Rationalize the ScrollMetrics cloning logic (#14281)

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

@@ -130,12 +130,41 @@ class FixedExtentScrollController extends ScrollController {
 /// This is distinct from `Fixed` in the parent class name's [FixedScrollMetric]
 /// which refers to its immutability.
 class FixedExtentMetrics extends FixedScrollMetrics {
-  /// Creates fixed extent metrics that add the item index information to the
-  /// `parent` [FixedScrollMetrics].
+  /// Creates an immutable snapshot of values associated with a
+  /// [ListWheelScrollView].
   FixedExtentMetrics({
-    ScrollMetrics parent,
+    @required double minScrollExtent,
+    @required double maxScrollExtent,
+    @required double pixels,
+    @required double viewportDimension,
+    @required AxisDirection axisDirection,
     @required this.itemIndex,
-  }) : super.clone(parent);
+  }) : super(
+         minScrollExtent: minScrollExtent,
+         maxScrollExtent: maxScrollExtent,
+         pixels: pixels,
+         viewportDimension: viewportDimension,
+         axisDirection: axisDirection,
+       );
+
+  @override
+  FixedExtentMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+    int itemIndex,
+  }) {
+    return new FixedExtentMetrics(
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+      itemIndex: itemIndex ?? this.itemIndex,
+    );
+  }
 
   /// The scroll view's currently selected item index.
   final int itemIndex;
@@ -160,7 +189,7 @@ double _clipOffsetToScrollableRange(
 
 /// A [ScrollPositionWithSingleContext] that can only be created based on
 /// [_FixedExtentScrollable] and can access its `itemExtent` to derive [itemIndex].
-class _FixedExtentScrollPosition extends ScrollPositionWithSingleContext {
+class _FixedExtentScrollPosition extends ScrollPositionWithSingleContext implements FixedExtentMetrics {
   _FixedExtentScrollPosition({
     @required ScrollPhysics physics,
     @required ScrollContext context,
@@ -188,6 +217,7 @@ class _FixedExtentScrollPosition extends ScrollPositionWithSingleContext {
 
   double get itemExtent => _getItemExtentFromScrollContext(context);
 
+  @override
   int get itemIndex {
     return _getItemFromOffset(
       offset: pixels,
@@ -198,8 +228,22 @@ class _FixedExtentScrollPosition extends ScrollPositionWithSingleContext {
   }
 
   @override
-  FixedExtentMetrics cloneMetrics() {
-    return new FixedExtentMetrics(parent: this, itemIndex: itemIndex);
+  FixedExtentMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+    int itemIndex,
+  }) {
+    return new FixedExtentMetrics(
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+      itemIndex: itemIndex ?? this.itemIndex,
+    );
   }
 }
 

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

@@ -425,6 +425,29 @@ class _NestedScrollMetrics extends FixedScrollMetrics {
     axisDirection: axisDirection,
   );
 
+  @override
+  _NestedScrollMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+    double minRange,
+    double maxRange,
+    double correctionOffset,
+  }) {
+    return new _NestedScrollMetrics(
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+      minRange: minRange ?? this.minRange,
+      maxRange: maxRange ?? this.maxRange,
+      correctionOffset: correctionOffset ?? this.correctionOffset,
+    );
+  }
+
   final double minRange;
 
   final double maxRange;

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

@@ -178,18 +178,54 @@ class PageController extends ScrollController {
 /// The metrics are available on [ScrollNotification]s generated from
 /// [PageView]s.
 class PageMetrics extends FixedScrollMetrics {
-  /// Creates page metrics that add the given information to the `parent`
-  /// metrics.
+  /// Creates an immutable snapshot of values associated with a [PageView].
   PageMetrics({
-    ScrollMetrics parent,
-    this.page,
-  }) : super.clone(parent);
+    @required double minScrollExtent,
+    @required double maxScrollExtent,
+    @required double pixels,
+    @required double viewportDimension,
+    @required AxisDirection axisDirection,
+    @required this.viewportFraction,
+  }) : super(
+         minScrollExtent: minScrollExtent,
+         maxScrollExtent: maxScrollExtent,
+         pixels: pixels,
+         viewportDimension: viewportDimension,
+         axisDirection: axisDirection,
+       );
+
+  @override
+  PageMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+    double viewportFraction,
+  }) {
+    return new PageMetrics(
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+      viewportFraction: viewportFraction ?? this.viewportFraction,
+    );
+  }
 
   /// The current page displayed in the [PageView].
-  final double page;
+  double get page {
+    return math.max(0.0, pixels.clamp(minScrollExtent, maxScrollExtent)) /
+           math.max(1.0, viewportDimension * viewportFraction);
+  }
+
+  /// The fraction of the viewport that each page occupies.
+  ///
+  /// Used to compute [page] from the current [pixels].
+  final double viewportFraction;
 }
 
-class _PagePosition extends ScrollPositionWithSingleContext {
+class _PagePosition extends ScrollPositionWithSingleContext implements PageMetrics {
   _PagePosition({
     ScrollPhysics physics,
     ScrollContext context,
@@ -214,6 +250,7 @@ class _PagePosition extends ScrollPositionWithSingleContext {
   final int initialPage;
   double _pageToUseOnStartup;
 
+  @override
   double get viewportFraction => _viewportFraction;
   double _viewportFraction;
   set viewportFraction(double value) {
@@ -233,6 +270,7 @@ class _PagePosition extends ScrollPositionWithSingleContext {
     return page * viewportDimension * viewportFraction;
   }
 
+  @override
   double get page => pixels == null ? null : getPageFromPixels(pixels.clamp(minScrollExtent, maxScrollExtent), viewportDimension);
 
   @override
@@ -264,10 +302,21 @@ class _PagePosition extends ScrollPositionWithSingleContext {
   }
 
   @override
-  PageMetrics cloneMetrics() {
+  PageMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+    double viewportFraction,
+  }) {
     return new PageMetrics(
-      parent: this,
-      page: page,
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+      viewportFraction: viewportFraction ?? this.viewportFraction,
     );
   }
 }

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

@@ -36,7 +36,25 @@ abstract class ScrollMetrics {
   ///
   /// This is useful if this object is mutable, but you want to get a snapshot
   /// of the current state.
-  ScrollMetrics cloneMetrics() => new FixedScrollMetrics.clone(this);
+  ///
+  /// The named arguments allow the values to be adjusted in the process. This
+  /// is useful to examine hypothetical situations, for example "would applying
+  /// this delta unmodified take the position [outOfRange]?".
+  ScrollMetrics copyWith({
+    double minScrollExtent,
+    double maxScrollExtent,
+    double pixels,
+    double viewportDimension,
+    AxisDirection axisDirection,
+  }) {
+    return new FixedScrollMetrics(
+      minScrollExtent: minScrollExtent ?? this.minScrollExtent,
+      maxScrollExtent: maxScrollExtent ?? this.maxScrollExtent,
+      pixels: pixels ?? this.pixels,
+      viewportDimension: viewportDimension ?? this.viewportDimension,
+      axisDirection: axisDirection ?? this.axisDirection,
+    );
+  }
 
   /// The minimum in-range value for [pixels].
   ///
@@ -93,7 +111,6 @@ abstract class ScrollMetrics {
 /// An immutable snapshot of values associated with a [Scrollable] viewport.
 ///
 /// For details, see [ScrollMetrics], which defines this object's interfaces.
-@immutable
 class FixedScrollMetrics extends ScrollMetrics {
   /// Creates an immutable snapshot of values associated with a [Scrollable] viewport.
   FixedScrollMetrics({
@@ -104,14 +121,6 @@ class FixedScrollMetrics extends ScrollMetrics {
     @required this.axisDirection,
   });
 
-  /// Creates an immutable snapshot of the given metrics.
-  FixedScrollMetrics.clone(ScrollMetrics parent) :
-    minScrollExtent = parent.minScrollExtent,
-    maxScrollExtent = parent.maxScrollExtent,
-    pixels = parent.pixels,
-    viewportDimension = parent.viewportDimension,
-    axisDirection = parent.axisDirection;
-
   @override
   final double minScrollExtent;
 

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

@@ -589,19 +589,19 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
 
   /// Called by [beginActivity] to report when an activity has started.
   void didStartScroll() {
-    activity.dispatchScrollStartNotification(cloneMetrics(), context.notificationContext);
+    activity.dispatchScrollStartNotification(copyWith(), context.notificationContext);
   }
 
   /// Called by [setPixels] to report a change to the [pixels] position.
   void didUpdateScrollPositionBy(double delta) {
-    activity.dispatchScrollUpdateNotification(cloneMetrics(), context.notificationContext, delta);
+    activity.dispatchScrollUpdateNotification(copyWith(), context.notificationContext, delta);
   }
 
   /// Called by [beginActivity] to report when an activity has ended.
   ///
   /// This also saves the scroll offset using [saveScrollOffset].
   void didEndScroll() {
-    activity.dispatchScrollEndNotification(cloneMetrics(), context.notificationContext);
+    activity.dispatchScrollEndNotification(copyWith(), context.notificationContext);
     if (keepScrollOffset)
       saveScrollOffset();
   }
@@ -611,14 +611,14 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
   /// not applied to the [pixels] value.
   void didOverscrollBy(double value) {
     assert(activity.isScrolling);
-    activity.dispatchOverscrollNotification(cloneMetrics(), context.notificationContext, value);
+    activity.dispatchOverscrollNotification(copyWith(), context.notificationContext, value);
   }
 
   /// Dispatches a notification that the [userScrollDirection] has changed.
   ///
   /// Subclasses should call this function when they change [userScrollDirection].
   void didUpdateScrollDirection(ScrollDirection direction) {
-    new UserScrollNotification(metrics: cloneMetrics(), context: context.notificationContext, direction: direction).dispatch(context.notificationContext);
+    new UserScrollNotification(metrics: copyWith(), context: context.notificationContext, direction: direction).dispatch(context.notificationContext);
   }
 
   @override

packages/flutter/test/widgets/page_view_test.dart

@@ -712,4 +712,20 @@ void main() {
 
     semantics.dispose();
   });
+
+  testWidgets('PageMetrics', (WidgetTester tester) async {
+    final PageMetrics page = new PageMetrics(
+      minScrollExtent: 100.0,
+      maxScrollExtent: 200.0,
+      pixels: 150.0,
+      viewportDimension: 25.0,
+      axisDirection: AxisDirection.right,
+      viewportFraction: 1.0,
+    );
+    expect(page.page, 6);
+    final PageMetrics page2 = page.copyWith(
+      pixels: page.pixels - 100.0,
+    );
+    expect(page2.page, 4.0);
+  });
 }

packages/flutter/test/widgets/scroll_behavior_test.dart

@@ -48,7 +48,7 @@ void main() {
     expect(behavior, isNotNull);
     expect(behavior.flag, isTrue);
     expect(position.physics, const isInstanceOf<ClampingScrollPhysics>());
-    ScrollMetrics metrics = position.cloneMetrics();
+    ScrollMetrics metrics = position.copyWith();
     expect(metrics.extentAfter, equals(400.0));
     expect(metrics.viewportDimension, equals(600.0));
 
@@ -64,7 +64,7 @@ void main() {
     expect(behavior.flag, isFalse);
     expect(position.physics, const isInstanceOf<BouncingScrollPhysics>());
     // Regression test for https://github.com/flutter/flutter/issues/5856
-    metrics = position.cloneMetrics();
+    metrics = position.copyWith();
     expect(metrics.extentAfter, equals(400.0));
     expect(metrics.viewportDimension, equals(600.0));
   });