Do not implicitly scroll a PageView when showOnScreen is called (#19735)

Fixes #19523

packages/flutter/lib/src/rendering/viewport.dart

@@ -877,6 +877,15 @@ abstract class RenderViewportBase<ParentDataClass extends ContainerParentDataMix
     Duration duration = Duration.zero,
     Curve curve = Curves.ease,
   }) {
+    if (!offset.allowImplicitScrolling) {
+      return super.showOnScreen(
+        descendant: descendant,
+        rect: rect,
+        duration: duration,
+        curve: curve,
+      );
+    }
+
     final Rect newRect = RenderViewportBase.showInViewport(
       descendant: descendant,
       viewport: this,

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -195,6 +195,15 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// offset direction.
   ScrollDirection get userScrollDirection;
 
+  /// Whether a viewport is allowed to change [pixels] implicitly to respond to
+  /// a call to [RenderObject.showOnScreen].
+  ///
+  /// [RenderObject.showOnScreen] is for example used to bring a text field
+  /// fully on screen after it has received focus. This property controls
+  /// whether the viewport associated with this offset is allowed to change the
+  /// offset's [pixels] value to fulfill such a request.
+  bool get allowImplicitScrolling;
+
   @override
   String toString() {
     final List<String> description = <String>[];
@@ -250,4 +259,7 @@ class _FixedViewportOffset extends ViewportOffset {
 
   @override
   ScrollDirection get userScrollDirection => ScrollDirection.idle;
+
+  @override
+  bool get allowImplicitScrolling => false;
 }

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

@@ -372,6 +372,9 @@ class PageScrollPhysics extends ScrollPhysics {
       return new ScrollSpringSimulation(spring, position.pixels, target, velocity, tolerance: tolerance);
     return null;
   }
+
+  @override
+  bool get allowImplicitScrolling => false;
 }
 
 // Having this global (mutable) page controller is a bit of a hack. We need it

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

@@ -225,6 +225,15 @@ class ScrollPhysics {
   /// If null, no minimum threshold is enforced.
   double get dragStartDistanceMotionThreshold => parent?.dragStartDistanceMotionThreshold;
 
+  /// Whether a viewport is allowed to change its scroll position implicitly in
+  /// responds to a call to [RenderObject.showOnScreen].
+  ///
+  /// [RenderObject.showOnScreen] is for example used to bring a text field
+  /// fully on screen after it has received focus. This property controls
+  /// whether the viewport associated with this object is allowed to change the
+  /// scroll position to fulfill such a request.
+  bool get allowImplicitScrolling => true;
+
   @override
   String toString() {
     if (parent == null)
@@ -485,4 +494,7 @@ class NeverScrollableScrollPhysics extends ScrollPhysics {
 
   @override
   bool shouldAcceptUserOffset(ScrollMetrics position) => false;
+
+  @override
+  bool get allowImplicitScrolling => false;
 }

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

@@ -562,6 +562,9 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   @override
   void jumpTo(double value);
 
+  @override
+  bool get allowImplicitScrolling => physics.allowImplicitScrolling;
+
   /// Deprecated. Use [jumpTo] or a custom [ScrollPosition] instead.
   @Deprecated('This will lead to bugs.')
   void jumpToWithoutSettling(double value);

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

@@ -596,6 +596,15 @@ class _RenderSingleChildViewport extends RenderBox with RenderObjectWithChildMix
     Duration duration = Duration.zero,
     Curve curve = Curves.ease,
   }) {
+    if (!offset.allowImplicitScrolling) {
+      return super.showOnScreen(
+        descendant: descendant,
+        rect: rect,
+        duration: duration,
+        curve: curve,
+      );
+    }
+
     final Rect newRect = RenderViewportBase.showInViewport(
       descendant: descendant,
       viewport: this,

packages/flutter/test/widgets/editable_text_show_on_screen_test.dart

@@ -142,6 +142,97 @@ void main() {
     expect(find.byType(EditableText), findsOneWidget);
   });
 
+  testWidgets('entering text does not scroll when scrollPhysics.allowImplicitScrolling = false', (WidgetTester tester) async {
+    // regression test for https://github.com/flutter/flutter/issues/19523
+
+    final ScrollController scrollController = new ScrollController(initialScrollOffset: 100.0);
+    final TextEditingController controller = new TextEditingController();
+    final FocusNode focusNode = new FocusNode();
+
+    await tester.pumpWidget(new MaterialApp(
+      home: new Center(
+        child: new Container(
+          height: 300.0,
+          child: new ListView(
+            physics: const NoImplicitScrollPhysics(),
+            controller: scrollController,
+            children: <Widget>[
+              new Container(
+                height: 350.0,
+              ),
+              new EditableText(
+                controller: controller,
+                focusNode: focusNode,
+                style: textStyle,
+                cursorColor: cursorColor,
+              ),
+              new Container(
+                height: 350.0,
+              ),
+            ],
+          ),
+        ),
+      ),
+    ));
+
+    // Focus the EditableText and scroll it off screen.
+    await tester.showKeyboard(find.byType(EditableText));
+    await tester.pumpAndSettle();
+    expect(focusNode.hasFocus, isTrue);
+    scrollController.jumpTo(0.0);
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(find.byType(EditableText), findsNothing);
+
+    // Entering text brings it not back on screen.
+    tester.testTextInput.enterText('Hello');
+    await tester.pumpAndSettle();
+    expect(scrollController.offset, 0.0);
+    expect(find.byType(EditableText), findsNothing);
+  });
+
+  testWidgets('entering text does not scroll a sourrounding PageView', (WidgetTester tester) async {
+    // regression test for https://github.com/flutter/flutter/issues/19523
+
+    final TextEditingController textController = new TextEditingController();
+    final PageController pageController = new PageController(initialPage: 1);
+
+    await tester.pumpWidget(new Directionality(
+      textDirection: TextDirection.ltr,
+      child: new Material(
+        child: new PageView(
+          controller: pageController,
+          children: <Widget>[
+            new Container(
+              color: Colors.red,
+            ),
+            new Container(
+              child: new TextField(
+                controller: textController,
+              ),
+              color: Colors.green,
+            ),
+            new Container(
+              color: Colors.red,
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.showKeyboard(find.byType(EditableText));
+    await tester.pumpAndSettle();
+    expect(textController.text, '');
+    tester.testTextInput.enterText('H');
+    final int frames = await tester.pumpAndSettle();
+
+    // The text input should not trigger any animations, which would indicate
+    // that the surrounding PageView is incorrectly scrolling back-and-forth.
+    expect(frames, 1);
+
+    expect(textController.text, 'H');
+  });
+
   testWidgets('focused multi-line editable scrolls caret back into view when typing', (WidgetTester tester) async {
     final ScrollController scrollController = new ScrollController();
     final TextEditingController controller = new TextEditingController();
@@ -234,3 +325,15 @@ void main() {
     expect(find.byKey(container), findsNothing);
   });
 }
+
+class NoImplicitScrollPhysics extends AlwaysScrollableScrollPhysics {
+  const NoImplicitScrollPhysics({ ScrollPhysics parent }) : super(parent: parent);
+
+  @override
+  bool get allowImplicitScrolling => false;
+
+  @override
+  NoImplicitScrollPhysics applyTo(ScrollPhysics ancestor) {
+    return new NoImplicitScrollPhysics(parent: buildParent(ancestor));
+  }
+}