Add onSelectionChanged into SelectableText widget (#65320)

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

@@ -199,6 +199,7 @@ class SelectableText extends StatefulWidget {
     this.scrollPhysics,
     this.textHeightBehavior,
     this.textWidthBasis,
+    this.onSelectionChanged,
   }) :  assert(showCursor != null),
         assert(autofocus != null),
         assert(dragStartBehavior != null),
@@ -250,6 +251,7 @@ class SelectableText extends StatefulWidget {
     this.scrollPhysics,
     this.textHeightBehavior,
     this.textWidthBasis,
+    this.onSelectionChanged,
   }) :  assert(showCursor != null),
     assert(autofocus != null),
     assert(dragStartBehavior != null),
@@ -392,6 +394,9 @@ class SelectableText extends StatefulWidget {
   /// {@macro flutter.painting.textPainter.textWidthBasis}
   final TextWidthBasis textWidthBasis;
 
+  /// {@macro flutter.widgets.editableText.onSelectionChanged}
+  final SelectionChangedCallback onSelectionChanged;
+
   @override
   _SelectableTextState createState() => _SelectableTextState();
 
@@ -494,6 +499,10 @@ class _SelectableTextState extends State<SelectableText> with AutomaticKeepAlive
       });
     }
 
+    if (widget.onSelectionChanged != null) {
+      widget.onSelectionChanged(selection, cause);
+    }
+
     switch (Theme.of(context).platform) {
       case TargetPlatform.iOS:
       case TargetPlatform.macOS:

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

@@ -1044,8 +1044,10 @@ class EditableText extends StatefulWidget {
   /// {@endtemplate}
   final AppPrivateCommandCallback onAppPrivateCommand;
 
+  /// {@template flutter.widgets.editableText.onSelectionChanged}
   /// Called when the user changes the selection of text (including the cursor
   /// location).
+  /// {@endtemplate}
   final SelectionChangedCallback onSelectionChanged;
 
   /// {@macro flutter.widgets.textSelection.onSelectionHandleTapped}

packages/flutter/test/widgets/selectable_text_test.dart

@@ -4012,4 +4012,33 @@ void main() {
       expect(state.selectionOverlay.handlesAreVisible, isFalse);
     }
   });
+
+  testWidgets('onSelectionChanged is called when selection changes', (WidgetTester tester) async {
+    int onSelectionChangedCallCount = 0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: SelectableText(
+            'abc def ghi',
+            onSelectionChanged: (TextSelection selection, SelectionChangedCause cause) {
+              onSelectionChangedCallCount += 1;
+            },
+          ),
+        ),
+      ),
+    );
+
+    // Long press to select 'abc'.
+    final Offset aLocation = textOffsetToPosition(tester, 1);
+    await tester.longPressAt(aLocation);
+    await tester.pump();
+    expect(onSelectionChangedCallCount, equals(1));
+
+    // Tap on 'Select all' option to select the whole text.
+    await tester.longPressAt(textOffsetToPosition(tester, 5));
+    await tester.pump();
+    await tester.tap(find.text('Select all'));
+    expect(onSelectionChangedCallCount, equals(2));
+  });
 }