Make textfields restorable (#63401)

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

@@ -273,6 +273,7 @@ class CupertinoTextField extends StatefulWidget {
     this.scrollController,
     this.scrollPhysics,
     this.autofillHints,
+    this.restorationId,
   }) : assert(textAlign != null),
        assert(readOnly != null),
        assert(autofocus != null),
@@ -600,6 +601,9 @@ class CupertinoTextField extends StatefulWidget {
   /// {@macro flutter.services.autofill.autofillHints}
   final Iterable<String> autofillHints;
 
+  /// {@macro flutter.material.textfield.restorationId}
+  final String restorationId;
+
   @override
   _CupertinoTextFieldState createState() => _CupertinoTextFieldState();
 
@@ -641,11 +645,11 @@ class CupertinoTextField extends StatefulWidget {
   }
 }
 
-class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticKeepAliveClientMixin implements TextSelectionGestureDetectorBuilderDelegate {
+class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin implements TextSelectionGestureDetectorBuilderDelegate {
   final GlobalKey _clearGlobalKey = GlobalKey();
 
-  TextEditingController _controller;
-  TextEditingController get _effectiveController => widget.controller ?? _controller;
+  RestorableTextEditingController _controller;
+  TextEditingController get _effectiveController => widget.controller ?? _controller.value;
 
   FocusNode _focusNode;
   FocusNode get _effectiveFocusNode => widget.focusNode ?? (_focusNode ??= FocusNode());
@@ -670,8 +674,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
     super.initState();
     _selectionGestureDetectorBuilder = _CupertinoTextFieldSelectionGestureDetectorBuilder(state: this);
     if (widget.controller == null) {
-      _controller = TextEditingController();
-      _controller.addListener(updateKeepAlive);
+      _createLocalController();
     }
   }
 
@@ -679,9 +682,10 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
   void didUpdateWidget(CupertinoTextField oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.controller == null && oldWidget.controller != null) {
-      _controller = TextEditingController.fromValue(oldWidget.controller.value);
-      _controller.addListener(updateKeepAlive);
+      _createLocalController(oldWidget.controller.value);
     } else if (widget.controller != null && oldWidget.controller == null) {
+      unregisterFromRestoration(_controller);
+      _controller.dispose();
       _controller = null;
     }
     final bool isEnabled = widget.enabled ?? true;
@@ -691,10 +695,36 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
     }
   }
 
+  @override
+  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+    if (_controller != null) {
+      _registerController();
+    }
+  }
+
+  void _registerController() {
+    assert(_controller != null);
+    registerForRestoration(_controller, 'controller');
+    _controller.value.addListener(updateKeepAlive);
+  }
+
+  void _createLocalController([TextEditingValue value]) {
+    assert(_controller == null);
+    _controller = value == null
+        ? RestorableTextEditingController()
+        : RestorableTextEditingController.fromValue(value);
+    if (!restorePending) {
+      _registerController();
+    }
+  }
+
+  @override
+  String get restorationId => widget.restorationId;
+
   @override
   void dispose() {
     _focusNode?.dispose();
-    _controller?.removeListener(updateKeepAlive);
+    _controller?.dispose();
     super.dispose();
   }
 
@@ -736,7 +766,7 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
   }
 
   @override
-  bool get wantKeepAlive => _controller?.text?.isNotEmpty == true;
+  bool get wantKeepAlive => _controller?.value?.text?.isNotEmpty == true;
 
   bool _shouldShowAttachment({
     OverlayVisibilityMode attachment,
@@ -927,57 +957,61 @@ class _CupertinoTextFieldState extends State<CupertinoTextField> with AutomaticK
     final Widget paddedEditable = Padding(
       padding: widget.padding,
       child: RepaintBoundary(
-        child: EditableText(
-          key: editableTextKey,
-          controller: controller,
-          readOnly: widget.readOnly,
-          toolbarOptions: widget.toolbarOptions,
-          showCursor: widget.showCursor,
-          showSelectionHandles: _showSelectionHandles,
-          focusNode: _effectiveFocusNode,
-          keyboardType: widget.keyboardType,
-          textInputAction: widget.textInputAction,
-          textCapitalization: widget.textCapitalization,
-          style: textStyle,
-          strutStyle: widget.strutStyle,
-          textAlign: widget.textAlign,
-          autofocus: widget.autofocus,
-          obscuringCharacter: widget.obscuringCharacter,
-          obscureText: widget.obscureText,
-          autocorrect: widget.autocorrect,
-          smartDashesType: widget.smartDashesType,
-          smartQuotesType: widget.smartQuotesType,
-          enableSuggestions: widget.enableSuggestions,
-          maxLines: widget.maxLines,
-          minLines: widget.minLines,
-          expands: widget.expands,
-          selectionColor: selectionColor,
-          selectionControls: widget.selectionEnabled
-            ? cupertinoTextSelectionControls : null,
-          onChanged: widget.onChanged,
-          onSelectionChanged: _handleSelectionChanged,
-          onEditingComplete: widget.onEditingComplete,
-          onSubmitted: widget.onSubmitted,
-          inputFormatters: formatters,
-          rendererIgnoresPointer: true,
-          cursorWidth: widget.cursorWidth,
-          cursorHeight: widget.cursorHeight,
-          cursorRadius: widget.cursorRadius,
-          cursorColor: cursorColor,
-          cursorOpacityAnimates: true,
-          cursorOffset: cursorOffset,
-          paintCursorAboveText: true,
-          autocorrectionTextRectColor: selectionColor,
-          backgroundCursorColor: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context),
-          selectionHeightStyle: widget.selectionHeightStyle,
-          selectionWidthStyle: widget.selectionWidthStyle,
-          scrollPadding: widget.scrollPadding,
-          keyboardAppearance: keyboardAppearance,
-          dragStartBehavior: widget.dragStartBehavior,
-          scrollController: widget.scrollController,
-          scrollPhysics: widget.scrollPhysics,
-          enableInteractiveSelection: widget.enableInteractiveSelection,
-          autofillHints: widget.autofillHints,
+        child: UnmanagedRestorationScope(
+          bucket: bucket,
+          child: EditableText(
+            key: editableTextKey,
+            controller: controller,
+            readOnly: widget.readOnly,
+            toolbarOptions: widget.toolbarOptions,
+            showCursor: widget.showCursor,
+            showSelectionHandles: _showSelectionHandles,
+            focusNode: _effectiveFocusNode,
+            keyboardType: widget.keyboardType,
+            textInputAction: widget.textInputAction,
+            textCapitalization: widget.textCapitalization,
+            style: textStyle,
+            strutStyle: widget.strutStyle,
+            textAlign: widget.textAlign,
+            autofocus: widget.autofocus,
+            obscuringCharacter: widget.obscuringCharacter,
+            obscureText: widget.obscureText,
+            autocorrect: widget.autocorrect,
+            smartDashesType: widget.smartDashesType,
+            smartQuotesType: widget.smartQuotesType,
+            enableSuggestions: widget.enableSuggestions,
+            maxLines: widget.maxLines,
+            minLines: widget.minLines,
+            expands: widget.expands,
+            selectionColor: selectionColor,
+            selectionControls: widget.selectionEnabled
+              ? cupertinoTextSelectionControls : null,
+            onChanged: widget.onChanged,
+            onSelectionChanged: _handleSelectionChanged,
+            onEditingComplete: widget.onEditingComplete,
+            onSubmitted: widget.onSubmitted,
+            inputFormatters: formatters,
+            rendererIgnoresPointer: true,
+            cursorWidth: widget.cursorWidth,
+            cursorHeight: widget.cursorHeight,
+            cursorRadius: widget.cursorRadius,
+            cursorColor: cursorColor,
+            cursorOpacityAnimates: true,
+            cursorOffset: cursorOffset,
+            paintCursorAboveText: true,
+            autocorrectionTextRectColor: selectionColor,
+            backgroundCursorColor: CupertinoDynamicColor.resolve(CupertinoColors.inactiveGray, context),
+            selectionHeightStyle: widget.selectionHeightStyle,
+            selectionWidthStyle: widget.selectionWidthStyle,
+            scrollPadding: widget.scrollPadding,
+            keyboardAppearance: keyboardAppearance,
+            dragStartBehavior: widget.dragStartBehavior,
+            scrollController: widget.scrollController,
+            scrollPhysics: widget.scrollPhysics,
+            enableInteractiveSelection: widget.enableInteractiveSelection,
+            autofillHints: widget.autofillHints,
+            restorationId: 'editable',
+          ),
         ),
       ),
     );

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

@@ -372,6 +372,7 @@ class TextField extends StatefulWidget {
     this.scrollController,
     this.scrollPhysics,
     this.autofillHints,
+    this.restorationId,
   }) : assert(textAlign != null),
        assert(readOnly != null),
        assert(autofocus != null),
@@ -787,6 +788,25 @@ class TextField extends StatefulWidget {
   /// {@macro flutter.services.autofill.autofillHints}
   final Iterable<String> autofillHints;
 
+  /// {@template flutter.material.textfield.restorationId}
+  /// Restoration ID to save and restore the state of the text field.
+  ///
+  /// If non-null, the text field will persist and restore its current scroll
+  /// offset and - if no [controller] has been provided - the content of the
+  /// text field. If a [controller] has been provided, it is the responsibility
+  /// of the owner of that controller to persist and restore it, e.g. by using
+  /// a [RestorableTextEditingController].
+  ///
+  /// The state of this widget is persisted in a [RestorationBucket] claimed
+  /// from the surrounding [RestorationScope] using the provided restoration ID.
+  ///
+  /// See also:
+  ///
+  ///  * [RestorationManager], which explains how state restoration works in
+  ///    Flutter.
+  /// {@endtemplate}
+  final String restorationId;
+
   @override
   _TextFieldState createState() => _TextFieldState();
 
@@ -828,9 +848,9 @@ class TextField extends StatefulWidget {
   }
 }
 
-class _TextFieldState extends State<TextField> implements TextSelectionGestureDetectorBuilderDelegate {
-  TextEditingController _controller;
-  TextEditingController get _effectiveController => widget.controller ?? _controller;
+class _TextFieldState extends State<TextField> with RestorationMixin implements TextSelectionGestureDetectorBuilderDelegate {
+  RestorableTextEditingController _controller;
+  TextEditingController get _effectiveController => widget.controller ?? _controller.value;
 
   FocusNode _focusNode;
   FocusNode get _effectiveFocusNode => widget.focusNode ?? (_focusNode ??= FocusNode());
@@ -937,7 +957,7 @@ class _TextFieldState extends State<TextField> implements TextSelectionGestureDe
     super.initState();
     _selectionGestureDetectorBuilder = _TextFieldSelectionGestureDetectorBuilder(state: this);
     if (widget.controller == null) {
-      _controller = TextEditingController();
+      _createLocalController();
     }
     _effectiveFocusNode.canRequestFocus = _isEnabled;
   }
@@ -963,10 +983,13 @@ class _TextFieldState extends State<TextField> implements TextSelectionGestureDe
   @override
   void didUpdateWidget(TextField oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.controller == null && oldWidget.controller != null)
-      _controller = TextEditingController.fromValue(oldWidget.controller.value);
-    else if (widget.controller != null && oldWidget.controller == null)
+    if (widget.controller == null && oldWidget.controller != null) {
+      _createLocalController(oldWidget.controller.value);
+    } else if (widget.controller != null && oldWidget.controller == null) {
+      unregisterFromRestoration(_controller);
+      _controller.dispose();
       _controller = null;
+    }
     _effectiveFocusNode.canRequestFocus = _canRequestFocus;
     if (_effectiveFocusNode.hasFocus && widget.readOnly != oldWidget.readOnly && _isEnabled) {
       if(_effectiveController.selection.isCollapsed) {
@@ -975,9 +998,35 @@ class _TextFieldState extends State<TextField> implements TextSelectionGestureDe
     }
   }
 
+  @override
+  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+    if (_controller != null) {
+      _registerController();
+    }
+  }
+
+  void _registerController() {
+    assert(_controller != null);
+    registerForRestoration(_controller, 'controller');
+  }
+
+  void _createLocalController([TextEditingValue value]) {
+    assert(_controller == null);
+    _controller = value == null
+        ? RestorableTextEditingController()
+        : RestorableTextEditingController.fromValue(value);
+    if (!restorePending) {
+      _registerController();
+    }
+  }
+
+  @override
+  String get restorationId => widget.restorationId;
+
   @override
   void dispose() {
     _focusNode?.dispose();
+    _controller?.dispose();
     super.dispose();
   }
 
@@ -1122,60 +1171,64 @@ class _TextFieldState extends State<TextField> implements TextSelectionGestureDe
     }
 
     Widget child = RepaintBoundary(
-      child: EditableText(
-        key: editableTextKey,
-        readOnly: widget.readOnly || !_isEnabled,
-        toolbarOptions: widget.toolbarOptions,
-        showCursor: widget.showCursor,
-        showSelectionHandles: _showSelectionHandles,
-        controller: controller,
-        focusNode: focusNode,
-        keyboardType: widget.keyboardType,
-        textInputAction: widget.textInputAction,
-        textCapitalization: widget.textCapitalization,
-        style: style,
-        strutStyle: widget.strutStyle,
-        textAlign: widget.textAlign,
-        textDirection: widget.textDirection,
-        autofocus: widget.autofocus,
-        obscuringCharacter: widget.obscuringCharacter,
-        obscureText: widget.obscureText,
-        autocorrect: widget.autocorrect,
-        smartDashesType: widget.smartDashesType,
-        smartQuotesType: widget.smartQuotesType,
-        enableSuggestions: widget.enableSuggestions,
-        maxLines: widget.maxLines,
-        minLines: widget.minLines,
-        expands: widget.expands,
-        selectionColor: selectionColor,
-        selectionControls: widget.selectionEnabled ? textSelectionControls : null,
-        onChanged: widget.onChanged,
-        onSelectionChanged: _handleSelectionChanged,
-        onEditingComplete: widget.onEditingComplete,
-        onSubmitted: widget.onSubmitted,
-        onAppPrivateCommand: widget.onAppPrivateCommand,
-        onSelectionHandleTapped: _handleSelectionHandleTapped,
-        inputFormatters: formatters,
-        rendererIgnoresPointer: true,
-        mouseCursor: MouseCursor.defer, // TextField will handle the cursor
-        cursorWidth: widget.cursorWidth,
-        cursorHeight: widget.cursorHeight,
-        cursorRadius: cursorRadius,
-        cursorColor: cursorColor,
-        selectionHeightStyle: widget.selectionHeightStyle,
-        selectionWidthStyle: widget.selectionWidthStyle,
-        cursorOpacityAnimates: cursorOpacityAnimates,
-        cursorOffset: cursorOffset,
-        paintCursorAboveText: paintCursorAboveText,
-        backgroundCursorColor: CupertinoColors.inactiveGray,
-        scrollPadding: widget.scrollPadding,
-        keyboardAppearance: keyboardAppearance,
-        enableInteractiveSelection: widget.enableInteractiveSelection,
-        dragStartBehavior: widget.dragStartBehavior,
-        scrollController: widget.scrollController,
-        scrollPhysics: widget.scrollPhysics,
-        autofillHints: widget.autofillHints,
-        autocorrectionTextRectColor: autocorrectionTextRectColor,
+      child: UnmanagedRestorationScope(
+        bucket: bucket,
+        child: EditableText(
+          key: editableTextKey,
+          readOnly: widget.readOnly || !_isEnabled,
+          toolbarOptions: widget.toolbarOptions,
+          showCursor: widget.showCursor,
+          showSelectionHandles: _showSelectionHandles,
+          controller: controller,
+          focusNode: focusNode,
+          keyboardType: widget.keyboardType,
+          textInputAction: widget.textInputAction,
+          textCapitalization: widget.textCapitalization,
+          style: style,
+          strutStyle: widget.strutStyle,
+          textAlign: widget.textAlign,
+          textDirection: widget.textDirection,
+          autofocus: widget.autofocus,
+          obscuringCharacter: widget.obscuringCharacter,
+          obscureText: widget.obscureText,
+          autocorrect: widget.autocorrect,
+          smartDashesType: widget.smartDashesType,
+          smartQuotesType: widget.smartQuotesType,
+          enableSuggestions: widget.enableSuggestions,
+          maxLines: widget.maxLines,
+          minLines: widget.minLines,
+          expands: widget.expands,
+          selectionColor: selectionColor,
+          selectionControls: widget.selectionEnabled ? textSelectionControls : null,
+          onChanged: widget.onChanged,
+          onSelectionChanged: _handleSelectionChanged,
+          onEditingComplete: widget.onEditingComplete,
+          onSubmitted: widget.onSubmitted,
+          onAppPrivateCommand: widget.onAppPrivateCommand,
+          onSelectionHandleTapped: _handleSelectionHandleTapped,
+          inputFormatters: formatters,
+          rendererIgnoresPointer: true,
+          mouseCursor: MouseCursor.defer, // TextField will handle the cursor
+          cursorWidth: widget.cursorWidth,
+          cursorHeight: widget.cursorHeight,
+          cursorRadius: cursorRadius,
+          cursorColor: cursorColor,
+          selectionHeightStyle: widget.selectionHeightStyle,
+          selectionWidthStyle: widget.selectionWidthStyle,
+          cursorOpacityAnimates: cursorOpacityAnimates,
+          cursorOffset: cursorOffset,
+          paintCursorAboveText: paintCursorAboveText,
+          backgroundCursorColor: CupertinoColors.inactiveGray,
+          scrollPadding: widget.scrollPadding,
+          keyboardAppearance: keyboardAppearance,
+          enableInteractiveSelection: widget.enableInteractiveSelection,
+          dragStartBehavior: widget.dragStartBehavior,
+          scrollController: widget.scrollController,
+          scrollPhysics: widget.scrollPhysics,
+          autofillHints: widget.autofillHints,
+          autocorrectionTextRectColor: autocorrectionTextRectColor,
+          restorationId: 'editable',
+        ),
       ),
     );
 

packages/flutter/lib/src/widgets/editable_text.dart

@@ -445,6 +445,7 @@ class EditableText extends StatefulWidget {
     ),
     this.autofillHints,
     this.clipBehavior = Clip.hardEdge,
+    this.restorationId,
   }) : assert(controller != null),
        assert(focusNode != null),
        assert(obscuringCharacter != null && obscuringCharacter.length == 1),
@@ -1218,6 +1219,25 @@ class EditableText extends StatefulWidget {
   /// Defaults to [Clip.hardEdge].
   final Clip clipBehavior;
 
+  /// Restoration ID to save and restore the scroll offset of the
+  /// [EditableText].
+  ///
+  /// If a restoration id is provided, the [EditableText] will persist its
+  /// current scroll offset and restore it during state restoration.
+  ///
+  /// The scroll offset is persisted in a [RestorationBucket] claimed from
+  /// the surrounding [RestorationScope] using the provided restoration ID.
+  ///
+  /// Persisting and restoring the content of the [EditableText] is the
+  /// responsibilility of the owner of the [controller], who may use a
+  /// [RestorableTextEditingController] for that purpose.
+  ///
+  /// See also:
+  ///
+  ///  * [RestorationManager], which explains how state restoration works in
+  ///    Flutter.
+  final String restorationId;
+
   // Infer the keyboard type of an `EditableText` if it's not specified.
   static TextInputType _inferKeyboardType({
     @required Iterable<String> autofillHints,
@@ -2323,6 +2343,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
         controller: _scrollController,
         physics: widget.scrollPhysics,
         dragStartBehavior: widget.dragStartBehavior,
+        restorationId: widget.restorationId,
         viewportBuilder: (BuildContext context, ViewportOffset offset) {
           return CompositedTransformTarget(
             link: _toolbarLayerLink,

packages/flutter/lib/src/widgets/restoration_properties.dart

@@ -4,6 +4,8 @@
 
 // @dart = 2.8
 
+import 'dart:async';
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
@@ -257,11 +259,26 @@ class RestorableTextEditingController extends RestorableListenable<TextEditingCo
     return value.text;
   }
 
+  TextEditingController _controller;
+
+  @override
+  void initWithValue(TextEditingController value) {
+    _disposeControllerIfNecessary();
+    _controller = value;
+    super.initWithValue(value);
+  }
+
   @override
   void dispose() {
-    if (isRegistered) {
-      value.dispose();
-    }
     super.dispose();
+    _disposeControllerIfNecessary();
+  }
+
+  void _disposeControllerIfNecessary() {
+    if (_controller != null) {
+      // Scheduling a microtask for dispose to give other entities a chance
+      // to remove their listeners first.
+      scheduleMicrotask(_controller.dispose);
+    }
   }
 }

packages/flutter/test/cupertino/text_field_restoration_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.8
+
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+const String text = 'Hello World! How are you? Life is good!';
+const String alternativeText = 'Everything is awesome!!';
+
+void main() {
+  testWidgets('CupertinoTextField restoration', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const RootRestorationScope(
+        child: TestWidget(),
+        restorationId: 'root',
+      ),
+    );
+
+    await restoreAndVerify(tester);
+  });
+
+  testWidgets('CupertinoTextField restoration with external controller', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const RootRestorationScope(
+        child: TestWidget(
+          useExternal: true,
+        ),
+        restorationId: 'root',
+      ),
+    );
+
+    await restoreAndVerify(tester);
+  });
+}
+
+Future<void> restoreAndVerify(WidgetTester tester) async {
+  expect(find.text(text), findsNothing);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 0);
+
+  await tester.enterText(find.byType(CupertinoTextField), text);
+  await skipPastScrollingAnimation(tester);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 0);
+
+  await tester.drag(find.byType(Scrollable), const Offset(0, -80));
+  await skipPastScrollingAnimation(tester);
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+
+  await tester.restartAndRestore();
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+
+  final TestRestorationData data = await tester.getRestorationData();
+
+  await tester.enterText(find.byType(CupertinoTextField), alternativeText);
+  await skipPastScrollingAnimation(tester);
+  await tester.drag(find.byType(Scrollable), const Offset(0, 80));
+  await skipPastScrollingAnimation(tester);
+
+  expect(find.text(text), findsNothing);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, isNot(60));
+
+  await tester.restoreFrom(data);
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+}
+
+class TestWidget extends StatefulWidget {
+  const TestWidget({Key key, this.useExternal = false}) : super(key: key);
+
+  final bool useExternal;
+
+  @override
+  TestWidgetState createState() => TestWidgetState();
+}
+
+class TestWidgetState extends State<TestWidget> with RestorationMixin {
+  final RestorableTextEditingController controller = RestorableTextEditingController();
+
+  @override
+  String get restorationId => 'widget';
+
+  @override
+  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+    registerForRestoration(controller, 'controller');
+  }
+
+  @override
+  void dispose() {
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Material(
+        child: Align(
+          alignment: Alignment.center,
+          child: SizedBox(
+            width: 50,
+            child: CupertinoTextField(
+              restorationId: 'text',
+              maxLines: 3,
+              controller: widget.useExternal ? controller.value : null,
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+Future<void> skipPastScrollingAnimation(WidgetTester tester) async {
+  await tester.pump();
+  await tester.pump(const Duration(milliseconds: 200));
+}

packages/flutter/test/material/text_field_restoration_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.8
+
+import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/widgets.dart';
+
+const String text = 'Hello World! How are you? Life is good!';
+const String alternativeText = 'Everything is awesome!!';
+
+void main() {
+  testWidgets('TextField restoration', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const RootRestorationScope(
+        child: TestWidget(),
+        restorationId: 'root',
+      ),
+    );
+
+    await restoreAndVerify(tester);
+  });
+
+  testWidgets('TextField restoration with external controller', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const RootRestorationScope(
+        child: TestWidget(
+          useExternal: true,
+        ),
+        restorationId: 'root',
+      ),
+    );
+
+    await restoreAndVerify(tester);
+  });
+}
+
+Future<void> restoreAndVerify(WidgetTester tester) async {
+  expect(find.text(text), findsNothing);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 0);
+
+  await tester.enterText(find.byType(TextField), text);
+  await skipPastScrollingAnimation(tester);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 0);
+
+  await tester.drag(find.byType(Scrollable), const Offset(0, -80));
+  await skipPastScrollingAnimation(tester);
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+
+  await tester.restartAndRestore();
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+
+  final TestRestorationData data = await tester.getRestorationData();
+
+  await tester.enterText(find.byType(TextField), alternativeText);
+  await skipPastScrollingAnimation(tester);
+  await tester.drag(find.byType(Scrollable), const Offset(0, 80));
+  await skipPastScrollingAnimation(tester);
+
+  expect(find.text(text), findsNothing);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, isNot(60));
+
+  await tester.restoreFrom(data);
+
+  expect(find.text(text), findsOneWidget);
+  expect(tester.state<ScrollableState>(find.byType(Scrollable)).position.pixels, 60);
+}
+
+class TestWidget extends StatefulWidget {
+  const TestWidget({Key key, this.useExternal = false}) : super(key: key);
+
+  final bool useExternal;
+
+  @override
+  TestWidgetState createState() => TestWidgetState();
+}
+
+class TestWidgetState extends State<TestWidget> with RestorationMixin {
+  final RestorableTextEditingController controller = RestorableTextEditingController();
+
+  @override
+  String get restorationId => 'widget';
+
+  @override
+  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
+    registerForRestoration(controller, 'controller');
+  }
+
+  @override
+  void dispose() {
+    controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Material(
+        child: Align(
+          alignment: Alignment.center,
+          child: SizedBox(
+            width: 50,
+            child: TextField(
+              restorationId: 'text',
+              maxLines: 3,
+              controller: widget.useExternal ? controller.value : null,
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+Future<void> skipPastScrollingAnimation(WidgetTester tester) async {
+  await tester.pump();
+  await tester.pump(const Duration(milliseconds: 200));
+}