Improve unit conversions in Scrollable

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

@@ -922,12 +922,12 @@ class _TabBarViewState extends PageableListState<TabBarView> implements TabBarSe
       controller.value = scrollOffset / 2.0;
   }
 
-  Future fling(Offset scrollVelocity) {
+  Future fling(double scrollVelocity) {
     if (_selection == null || _selection.valueIsChanging)
       return new Future.value();
 
-    if (scrollVelocity.dx.abs() > _kMinFlingVelocity) {
-      final int selectionDelta = scrollVelocity.dx > 0 ? -1 : 1;
+    if (scrollVelocity.abs() > _kMinFlingVelocity) {
+      final int selectionDelta = scrollVelocity.sign.truncate();
       final int targetIndex = (_selection.index + selectionDelta).clamp(0, _tabCount - 1);
       _selection.value = _selection.values[targetIndex];
       return new Future.value();

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

@@ -9,6 +9,11 @@ import 'package:vector_math/vector_math_64.dart';
 import 'box.dart';
 import 'object.dart';
 
+enum ViewportAnchor {
+  start,
+  end,
+}
+
 abstract class HasScrollDirection {
   Axis get scrollDirection;
 }

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

@@ -5,7 +5,6 @@
 import 'dart:async';
 
 import 'package:flutter/animation.dart';
-import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';
 
 import 'basic.dart';
@@ -65,12 +64,25 @@ class PageableListState<T extends PageableList> extends ScrollableState<T> {
   int get itemCount => config.children?.length ?? 0;
   int _previousItemCount;
 
-  double pixelToScrollOffset(double value) {
+  double get _pixelsPerScrollUnit {
     final RenderBox box = context.findRenderObject();
     if (box == null || !box.hasSize)
       return 0.0;
-    final double pixelScrollExtent = config.scrollDirection == Axis.vertical ? box.size.height : box.size.width;
-    return pixelScrollExtent == 0.0 ? 0.0 : value / pixelScrollExtent;
+    switch (config.scrollDirection) {
+      case Axis.horizontal:
+        return box.size.width;
+      case Axis.vertical:
+        return box.size.height;
+    }
+  }
+
+  double pixelOffsetToScrollOffset(double pixelOffset) {
+    final double pixelsPerScrollUnit = _pixelsPerScrollUnit;
+    return super.pixelOffsetToScrollOffset(pixelsPerScrollUnit == 0.0 ? 0.0 : pixelOffset / pixelsPerScrollUnit);
+  }
+
+  double scrollOffsetToPixelOffset(double scrollOffset) {
+    return super.scrollOffsetToPixelOffset(scrollOffset * _pixelsPerScrollUnit);
   }
 
   void initState() {
@@ -143,7 +155,7 @@ class PageableListState<T extends PageableList> extends ScrollableState<T> {
 
   ScrollBehavior createScrollBehavior() => scrollBehavior;
 
-  bool get snapScrollOffsetChanges => config.itemsSnapAlignment == ItemsSnapAlignment.item;
+  bool get shouldSnapScrollOffset => config.itemsSnapAlignment == ItemsSnapAlignment.item;
 
   double snapScrollOffset(double newScrollOffset) {
     final double previousItemOffset = newScrollOffset.floorToDouble();
@@ -152,20 +164,19 @@ class PageableListState<T extends PageableList> extends ScrollableState<T> {
       .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
   }
 
-  Future _flingToAdjacentItem(Offset velocity) {
-    final double scrollVelocity = scrollDirectionVelocity(velocity);
+  Future _flingToAdjacentItem(double scrollVelocity) {
     final double newScrollOffset = snapScrollOffset(scrollOffset + scrollVelocity.sign)
       .clamp(snapScrollOffset(scrollOffset - 0.5), snapScrollOffset(scrollOffset + 0.5));
     return scrollTo(newScrollOffset, duration: config.duration, curve: config.curve)
       .then(_notifyPageChanged);
   }
 
-  Future fling(Offset velocity) {
+  Future fling(double scrollVelocity) {
     switch(config.itemsSnapAlignment) {
       case ItemsSnapAlignment.adjacentItem:
-        return _flingToAdjacentItem(velocity);
+        return _flingToAdjacentItem(scrollVelocity);
       default:
-        return super.fling(velocity).then(_notifyPageChanged);
+        return super.fling(scrollVelocity).then(_notifyPageChanged);
     }
   }
 

packages/flutter/test/widget/snap_scrolling_test.dart

@@ -48,14 +48,11 @@ void set scrollOffset(double value) {
 }
 
 Future fling(double velocity) {
-  Offset velocityOffset = scrollDirection == Axis.vertical
-    ? new Offset(0.0, velocity)
-    : new Offset(velocity, 0.0);
-  return scrollableState.fling(velocityOffset);
+  return scrollableState.fling(velocity);
 }
 
 void main() {
-  test('ScrollableList snap scrolling, fling(-0.8)', () {
+  test('ScrollableList snap scrolling, fling(0.8)', () {
     testWidgets((WidgetTester tester) {
       tester.pumpWidget(buildFrame());
 
@@ -65,7 +62,7 @@ void main() {
 
       Duration dt = const Duration(seconds: 2);
 
-      fling(-0.8);
+      fling(0.8);
       tester.pump(); // Start the scheduler at 0.0
       tester.pump(dt);
       expect(scrollOffset, closeTo(200.0, 1.0));
@@ -74,7 +71,7 @@ void main() {
       tester.pump();
       expect(scrollOffset, 0.0);
 
-      fling(-2.0);
+      fling(2.0);
       tester.pump();
       tester.pump(dt);
       expect(scrollOffset, closeTo(400.0, 1.0));
@@ -83,7 +80,7 @@ void main() {
       tester.pump();
       expect(scrollOffset, 400.0);
 
-      fling(0.8);
+      fling(-0.8);
       tester.pump();
       tester.pump(dt);
       expect(scrollOffset, closeTo(0.0, 1.0));
@@ -92,7 +89,7 @@ void main() {
       tester.pump();
       expect(scrollOffset, 800.0);
 
-      fling(2.0);
+      fling(-2.0);
       tester.pump();
       tester.pump(dt);
       expect(scrollOffset, closeTo(200.0, 1.0));
@@ -102,7 +99,7 @@ void main() {
       expect(scrollOffset, 800.0);
 
       bool completed = false;
-      fling(2.0).then((_) {
+      fling(-2.0).then((_) {
         completed = true;
         expect(scrollOffset, closeTo(200.0, 1.0));
       });