect((){
+      MaterialApp(
+        theme: themeData,
+        home: Scaffold(
+          body: DropdownMenu<TestMenu>(
+            requestFocusOnTap: true,
+            filterCallback: (List<DropdownMenuEntry<TestMenu>> entries, String filter) {
+              return entries.where((DropdownMenuEntry<TestMenu> element) => element.label.contains(filter)).toList();
+            },
+            dropdownMenuEntries: menuChildren,
+            controller: controller,
+          ),
+        ),
+      );
+    }, throwsAssertionError);
+  });
+
   testWidgets('The controller can access the value in the input field', (WidgetTester tester) async {
     final ThemeData themeData = ThemeData();
     final TextEditingController controller = TextEditingController();