PageView resize from zero-size viewport should not lose state (#65015)

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

@@ -193,6 +193,11 @@ class PageController extends ScrollController {
     required Curve curve,
   }) {
     final _PagePosition position = this.position as _PagePosition;
+    if (position._cachedPage != null) {
+      position._cachedPage = page.toDouble();
+      return Future<void>.value();
+    }
+
     return position.animateTo(
       position.getPixelsFromPage(page.toDouble()),
       duration: duration,
@@ -206,6 +211,11 @@ class PageController extends ScrollController {
   /// without animation, and without checking if the new value is in range.
   void jumpToPage(int page) {
     final _PagePosition position = this.position as _PagePosition;
+    if (position._cachedPage != null) {
+      position._cachedPage = page.toDouble();
+      return;
+    }
+
     position.jumpTo(position.getPixelsFromPage(page.toDouble()));
   }
 
@@ -325,6 +335,10 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
 
   final int initialPage;
   double _pageToUseOnStartup;
+  // When the viewport has a zero-size, the `page` can not
+  // be retrieved by `getPageFromPixels`, so we need to cache the page
+  // for use when resizing the viewport to non-zero next time.
+  double? _cachedPage;
 
   @override
   Future<void> ensureVisible(
@@ -370,7 +384,8 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
   double get _initialPageOffset => math.max(0, viewportDimension * (viewportFraction - 1) / 2);
 
   double getPageFromPixels(double pixels, double viewportDimension) {
-    final double actual = math.max(0.0, pixels - _initialPageOffset) / math.max(1.0, viewportDimension * viewportFraction);
+    assert(viewportDimension > 0.0);
+    final double actual = math.max(0.0, pixels - _initialPageOffset) / (viewportDimension * viewportFraction);
     final double round = actual.roundToDouble();
     if ((actual - round).abs() < precisionErrorTolerance) {
       return round;
@@ -390,12 +405,12 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
     );
     return !hasPixels || !hasContentDimensions
       ? null
-      : getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent), viewportDimension);
+      : _cachedPage ?? getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent), viewportDimension);
   }
 
   @override
   void saveScrollOffset() {
-    PageStorage.of(context.storageContext)?.writeState(context.storageContext, getPageFromPixels(pixels, viewportDimension));
+    PageStorage.of(context.storageContext)?.writeState(context.storageContext, _cachedPage ?? getPageFromPixels(pixels, viewportDimension));
   }
 
   @override
@@ -409,7 +424,7 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
 
   @override
   void saveOffset() {
-    context.saveOffset(getPageFromPixels(pixels, viewportDimension));
+    context.saveOffset(_cachedPage ?? getPageFromPixels(pixels, viewportDimension));
   }
 
   @override
@@ -431,9 +446,21 @@ class _PagePosition extends ScrollPositionWithSingleContext implements PageMetri
     }
     final bool result = super.applyViewportDimension(viewportDimension);
     final double? oldPixels = hasPixels ? pixels : null;
-    final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? _pageToUseOnStartup : getPageFromPixels(oldPixels, oldViewportDimensions!);
+    double page;
+    if (oldPixels == null) {
+      page = _pageToUseOnStartup;
+    } else if (oldViewportDimensions == 0.0) {
+      // If resize from zero, we should use the _cachedPage to recover the state.
+      page = _cachedPage!;
+    } else {
+      page = getPageFromPixels(oldPixels, oldViewportDimensions!);
+    }
     final double newPixels = getPixelsFromPage(page);
 
+    // If the viewportDimension is zero, cache the page
+    // in case the viewport is resized to be non-zero.
+    _cachedPage = (viewportDimension == 0.0) ? page : null;
+
     if (newPixels != oldPixels) {
       correctPixels(newPixels);
       return false;

packages/flutter/test/widgets/page_view_test.dart

@@ -14,6 +14,91 @@ import 'states.dart';
 const Duration _frameDuration = Duration(milliseconds: 100);
 
 void main() {
+  testWidgets('PageView resize from zero-size viewport should not lose state', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/88956
+    final PageController controller = PageController(
+      initialPage: 1,
+    );
+
+    Widget build(Size size) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: SizedBox.fromSize(
+            size: size,
+            child: PageView(
+              children: kStates.map<Widget>((String state) => Text(state)).toList(),
+              controller: controller,
+              onPageChanged: (int page) { },
+            ),
+          ),
+        ),
+      );
+    }
+
+    // The pageView have a zero viewport, so nothing display.
+    await tester.pumpWidget(build(Size.zero));
+    expect(find.text('Alabama'), findsNothing);
+    expect(find.text('Alabama', skipOffstage: false), findsOneWidget);
+
+    // Resize from zero viewport to non-zero, the controller's initialPage 1 will display.
+    await tester.pumpWidget(build(const Size(200.0, 200.0)));
+    expect(find.text('Alaska'), findsOneWidget);
+
+    // Jump to page 'Iowa'.
+    controller.jumpToPage(kStates.indexOf('Iowa'));
+    await tester.pump();
+    expect(find.text('Iowa'), findsOneWidget);
+
+    // Resize to zero viewport again, nothing display.
+    await tester.pumpWidget(build(Size.zero));
+    expect(find.text('Iowa'), findsNothing);
+
+    // Resize from zero to non-zero, the pageView should not lose state, so the page 'Iowa' show again.
+    await tester.pumpWidget(build(const Size(200.0, 200.0)));
+    expect(find.text('Iowa'), findsOneWidget);
+  });
+
+  testWidgets('Change the page through the controller when zero-size viewport', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/88956
+    final PageController controller = PageController(
+      initialPage: 1,
+    );
+
+    Widget build(Size size) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: SizedBox.fromSize(
+            size: size,
+            child: PageView(
+              children: kStates.map<Widget>((String state) => Text(state)).toList(),
+              controller: controller,
+              onPageChanged: (int page) { },
+            ),
+          ),
+        ),
+      );
+    }
+
+    // The pageView have a zero viewport, so nothing display.
+    await tester.pumpWidget(build(Size.zero));
+    expect(find.text('Alabama'), findsNothing);
+    expect(find.text('Alabama', skipOffstage: false), findsOneWidget);
+
+    // Change the page through the page controller when zero viewport
+    controller.animateToPage(kStates.indexOf('Iowa'), duration: kTabScrollDuration, curve: Curves.ease);
+    expect(controller.page, kStates.indexOf('Iowa'));
+
+    controller.jumpToPage(kStates.indexOf('Illinois'));
+    expect(controller.page, kStates.indexOf('Illinois'));
+
+    // Resize from zero viewport to non-zero, the latest state should not lost.
+    await tester.pumpWidget(build(const Size(200.0, 200.0)));
+    expect(controller.page, kStates.indexOf('Illinois'));
+    expect(find.text('Illinois'), findsOneWidget);
+  });
+
   testWidgets('PageController cannot return page while unattached',
       (WidgetTester tester) async {
     final PageController controller = PageController();