Changes to initialDate for CalendarDatePicker should update the state. (#64115)

packages/flutter/lib/src/material/pickers/calendar_date_picker.dart

@@ -12,6 +12,7 @@ import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
 import '../color_scheme.dart';
+import '../debug.dart';
 import '../divider.dart';
 import '../icon_button.dart';
 import '../icons.dart';
@@ -164,14 +165,21 @@ class _CalendarDatePickerState extends State<CalendarDatePicker> {
   @override
   void initState() {
     super.initState();
-    _mode = widget.initialCalendarMode;
-    _currentDisplayedMonthDate = DateTime(widget.initialDate.year, widget.initialDate.month);
-    _selectedDate = widget.initialDate;
+    _initWidgetState();
+  }
+
+  @override
+  void didUpdateWidget(CalendarDatePicker oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    _initWidgetState();
   }
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
+    assert(debugCheckHasMaterial(context));
+    assert(debugCheckHasMaterialLocalizations(context));
+    assert(debugCheckHasDirectionality(context));
     _localizations = MaterialLocalizations.of(context);
     _textDirection = Directionality.of(context);
     if (!_announcedInitialDate) {
@@ -183,6 +191,12 @@ class _CalendarDatePickerState extends State<CalendarDatePicker> {
     }
   }
 
+  void _initWidgetState() {
+    _mode = widget.initialCalendarMode;
+    _currentDisplayedMonthDate = DateTime(widget.initialDate.year, widget.initialDate.month);
+    _selectedDate = widget.initialDate;
+  }
+
   void _vibrate() {
     switch (Theme.of(context).platform) {
       case TargetPlatform.android:
@@ -281,6 +295,9 @@ class _CalendarDatePickerState extends State<CalendarDatePicker> {
 
   @override
   Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
+    assert(debugCheckHasMaterialLocalizations(context));
+    assert(debugCheckHasDirectionality(context));
     return Stack(
       children: <Widget>[
         SizedBox(
@@ -514,6 +531,14 @@ class _MonthPickerState extends State<_MonthPicker> {
     _textDirection = Directionality.of(context);
   }
 
+  @override
+  void didUpdateWidget(_MonthPicker oldWidget) {
+    super.didUpdateWidget(oldWidget);
+    if (widget.initialMonth != oldWidget.initialMonth) {
+      _showMonth(widget.initialMonth);
+    }
+  }
+
   @override
   void dispose() {
     _pageController?.dispose();

packages/flutter/test/material/date_picker_test.dart

@@ -825,6 +825,62 @@ void main() {
     });
   });
 
+  group('CalendarDatePicker', () {
+    // Tests for the standalone CalendarDatePicker class
+    testWidgets('Updates to initialDate parameter is reflected in the state', (WidgetTester tester) async {
+      final Key pickerKey = UniqueKey();
+      final DateTime initialDate = DateTime(2020, 1, 21);
+      final DateTime updatedDate = DateTime(1976, 2, 23);
+      const Color selectedColor = Color(0xff2196f3); // default primary color
+
+      await tester.pumpWidget(MaterialApp(
+        home: Material(
+          child: CalendarDatePicker(
+            key: pickerKey,
+            initialDate: initialDate,
+            firstDate: DateTime(1970, 1, 1),
+            lastDate: DateTime(2099, 31, 12),
+            onDateChanged: (DateTime value) {},
+          ),
+        ),
+      ));
+      await tester.pumpAndSettle();
+
+      // Month should show as January 2020
+      expect(find.text('January 2020'), findsOneWidget);
+      // Selected date should be painted with a colored circle
+      expect(
+        Material.of(tester.element(find.text('21'))),
+        paints..circle(color: selectedColor, style: PaintingStyle.fill)
+      );
+
+      // Change to the updated initialDate
+      await tester.pumpWidget(MaterialApp(
+        home: Material(
+          child: CalendarDatePicker(
+            key: pickerKey,
+            initialDate: updatedDate,
+            firstDate: DateTime(1970, 1, 1),
+            lastDate: DateTime(2099, 31, 12),
+            onDateChanged: (DateTime value) {},
+          ),
+        ),
+      ));
+      // Wait for the page scroll animation to finish
+      await tester.pumpAndSettle(const Duration(milliseconds: 200));
+
+      // Month should show as February 1976
+      expect(find.text('January 2020'), findsNothing);
+      expect(find.text('February 1976'), findsOneWidget);
+      // Selected date should be painted with a colored circle
+      expect(
+          Material.of(tester.element(find.text('23'))),
+          paints..circle(color: selectedColor, style: PaintingStyle.fill)
+      );
+
+    });
+  });
+
   group('Haptic feedback', () {
     const Duration hapticFeedbackInterval = Duration(milliseconds: 10);
     FeedbackTester feedback;