PageView shouldn't squish when overscrolled (#7870)

This patch passes down the viewportMainAxisExtent so that RenderSliverPage can
pick it up and size its children appropriately.

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

@@ -139,6 +139,7 @@ class SliverConstraints extends Constraints {
     @required this.overlap,
     @required this.remainingPaintExtent,
     @required this.crossAxisExtent,
+    @required this.viewportMainAxisExtent,
   });
 
   SliverConstraints copyWith({
@@ -149,6 +150,7 @@ class SliverConstraints extends Constraints {
     double overlap,
     double remainingPaintExtent,
     double crossAxisExtent,
+    double viewportMainAxisExtent,
   }) {
     return new SliverConstraints(
       axisDirection: axisDirection ?? this.axisDirection,
@@ -158,6 +160,7 @@ class SliverConstraints extends Constraints {
       overlap: overlap ?? this.overlap,
       remainingPaintExtent: remainingPaintExtent ?? this.remainingPaintExtent,
       crossAxisExtent: crossAxisExtent ?? this.crossAxisExtent,
+      viewportMainAxisExtent: viewportMainAxisExtent ?? this.viewportMainAxisExtent,
     );
   }
 
@@ -239,10 +242,16 @@ class SliverConstraints extends Constraints {
   /// bottom of a downwards vertical viewport.
   final double remainingPaintExtent;
 
-  /// The number of pixels in the cross-axis. For a vertical list, this is the
-  /// width of the viewport.
+  /// The number of pixels in the cross-axis.
+  ///
+  /// For a vertical list, this is the width of the sliver..
   final double crossAxisExtent;
 
+  /// The number of pixels the viewport can display in the main axis.
+  ///
+  /// For a vertical list, this is the height of the viewport.
+  final double viewportMainAxisExtent;
+
   Axis get axis => axisDirectionToAxis(axisDirection);
 
   /// Return what the [growthDirection] would be if the [axisDirection] was
@@ -280,6 +289,7 @@ class SliverConstraints extends Constraints {
   bool get isNormalized {
     return scrollOffset >= 0.0
         && crossAxisExtent >= 0.0
+        && viewportMainAxisExtent >= 0.0
         && remainingPaintExtent >= 0.0;
   }
 
@@ -320,8 +330,10 @@ class SliverConstraints extends Constraints {
     assert(overlap != null);
     assert(remainingPaintExtent != null);
     assert(crossAxisExtent != null);
+    assert(viewportMainAxisExtent != null);
     assert(scrollOffset >= 0.0);
     assert(crossAxisExtent >= 0.0);
+    assert(viewportMainAxisExtent >= 0.0);
     assert(remainingPaintExtent >= 0.0);
     assert(isNormalized); // should be redundant with earlier checks
     return true;
@@ -340,12 +352,13 @@ class SliverConstraints extends Constraints {
            scrollOffset == typedOther.scrollOffset &&
            overlap == typedOther.overlap &&
            remainingPaintExtent == typedOther.remainingPaintExtent &&
-           crossAxisExtent == typedOther.crossAxisExtent;
+           crossAxisExtent == typedOther.crossAxisExtent &&
+           viewportMainAxisExtent == typedOther.viewportMainAxisExtent;
   }
 
   @override
   int get hashCode {
-    return hashValues(axisDirection, growthDirection, scrollOffset, overlap, remainingPaintExtent, crossAxisExtent);
+    return hashValues(axisDirection, growthDirection, scrollOffset, overlap, remainingPaintExtent, crossAxisExtent, viewportMainAxisExtent);
   }
 
   @override
@@ -357,7 +370,8 @@ class SliverConstraints extends Constraints {
              'scrollOffset: ${scrollOffset.toStringAsFixed(1)}, '
              'remainingPaintExtent: ${remainingPaintExtent.toStringAsFixed(1)}, ' +
              (overlap != 0.0 ? 'overlap: ${overlap.toStringAsFixed(1)}, ' : '') +
-             'crossAxisExtent: ${crossAxisExtent.toStringAsFixed(1)}'
+             'crossAxisExtent: ${crossAxisExtent.toStringAsFixed(1)}' +
+             'viewportMainAxisExtent: ${viewportMainAxisExtent.toStringAsFixed(1)}' +
            ')';
   }
 }
@@ -1178,6 +1192,7 @@ abstract class RenderViewportBase2<ParentDataClass extends ContainerParentDataMi
     double scrollOffset,
     double layoutOffset,
     double remainingPaintExtent,
+    double mainAxisExtent,
     double crossAxisExtent,
     GrowthDirection growthDirection,
     _Advancer advance,
@@ -1216,6 +1231,7 @@ abstract class RenderViewportBase2<ParentDataClass extends ContainerParentDataMi
         overlap: maxPaintOffset - layoutOffset,
         remainingPaintExtent: math.max(0.0, remainingPaintExtent - layoutOffset + initialLayoutOffset),
         crossAxisExtent: crossAxisExtent,
+        viewportMainAxisExtent: mainAxisExtent,
       ), parentUsesSize: true);
       // collect the child's objects
       final SliverGeometry childLayoutGeometry = child.geometry;
@@ -1624,6 +1640,7 @@ class RenderViewport2 extends RenderViewportBase2<SliverPhysicalContainerParentD
       math.max(mainAxisExtent, mainAxisExtent * anchor - correctedOffset) - mainAxisExtent,
       clampedReverseCenter,
       clampedForwardCenter,
+      mainAxisExtent,
       crossAxisExtent,
       GrowthDirection.reverse,
       childBefore,
@@ -1637,6 +1654,7 @@ class RenderViewport2 extends RenderViewportBase2<SliverPhysicalContainerParentD
       math.max(0.0, correctedOffset - mainAxisExtent * anchor),
       clampedForwardCenter,
       clampedReverseCenter,
+      mainAxisExtent,
       crossAxisExtent,
       GrowthDirection.forward,
       childAfter,
@@ -1897,6 +1915,7 @@ class RenderShrinkWrappingViewport extends RenderViewportBase2<SliverLogicalCont
       math.max(0.0, correctedOffset),
       0.0,
       mainAxisExtent,
+      mainAxisExtent,
       crossAxisExtent,
       GrowthDirection.forward,
       childAfter,

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

@@ -148,5 +148,5 @@ class RenderSliverFill extends RenderSliverFixedExtentBoxAdaptor {
   }) : super(childManager: childManager);
 
   @override
-  double get itemExtent => constraints.remainingPaintExtent;
+  double get itemExtent => constraints.viewportMainAxisExtent;
 }

packages/flutter/test/rendering/slivers_helpers_test.dart

@@ -25,6 +25,7 @@ void main() {
       overlap: 0.0,
       remainingPaintExtent: 0.0,
       crossAxisExtent: 0.0,
+      viewportMainAxisExtent: 0.0,
     );
     SliverConstraints b = a.copyWith();
     expect(a, equals(b));
@@ -41,6 +42,7 @@ void main() {
       overlap: 20.0,
       remainingPaintExtent: 30.0,
       crossAxisExtent: 40.0,
+      viewportMainAxisExtent: 30.0,
     );
     SliverConstraints d = new SliverConstraints(
       axisDirection: AxisDirection.up,
@@ -50,6 +52,7 @@ void main() {
       overlap: 20.0,
       remainingPaintExtent: 30.0,
       crossAxisExtent: 40.0,
+      viewportMainAxisExtent: 30.0,
     );
     expect(c, equals(d));
     expect(c.normalizedGrowthDirection, equals(GrowthDirection.forward));

packages/flutter/test/widgets/page_view_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter/widgets.dart';
 
 import 'states.dart';
@@ -73,4 +74,38 @@ void main() {
     expect(find.text('Alaska'), findsOneWidget);
     expect(find.text('Arizona'), findsNothing);
   });
+
+  testWidgets('PageView does not squish when overscrolled', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      theme: new ThemeData(platform: TargetPlatform.iOS),
+      home: new PageView(
+        children: new List<Widget>.generate(10, (int i) {
+          return new Container(
+            key: new ValueKey<int>(i),
+            decoration: const BoxDecoration(
+              backgroundColor: const Color(0xFF0000FF),
+            ),
+          );
+        }),
+      ),
+    ));
+
+    Size sizeOf(int i) => tester.getSize(find.byKey(new ValueKey<int>(i)));
+    double leftOf(int i) => tester.getTopLeft(find.byKey(new ValueKey<int>(i))).x;
+
+    expect(leftOf(0), equals(0.0));
+    expect(sizeOf(0), equals(const Size(800.0, 600.0)));
+
+    await tester.scroll(find.byType(PageView), const Offset(100.0, 0.0));
+    await tester.pump();
+
+    expect(leftOf(0), equals(100.0));
+    expect(sizeOf(0), equals(const Size(800.0, 600.0)));
+
+    await tester.scroll(find.byType(PageView), const Offset(-200.0, 0.0));
+    await tester.pump();
+
+    expect(leftOf(0), equals(-100.0));
+    expect(sizeOf(0), equals(const Size(800.0, 600.0)));
+  });
 }