Allow date pickers to not have selected date (#132343)

This enables our various date picker classes to have a null `initialDate`.

It also fixes the logic of some of the widgets which used to do something when you _changed_ the `initial*` parameters, which is wrong for `initial*` properties (they by definition should only impact the initial state) and wrong for properties in general (behaviour should not change based on whether the widget was built with a new value or not, that violates the reactive design principles).

Fixes https://github.com/flutter/flutter/issues/638.

dev/integration_tests/flutter_gallery/lib/demo/material/date_and_time_picker_demo.dart

@@ -62,7 +62,7 @@ class _DateTimePicker extends StatelessWidget {
   Future<void> _selectDate(BuildContext context) async {
     final DateTime? picked = await showDatePicker(
       context: context,
-      initialDate: selectedDate!,
+      initialDate: selectedDate,
       firstDate: DateTime(2015, 8),
       lastDate: DateTime(2101),
     );
@@ -120,9 +120,9 @@ class DateAndTimePickerDemo extends StatefulWidget {
 }
 
 class _DateAndTimePickerDemoState extends State<DateAndTimePickerDemo> {
-  DateTime _fromDate = DateTime.now();
+  DateTime? _fromDate = DateTime.now();
   TimeOfDay _fromTime = const TimeOfDay(hour: 7, minute: 28);
-  DateTime _toDate = DateTime.now();
+  DateTime? _toDate = DateTime.now();
   TimeOfDay _toTime = const TimeOfDay(hour: 8, minute: 28);
   final List<String> _allActivities = <String>['hiking', 'swimming', 'boating', 'fishing'];
   String? _activity = 'fishing';

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

@@ -53,18 +53,19 @@ const double _kMaxTextScaleFactor = 1.3;
 /// The returned [Future] resolves to the date selected by the user when the
 /// user confirms the dialog. If the user cancels the dialog, null is returned.
 ///
-/// When the date picker is first displayed, it will show the month of
-/// [initialDate], with [initialDate] selected.
+/// When the date picker is first displayed, if [initialDate] is not null, it
+/// will show the month of [initialDate], with [initialDate] selected. Otherwise
+/// it will show the [currentDate]'s month.
 ///
 /// The [firstDate] is the earliest allowable date. The [lastDate] is the latest
-/// allowable date. [initialDate] must either fall between these dates,
-/// or be equal to one of them. For each of these [DateTime] parameters, only
-/// their dates are considered. Their time fields are ignored. They must all
-/// be non-null.
+/// allowable date. If [initialDate] is not null, it must either fall between
+/// these dates, or be equal to one of them. For each of these [DateTime]
+/// parameters, only their dates are considered. Their time fields are ignored.
+/// They must all be non-null.
 ///
 /// The [currentDate] represents the current day (i.e. today). This
 /// date will be highlighted in the day grid. If null, the date of
-/// `DateTime.now()` will be used.
+/// [DateTime.now] will be used.
 ///
 /// An optional [initialEntryMode] argument can be used to display the date
 /// picker in the [DatePickerEntryMode.calendar] (a calendar month grid)
@@ -123,8 +124,7 @@ const double _kMaxTextScaleFactor = 1.3;
 ///
 /// An optional [initialDatePickerMode] argument can be used to have the
 /// calendar date picker initially appear in the [DatePickerMode.year] or
-/// [DatePickerMode.day] mode. It defaults to [DatePickerMode.day], and
-/// must be non-null.
+/// [DatePickerMode.day] mode. It defaults to [DatePickerMode.day].
 ///
 /// {@macro flutter.widgets.RawDialogRoute}
 ///
@@ -157,10 +157,9 @@ const double _kMaxTextScaleFactor = 1.3;
 ///  * [DisplayFeatureSubScreen], which documents the specifics of how
 ///    [DisplayFeature]s can split the screen into sub-screens.
 ///  * [showTimePicker], which shows a dialog that contains a Material Design time picker.
-///
 Future<DateTime?> showDatePicker({
   required BuildContext context,
-  required DateTime initialDate,
+  DateTime? initialDate,
   required DateTime firstDate,
   required DateTime lastDate,
   DateTime? currentDate,
@@ -188,7 +187,7 @@ Future<DateTime?> showDatePicker({
   final Icon? switchToInputEntryModeIcon,
   final Icon? switchToCalendarEntryModeIcon,
 }) async {
-  initialDate = DateUtils.dateOnly(initialDate);
+  initialDate = initialDate == null ? null : DateUtils.dateOnly(initialDate);
   firstDate = DateUtils.dateOnly(firstDate);
   lastDate = DateUtils.dateOnly(lastDate);
   assert(
@@ -196,15 +195,15 @@ Future<DateTime?> showDatePicker({
     'lastDate $lastDate must be on or after firstDate $firstDate.',
   );
   assert(
-    !initialDate.isBefore(firstDate),
+    initialDate == null || !initialDate.isBefore(firstDate),
     'initialDate $initialDate must be on or after firstDate $firstDate.',
   );
   assert(
-    !initialDate.isAfter(lastDate),
+    initialDate == null || !initialDate.isAfter(lastDate),
     'initialDate $initialDate must be on or before lastDate $lastDate.',
   );
   assert(
-    selectableDayPredicate == null || selectableDayPredicate(initialDate),
+    selectableDayPredicate == null || initialDate == null || selectableDayPredicate(initialDate),
     'Provided initialDate $initialDate must satisfy provided selectableDayPredicate.',
   );
   assert(debugCheckHasMaterialLocalizations(context));
@@ -272,7 +271,7 @@ class DatePickerDialog extends StatefulWidget {
   /// A Material-style date picker dialog.
   DatePickerDialog({
     super.key,
-    required DateTime initialDate,
+    DateTime? initialDate,
     required DateTime firstDate,
     required DateTime lastDate,
     DateTime? currentDate,
@@ -291,7 +290,7 @@ class DatePickerDialog extends StatefulWidget {
     this.onDatePickerModeChange,
     this.switchToInputEntryModeIcon,
     this.switchToCalendarEntryModeIcon,
-  }) : initialDate = DateUtils.dateOnly(initialDate),
+  }) : initialDate = initialDate == null ? null : DateUtils.dateOnly(initialDate),
        firstDate = DateUtils.dateOnly(firstDate),
        lastDate = DateUtils.dateOnly(lastDate),
        currentDate = DateUtils.dateOnly(currentDate ?? DateTime.now()) {
@@ -300,21 +299,24 @@ class DatePickerDialog extends StatefulWidget {
       'lastDate ${this.lastDate} must be on or after firstDate ${this.firstDate}.',
     );
     assert(
-      !this.initialDate.isBefore(this.firstDate),
+      initialDate == null || !this.initialDate!.isBefore(this.firstDate),
       'initialDate ${this.initialDate} must be on or after firstDate ${this.firstDate}.',
     );
     assert(
-      !this.initialDate.isAfter(this.lastDate),
+      initialDate == null || !this.initialDate!.isAfter(this.lastDate),
       'initialDate ${this.initialDate} must be on or before lastDate ${this.lastDate}.',
     );
     assert(
-      selectableDayPredicate == null || selectableDayPredicate!(this.initialDate),
+      selectableDayPredicate == null || initialDate == null || selectableDayPredicate!(this.initialDate!),
       'Provided initialDate ${this.initialDate} must satisfy provided selectableDayPredicate',
     );
   }
 
   /// The initially selected [DateTime] that the picker should display.
-  final DateTime initialDate;
+  ///
+  /// If this is null, there is no selected date. A date must be selected to
+  /// submit the dialog.
+  final DateTime? initialDate;
 
   /// The earliest allowable [DateTime] that the user can select.
   final DateTime firstDate;
@@ -410,7 +412,7 @@ class DatePickerDialog extends StatefulWidget {
 }
 
 class _DatePickerDialogState extends State<DatePickerDialog> with RestorationMixin {
-  late final RestorableDateTime _selectedDate = RestorableDateTime(widget.initialDate);
+  late final RestorableDateTimeN _selectedDate = RestorableDateTimeN(widget.initialDate);
   late final _RestorableDatePickerEntryMode _entryMode = _RestorableDatePickerEntryMode(widget.initialEntryMode);
   final _RestorableAutovalidateMode _autovalidateMode = _RestorableAutovalidateMode(AutovalidateMode.disabled);
 
@@ -639,7 +641,7 @@ class _DatePickerDialogState extends State<DatePickerDialog> with RestorationMix
           ? localizations.datePickerHelpText
           : localizations.datePickerHelpText.toUpperCase()
       ),
-      titleText: localizations.formatMediumDate(_selectedDate.value),
+      titleText: _selectedDate.value == null ? '' : localizations.formatMediumDate(_selectedDate.value!),
       titleStyle: headlineStyle,
       orientation: orientation,
       isShort: orientation == Orientation.landscape,

packages/flutter/test/material/date_picker_test.dart

@@ -14,7 +14,7 @@ void main() {
 
   late DateTime firstDate;
   late DateTime lastDate;
-  late DateTime initialDate;
+  late DateTime? initialDate;
   late DateTime today;
   late SelectableDayPredicate? selectableDayPredicate;
   late DatePickerEntryMode initialEntryMode;
@@ -1044,6 +1044,37 @@ void main() {
       });
     });
 
+    testWidgets('Can select a day with no initial date', (WidgetTester tester) async {
+      initialDate = null;
+      await prepareDatePicker(tester, (Future<DateTime?> date) async {
+        await tester.tap(find.text('12'));
+        await tester.tap(find.text('OK'));
+        expect(await date, equals(DateTime(2016, DateTime.january, 12)));
+      });
+    });
+
+    testWidgets('Can select a month with no initial date', (WidgetTester tester) async {
+      initialDate = null;
+      await prepareDatePicker(tester, (Future<DateTime?> date) async {
+        await tester.tap(previousMonthIcon);
+        await tester.pumpAndSettle(const Duration(seconds: 1));
+        await tester.tap(find.text('25'));
+        await tester.tap(find.text('OK'));
+        expect(await date, DateTime(2015, DateTime.december, 25));
+      });
+    });
+
+    testWidgets('Can select a year with no initial date', (WidgetTester tester) async {
+      initialDate = null;
+      await prepareDatePicker(tester, (Future<DateTime?> date) async {
+        await tester.tap(find.text('January 2016')); // Switch to year mode.
+        await tester.pump();
+        await tester.tap(find.text('2018'));
+        await tester.pump();
+        expect(find.text('January 2018'), findsOneWidget);
+      });
+    });
+
     testWidgets('Selecting date does not change displayed month', (WidgetTester tester) async {
       initialDate = DateTime(2020, DateTime.march, 15);
       await prepareDatePicker(tester, (Future<DateTime?> date) async {
@@ -1105,8 +1136,8 @@ void main() {
 
     testWidgets('Cannot select a day outside bounds', (WidgetTester tester) async {
       initialDate = DateTime(2017, DateTime.january, 15);
-      firstDate = initialDate;
-      lastDate = initialDate;
+      firstDate = initialDate!;
+      lastDate = initialDate!;
       await prepareDatePicker(tester, (Future<DateTime?> date) async {
         // Earlier than firstDate. Should be ignored.
         await tester.tap(find.text('10'));
@@ -1120,7 +1151,7 @@ void main() {
 
     testWidgets('Cannot select a month past last date', (WidgetTester tester) async {
       initialDate = DateTime(2017, DateTime.january, 15);
-      firstDate = initialDate;
+      firstDate = initialDate!;
       lastDate = DateTime(2017, DateTime.february, 20);
       await prepareDatePicker(tester, (Future<DateTime?> date) async {
         await tester.tap(nextMonthIcon);
@@ -1133,7 +1164,7 @@ void main() {
     testWidgets('Cannot select a month before first date', (WidgetTester tester) async {
       initialDate = DateTime(2017, DateTime.january, 15);
       firstDate = DateTime(2016, DateTime.december, 10);
-      lastDate = initialDate;
+      lastDate = initialDate!;
       await prepareDatePicker(tester, (Future<DateTime?> date) async {
         await tester.tap(previousMonthIcon);
         await tester.pumpAndSettle(const Duration(seconds: 1));