Fix immediately overriding the user's chosen AM/PM selection (#26904)

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

@@ -351,16 +351,35 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
   // in the widget after first build is ignored.
   DateTime initialDateTime;
 
-  // The currently selected values of the date picker.
-  int selectedDayFromInitial; // The difference in days between the initial date and the currently selected date.
+  // The difference in days between the initial date and the currently selected date.
+  int selectedDayFromInitial;
+
+  // The current selection of the hour picker.
+  //
+  // If [widget.use24hFormat] is true, values range from 1-24. Otherwise values
+  // range from 1-12.
   int selectedHour;
+
+  // The previous selection index of the hour column.
+  //
+  // This ranges from 0-23 even if [widget.use24hFormat] is false. As a result,
+  // it can be used for determining if we just changed from AM -> PM or vice
+  // versa.
+  int previousHourIndex;
+
+  // The current selection of the minute picker. Values range from 0 to 59.
   int selectedMinute;
-  int selectedAmPm; // 0 means AM, 1 means PM.
+
+  // The current selection of the AM/PM picker.
+  //
+  // - 0 means AM
+  // - 1 means PM
+  int selectedAmPm;
 
   // The controller of the AM/PM column.
   FixedExtentScrollController amPmController;
 
-  // Estimated width of columns.
+  // The estimated width of columns.
   final Map<int, double> estimatedColumnWidths = <int, double>{};
 
   @override
@@ -380,6 +399,8 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
 
       amPmController = FixedExtentScrollController(initialItem: selectedAmPm);
     }
+
+    previousHourIndex = selectedHour;
   }
 
   @override
@@ -477,25 +498,29 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
         if (widget.use24hFormat) {
           selectedHour = index;
           widget.onDateTimeChanged(_getDateTime());
-        }
-        else {
-          final int currentHourIn24h = selectedHour + selectedAmPm * 12;
+        } else {
+          selectedHour = index % 12;
+
           // Automatically scrolls the am/pm column when the hour column value
-          // goes far enough. This behavior is similar to
-          // iOS picker version.
-          if (currentHourIn24h ~/ 12 != index ~/ 12) {
-            selectedHour = index % 12;
+          // goes far enough.
+
+          final bool wasAm = previousHourIndex >=0 && previousHourIndex <= 11;
+          final bool isAm = index >= 0 && index <= 11;
+
+          if (wasAm != isAm) {
+            // Animation values obtained by comparing with iOS version.
             amPmController.animateToItem(
               1 - amPmController.selectedItem,
-              duration: const Duration(milliseconds: 300), // Set by comparing with iOS version.
+              duration: const Duration(milliseconds: 300),
               curve: Curves.easeOut,
-            ); // Set by comparing with iOS version.
+            );
           }
           else {
-            selectedHour = index % 12;
             widget.onDateTimeChanged(_getDateTime());
           }
         }
+
+        previousHourIndex = index;
       },
       children: List<Widget>.generate(24, (int index) {
         int hour = index;

packages/flutter/test/cupertino/date_picker_test.dart

@@ -7,6 +7,9 @@ import 'package:flutter/cupertino.dart';
 import 'package:flutter/semantics.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+// scrolling by this offset will move the picker to the next item
+const Offset _kRowOffset = Offset(0.0, -32.0);
+
 void main() {
   group('Countdown timer picker', () {
     testWidgets('onTimerDurationChanged is not null', (WidgetTester tester) async {
@@ -524,6 +527,53 @@ void main() {
       );
     });
 
+    testWidgets('picker persists am/pm value when scrolling hours', (WidgetTester tester) async {
+      DateTime date;
+      await tester.pumpWidget(
+        CupertinoApp(
+          home: SizedBox(
+            height: 400.0,
+            width: 400.0,
+            child: CupertinoDatePicker(
+              mode: CupertinoDatePickerMode.time,
+              onDateTimeChanged: (DateTime newDate) {
+                date = newDate;
+              },
+              initialDateTime: DateTime(2019, 1, 1, 3),
+            ),
+          ),
+        ),
+      );
+
+      // 3:00 -> 15:00
+      await tester.drag(find.text('AM'), _kRowOffset);
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 500));
+
+      expect(date, DateTime(2019, 1, 1, 15));
+
+      // 15:00 -> 16:00
+      await tester.drag(find.text('3'), _kRowOffset);
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 500));
+
+      expect(date, DateTime(2019, 1, 1, 16));
+
+      // 16:00 -> 4:00
+      await tester.drag(find.text('PM'), -_kRowOffset);
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 500));
+
+      expect(date, DateTime(2019, 1, 1, 4));
+
+      // 4:00 -> 3:00
+      await tester.drag(find.text('4'), -_kRowOffset);
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 500));
+
+      expect(date, DateTime(2019, 1, 1, 3));
+    });
+
     testWidgets('picker automatically scrolls the am/pm column when the hour column changes enough', (WidgetTester tester) async {
       DateTime date;
       await tester.pumpWidget(
@@ -542,25 +592,29 @@ void main() {
         ),
       );
 
-      await tester.drag(find.text('11'), const Offset(0.0, -32.0));
+      // 11:59 -> 12:59
+      await tester.drag(find.text('11'), _kRowOffset);
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 500));
 
       expect(date, DateTime(2018, 1, 1, 12, 59));
 
-      await tester.drag(find.text('12'), const Offset(0.0, 32.0));
+      // 12:59 -> 11:59
+      await tester.drag(find.text('12'), -_kRowOffset);
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 500));
 
       expect(date, DateTime(2018, 1, 1, 11, 59));
 
-      await tester.drag(find.text('11'), const Offset(0.0, 64.0));
+      // 11:59 -> 9:59
+      await tester.drag(find.text('11'), -_kRowOffset * 2);
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 500));
 
       expect(date, DateTime(2018, 1, 1, 9, 59));
 
-      await tester.drag(find.text('9'), const Offset(0.0, -192.0));
+      // 9:59 -> 15:59
+      await tester.drag(find.text('9'), _kRowOffset * 6);
       await tester.pump();
       await tester.pump(const Duration(milliseconds: 500));