feat: Added support for startWeekOnMonday to MacosDatePicker (#414)

* feat: Added support for `startWeekOnMonday` to `MacosDatePicker`

* Update lib/src/selectors/date_picker.dart

---------

Co-authored-by: Reuben Turner <groovinchip@gmail.com>

Author: rklos

CHANGELOG.md

@@ -1,4 +1,7 @@
 ## [2.0.0-beta.3]
+✨ New ✨
+* Added support for `startWeekOnMonday` to `MacosDatePicker`.
+
 🛠️ Fixed 🛠️
 * Better UX of the click on the calendar elements in `MacosDatePicker`
 

lib/src/selectors/date_picker.dart

@@ -44,6 +44,7 @@ class MacosDatePicker extends StatefulWidget {
     this.style = DatePickerStyle.combined,
     required this.onDateChanged,
     this.initialDate,
+    this.startWeekOnMonday,
   });
 
   /// The [DatePickerStyle] to use.
@@ -59,6 +60,14 @@ class MacosDatePicker extends StatefulWidget {
   /// Defaults to `DateTime.now()`.
   final DateTime? initialDate;
 
+  /// Allows for changing the order of day headers in the graphical Date Picker
+  /// to Mo, Tu, We, Th, Fr, Sa, Su.
+  /// 
+  /// This is useful for internationalization purposes, as many countries begin their weeks on Mondays.
+  /// 
+  /// Defaults to `false`.
+  final bool? startWeekOnMonday;
+
   @override
   State<MacosDatePicker> createState() => _MacosDatePickerState();
 }
@@ -153,12 +162,23 @@ class _MacosDatePickerState extends State<MacosDatePicker> {
   }
 
   // Creates the day headers - Su, Mo, Tu, We, Th, Fr, Sa
+  // or Mo, Tu, We, Th, Fr, Sa, Su depending on the value of [startWeekOnMonday]
   List<Widget> _dayHeaders(
     TextStyle? headerStyle,
     MaterialLocalizations localizations,
   ) {
     final result = <Widget>[];
-    for (int i = localizations.firstDayOfWeekIndex; true; i = (i + 1) % 7) {
+
+    // Hack due to invalid "firstDayOfWeekIndex" implementation in MaterialLocalizations
+    // issue: https://github.com/flutter/flutter/issues/122274
+    // TODO: remove this workaround once the issue is fixed.
+    //  Then, "firstDayOfWeekIndex" can be controlled by passing "localizationsDelegates" and "supportedLocales" to MacosApp
+    int firstDayOfWeekIndex = localizations.firstDayOfWeekIndex;
+    if (widget.startWeekOnMonday == true) {
+      firstDayOfWeekIndex = 1;
+    }
+
+    for (int i = firstDayOfWeekIndex; result.length < 7; i = (i + 1) % 7) {
       final weekday = _narrowWeekdays[i];
       result.add(
         ExcludeSemantics(
@@ -170,7 +190,6 @@ class _MacosDatePickerState extends State<MacosDatePicker> {
           ),
         ),
       );
-      if (i == (localizations.firstDayOfWeekIndex - 1) % 7) break;
     }
     return result;
   }
@@ -474,9 +493,19 @@ class _MacosDatePickerState extends State<MacosDatePicker> {
           ),
       localizations,
     );
+
+    // Hack due to invalid "firstDayOfWeekIndex" implementation in MaterialLocalizations
+    // issue: https://github.com/flutter/flutter/issues/122274
+    // TODO: remove this workaround once the issue is fixed.
+    //  Then, DateUtils.getDaysInMonth will work as expected when proper "localizationsDelegates" and "supportedLocales" are provided to MacosApp
+    int fixedDayOffset = dayOffset;
+    if (widget.startWeekOnMonday == true) {
+      fixedDayOffset = dayOffset - 1;
+    }
+
     // 1-based day of month, e.g. 1-31 for January, and 1-29 for February on
     // a leap year.
-    int day = -dayOffset;
+    int day = -fixedDayOffset;
 
     final dayItems = <Widget>[];
 

test/selectors/date_picker_test.dart

@@ -308,4 +308,96 @@ void main() {
       },
     );
   });
+
+  testWidgets(
+    'Graphical MacosDatePicker with "startWeekOnMonday" set to true shows Monday as the first day of the week',
+        (tester) async {
+      await tester.pumpWidget(
+        MacosApp(
+          home: MacosWindow(
+            child: MacosScaffold(
+              children: [
+                ContentArea(
+                  builder: (context, _) {
+                    return Center(
+                      child: MacosDatePicker(
+                        startWeekOnMonday: true,
+                        initialDate: DateTime.parse('2023-04-01'),
+                        onDateChanged: (date) {},
+                      ),
+                    );
+                  },
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      final dayHeadersRow = find.byType(GridView).first;
+      final dayHeaders = find.descendant(
+        of: dayHeadersRow,
+        matching: find.byType(Text),
+      );
+      final firstWeekday = dayHeaders.first;
+      final firstWeekdayText = (firstWeekday.evaluate().first.widget as Text).data;
+      await tester.pumpAndSettle();
+
+      expect(firstWeekdayText, 'Mo');
+
+      final calendarGrid = find.byType(GridView).last;
+      final dayOffsetWidgets = find.descendant(
+        of: calendarGrid,
+        matching: find.byType(SizedBox),
+      );
+      final dayOffset = dayOffsetWidgets.evaluate().length;
+
+      expect(dayOffset, 5);
+    },
+  );
+
+  // Regression test due to invalid "firstDayOfWeekIndex" implementation in MaterialLocalizations
+  // issue: https://github.com/flutter/flutter/issues/122274
+  // TODO: remove this once the issue is fixed and test starts failing
+  testWidgets(
+    'Graphical MacosDatePicker still needs "startWeekOnMonday" to show Monday as the first day of the week, even when the locale is set to something other than "en_US"',
+        (tester) async {
+      await tester.pumpWidget(
+        MacosApp(
+          supportedLocales: const [
+            Locale('en', 'PL'),
+          ],
+          home: MacosWindow(
+            child: MacosScaffold(
+              children: [
+                ContentArea(
+                  builder: (context, _) {
+                    return Center(
+                      child: MacosDatePicker(
+                        startWeekOnMonday: true,
+                        initialDate: DateTime.parse('2023-04-01'),
+                        onDateChanged: (date) {},
+                      ),
+                    );
+                  },
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+
+      final dayHeadersRow = find.byType(GridView).first;
+      final dayHeaders = find.descendant(
+        of: dayHeadersRow,
+        matching: find.byType(Text),
+      );
+      final firstWeekday = dayHeaders.first;
+      final firstWeekdayText = (firstWeekday.evaluate().first.widget as Text).data;
+      await tester.pumpAndSettle();
+
+      // The result will be 'Tu' if the fix is no longer needed and can be removed
+      expect(firstWeekdayText, 'Mo');
+    },
+  );
 }