Cupertino picker haptic (#14831)

* Add selection haptic feedback to Cupertino Pickers

* review

* don't haptic on android

examples/flutter_gallery/lib/demo/cupertino/cupertino_picker_demo.dart

@@ -60,8 +60,9 @@ class _CupertinoPickerDemoState extends State<CupertinoPickerDemo> {
     final FixedExtentScrollController scrollController =
         new FixedExtentScrollController(initialItem: _selectedItemIndex);
 
-    return new SizedBox(
+    return new Container(
       height: _kPickerSheetHeight,
+      color: CupertinoColors.white,
       child: new DefaultTextStyle(
         style: const TextStyle(
           color: CupertinoColors.black,
@@ -70,20 +71,22 @@ class _CupertinoPickerDemoState extends State<CupertinoPickerDemo> {
         child: new GestureDetector(
           // Blocks taps from propagating to the modal sheet and popping.
           onTap: () {},
-          child: new CupertinoPicker(
-            scrollController: scrollController,
-            itemExtent: _kPickerItemHeight,
-            backgroundColor: CupertinoColors.white,
-            onSelectedItemChanged: (int index) {
-              setState(() {
-                _selectedItemIndex = index;
-              });
-            },
-            children: new List<Widget>.generate(coolColorNames.length, (int index) {
-              return new Center(child:
-                new Text(coolColorNames[index]),
-              );
-            }),
+          child: new SafeArea(
+            child: new CupertinoPicker(
+              scrollController: scrollController,
+              itemExtent: _kPickerItemHeight,
+              backgroundColor: CupertinoColors.white,
+              onSelectedItemChanged: (int index) {
+                setState(() {
+                  _selectedItemIndex = index;
+                });
+              },
+              children: new List<Widget>.generate(coolColorNames.length, (int index) {
+                return new Center(child:
+                  new Text(coolColorNames[index]),
+                );
+              }),
+            ),
           ),
         ),
       ),

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

@@ -4,6 +4,7 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
 /// Color of the 'magnifier' lens border.
@@ -99,10 +100,12 @@ class _CupertinoPickerState extends State<CupertinoPicker> {
   int _lastHapticIndex;
 
   void _handleSelectedItemChanged(int index) {
-    if (index != _lastHapticIndex) {
-      // TODO(xster): Insert haptic feedback with lighter knock.
-      // https://github.com/flutter/flutter/issues/13710.
+    // Only the haptic engine hardware on iOS devices would produce the
+    // intended effects.
+    if (defaultTargetPlatform == TargetPlatform.iOS
+        && index != _lastHapticIndex) {
       _lastHapticIndex = index;
+      HapticFeedback.selectionClick();
     }
 
     if (widget.onSelectedItemChanged != null) {

packages/flutter/test/cupertino/picker_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
@@ -56,6 +57,99 @@ void main() {
   });
 
   group('scroll', () {
+    testWidgets(
+      'scrolling calls onSelectedItemChanged and triggers haptic feedback',
+      (WidgetTester tester) async {
+        debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+        final List<int> selectedItems = <int>[];
+        final List<MethodCall> systemCalls = <MethodCall>[];
+
+        SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+          systemCalls.add(methodCall);
+        });
+
+        await tester.pumpWidget(
+          new Directionality(
+            textDirection: TextDirection.ltr,
+            child: new CupertinoPicker(
+              itemExtent: 100.0,
+              onSelectedItemChanged: (int index) { selectedItems.add(index); },
+              children: new List<Widget>.generate(100, (int index) {
+                return new Center(
+                  child: new Container(
+                    width: 400.0,
+                    height: 100.0,
+                    child: new Text(index.toString()),
+                  ),
+                );
+              }),
+            ),
+          ),
+        );
+
+        await tester.drag(find.text('0'), const Offset(0.0, -100.0));
+        expect(selectedItems, <int>[1]);
+        expect(
+          systemCalls.single,
+          isMethodCall(
+            'HapticFeedback.vibrate',
+            arguments: 'HapticFeedbackType.selectionClick',
+          ),
+        );
+
+        await tester.drag(find.text('0'), const Offset(0.0, 100.0));
+        expect(selectedItems, <int>[1, 0]);
+        expect(systemCalls, hasLength(2));
+        expect(
+          systemCalls.last,
+          isMethodCall(
+            'HapticFeedback.vibrate',
+            arguments: 'HapticFeedbackType.selectionClick',
+          ),
+        );
+
+        debugDefaultTargetPlatformOverride = null;
+      },
+    );
+
+    testWidgets(
+      'do not trigger haptic effects on non-iOS devices',
+      (WidgetTester tester) async {
+        debugDefaultTargetPlatformOverride = TargetPlatform.android;
+        final List<int> selectedItems = <int>[];
+        final List<MethodCall> systemCalls = <MethodCall>[];
+
+        SystemChannels.platform.setMockMethodCallHandler((MethodCall methodCall) async {
+          systemCalls.add(methodCall);
+        });
+
+        await tester.pumpWidget(
+          new Directionality(
+            textDirection: TextDirection.ltr,
+            child: new CupertinoPicker(
+              itemExtent: 100.0,
+              onSelectedItemChanged: (int index) { selectedItems.add(index); },
+              children: new List<Widget>.generate(100, (int index) {
+                return new Center(
+                  child: new Container(
+                    width: 400.0,
+                    height: 100.0,
+                    child: new Text(index.toString()),
+                  ),
+                );
+              }),
+            ),
+          ),
+        );
+
+        await tester.drag(find.text('0'), const Offset(0.0, -100.0));
+        expect(selectedItems, <int>[1]);
+        expect(systemCalls, isEmpty);
+
+        debugDefaultTargetPlatformOverride = null;
+      },
+    );
+
     testWidgets('a drag in between items settles back', (WidgetTester tester) async {
       debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
       final FixedExtentScrollController controller =