SelectableText keep alive only when it has selection (#94493)

SelectableText defers to EditableText for wantKeepAlive, plus improved docs.

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

@@ -177,6 +177,8 @@ class _CupertinoTextFieldSelectionGestureDetectorBuilder extends TextSelectionGe
 /// rounded rectangle border around the text field. If you set the [decoration]
 /// property to null, the decoration will be removed entirely.
 ///
+/// {@macro flutter.material.textfield.wantKeepAlive}
+///
 /// Remember to call [TextEditingController.dispose] when it is no longer
 /// needed. This will ensure we discard any resources used by the object.
 ///

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

@@ -123,6 +123,8 @@ class _SelectableTextSelectionGestureDetectorBuilder extends TextSelectionGestur
 /// behavior is useful, for example, to make the text bold while using the
 /// default font family and size.
 ///
+/// {@macro flutter.material.textfield.wantKeepAlive}
+///
 /// {@tool snippet}
 ///
 /// ```dart
@@ -451,7 +453,7 @@ class SelectableText extends StatefulWidget {
   }
 }
 
-class _SelectableTextState extends State<SelectableText> with AutomaticKeepAliveClientMixin implements TextSelectionGestureDetectorBuilderDelegate {
+class _SelectableTextState extends State<SelectableText> implements TextSelectionGestureDetectorBuilderDelegate {
   EditableTextState? get _editableText => editableTextKey.currentState;
 
   late _TextSpanEditingController _controller;
@@ -579,12 +581,8 @@ class _SelectableTextState extends State<SelectableText> with AutomaticKeepAlive
     return false;
   }
 
-  @override
-  bool get wantKeepAlive => true;
-
   @override
   Widget build(BuildContext context) {
-    super.build(context); // See AutomaticKeepAliveClientMixin.
     // TODO(garyq): Assert to block WidgetSpans from being used here are removed,
     // but we still do not yet have nice handling of things like carets, clipboard,
     // and other features. We should add proper support. Currently, caret handling

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

@@ -168,6 +168,13 @@ class _TextFieldSelectionGestureDetectorBuilder extends TextSelectionGestureDete
 /// To integrate the [TextField] into a [Form] with other [FormField] widgets,
 /// consider using [TextFormField].
 ///
+/// {@template flutter.material.textfield.wantKeepAlive}
+/// When the widget has focus, it will prevent itself from disposing via its
+/// underlying [EditableText]'s [AutomaticKeepAliveClientMixin.wantKeepAlive] in
+/// order to avoid losing the selection. Removing the focus will allow it to be
+/// disposed.
+/// {@endtemplate}
+///
 /// Remember to call [TextEditingController.dispose] of the [TextEditingController]
 /// when it is no longer needed. This will ensure we discard any resources used
 /// by the object.

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

@@ -31,6 +31,8 @@ export 'package:flutter/services.dart' show SmartQuotesType, SmartDashesType;
 /// If a [controller] is not specified, [initialValue] can be used to give
 /// the automatically generated controller an initial value.
 ///
+/// {@macro flutter.material.textfield.wantKeepAlive}
+///
 /// Remember to call [TextEditingController.dispose] of the [TextEditingController]
 /// when it is no longer needed. This will ensure we discard any resources used
 /// by the object.

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

@@ -344,6 +344,10 @@ class ToolbarOptions {
 /// [onSubmitted] can be used to manually move focus to another input widget
 /// when a user finishes with the currently focused input widget.
 ///
+/// When the widget has focus, it will prevent itself from disposing via
+/// [AutomaticKeepAliveClientMixin.wantKeepAlive] in order to avoid losing the
+/// selection. Removing the focus will allow it to be disposed.
+///
 /// Rather than using this widget directly, consider using [TextField], which
 /// is a full-featured, material-design text input field with placeholder text,
 /// labels, and [Form] integration.

packages/flutter/test/widgets/selectable_text_test.dart

@@ -4869,4 +4869,91 @@ void main() {
       matchesGoldenFile('selectable_text_golden.TextSelectionStyle.2.png'),
     );
   });
+
+  testWidgets('keeps alive when has focus', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: DefaultTabController(
+          length: 2,
+          child: Scaffold(
+            body: NestedScrollView(
+              headerSliverBuilder:
+                (BuildContext context, bool innerBoxIsScrolled) {
+                  return <Widget>[
+                    SliverToBoxAdapter(
+                      child: Container(
+                        height: 200,
+                        color: Colors.black12,
+                        child: const Center(child: Text('Sliver 1')),
+                      ),
+                    ),
+                    const SliverToBoxAdapter(
+                      child: Center(
+                        child: TabBar(
+                          labelColor: Colors.black,
+                          tabs: <Tab>[
+                            Tab(text: 'Sliver Tab 1'),
+                            Tab(text: 'Sliver Tab 2'),
+                          ],
+                        ),
+                      )
+                    ),
+                  ];
+                },
+                body: const TabBarView(
+                  children: <Widget>[
+                    Padding(
+                      padding: EdgeInsets.only(top: 100.0),
+                      child: Text('Regular Text'),
+                    ),
+                    Padding(
+                      padding: EdgeInsets.only(top: 100.0),
+                      child: SelectableText('Selectable Text'),
+                    ),
+                  ],
+                ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // Without any selection, the offscreen widget is disposed and can't be
+    // found, for both Text and SelectableText.
+    expect(find.text('Regular Text', skipOffstage: false), findsOneWidget);
+    expect(find.byType(SelectableText, skipOffstage: false), findsNothing);
+
+    await tester.tap(find.text('Sliver Tab 2'));
+    await tester.pumpAndSettle();
+    expect(find.text('Regular Text', skipOffstage: false), findsNothing);
+    expect(find.byType(SelectableText, skipOffstage: false), findsOneWidget);
+
+    await tester.tap(find.text('Sliver Tab 1'));
+    await tester.pumpAndSettle();
+    expect(find.text('Regular Text', skipOffstage: false), findsOneWidget);
+    expect(find.byType(SelectableText, skipOffstage: false), findsNothing);
+
+    // Switch back to tab 2 and select some text in SelectableText.
+    await tester.tap(find.text('Sliver Tab 2'));
+    await tester.pumpAndSettle();
+    expect(find.text('Regular Text', skipOffstage: false), findsNothing);
+    expect(find.byType(SelectableText, skipOffstage: false), findsOneWidget);
+
+    final EditableText editableText = tester.widget(find.byType(EditableText));
+    expect(editableText.controller.selection.isValid, isFalse);
+    await tester.tapAt(textOffsetToPosition(tester, 4));
+    await tester.pump(const Duration(milliseconds: 50));
+    await tester.tapAt(textOffsetToPosition(tester, 4));
+    await tester.pumpAndSettle();
+    expect(editableText.controller.selection.isValid, isTrue);
+    expect(editableText.controller.selection.baseOffset, 0);
+    expect(editableText.controller.selection.extentOffset, 'Selectable'.length);
+
+    // Switch back to tab 1. The SelectableText remains because it is preserving
+    // its selection.
+    await tester.tap(find.text('Sliver Tab 1'));
+    await tester.pumpAndSettle();
+    expect(find.text('Regular Text', skipOffstage: false), findsOneWidget);
+    expect(find.byType(SelectableText, skipOffstage: false), findsOneWidget);
+  });
 }