Merge pull request #1710 from abarth/scroll_anchor

Preliminaries for scroll anchoring

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

@@ -777,7 +777,7 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
         onSizeChanged: _handleViewportSizeChanged,
         child: new Viewport(
           scrollDirection: Axis.horizontal,
-          scrollOffset: new Offset(scrollOffset, 0.0),
+          paintOffset: scrollOffsetToPixelDelta(scrollOffset),
           child: contents
         )
       );
@@ -929,12 +929,12 @@ 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/grid.dart

@@ -314,10 +314,12 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
     int virtualChildBase: 0,
     int virtualChildCount,
     Offset paintOffset: Offset.zero,
+    Painter overlayPainter,
     LayoutCallback callback
   }) : _delegate = delegate, _virtualChildBase = virtualChildBase, super(
     virtualChildCount: virtualChildCount,
     paintOffset: paintOffset,
+    overlayPainter: overlayPainter,
     callback: callback
   ) {
     assert(delegate != null);
@@ -338,6 +340,15 @@ class RenderGrid extends RenderVirtualViewport<GridParentData> {
     _delegate = newDelegate;
   }
 
+  void set scrollDirection(Axis value) {
+    assert(() {
+      if (value != Axis.vertical)
+        throw new RenderingError('RenderGrid doesn\'t yet support horizontal scrolling.');
+      return true;
+    });
+    super.scrollDirection = value;
+  }
+
   int get virtualChildCount => super.virtualChildCount ?? childCount;
 
   /// The virtual index of the first child.

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

@@ -11,7 +11,7 @@ import 'viewport.dart';
 /// Parent data for use with [RenderList].
 class ListParentData extends ContainerBoxParentDataMixin<RenderBox> { }
 
-class RenderList extends RenderVirtualViewport<ListParentData> implements HasScrollDirection {
+class RenderList extends RenderVirtualViewport<ListParentData> {
   RenderList({
     List<RenderBox> children,
     double itemExtent,
@@ -19,13 +19,15 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
     int virtualChildCount,
     Offset paintOffset: Offset.zero,
     Axis scrollDirection: Axis.vertical,
+    Painter overlayPainter,
     LayoutCallback callback
   }) : _itemExtent = itemExtent,
        _padding = padding,
-       _scrollDirection = scrollDirection,
        super(
          virtualChildCount: virtualChildCount,
          paintOffset: paintOffset,
+         scrollDirection: scrollDirection,
+         overlayPainter: overlayPainter,
          callback: callback
        ) {
     addAll(children);
@@ -50,15 +52,6 @@ class RenderList extends RenderVirtualViewport<ListParentData> implements HasScr
     markNeedsLayout();
   }
 
-  Axis get scrollDirection => _scrollDirection;
-  Axis _scrollDirection;
-  void set scrollDirection (Axis newValue) {
-    if (_scrollDirection == newValue)
-      return;
-    _scrollDirection = newValue;
-    markNeedsLayout();
-  }
-
   void setupParentData(RenderBox child) {
     if (child.parentData is! ListParentData)
       child.parentData = new ListParentData();

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

@@ -797,11 +797,12 @@ class Viewport extends OneChildRenderObjectWidget {
   Viewport({
     Key key,
     this.scrollDirection: Axis.vertical,
-    this.scrollOffset: Offset.zero,
+    this.paintOffset: Offset.zero,
+    this.overlayPainter,
     Widget child
   }) : super(key: key, child: child) {
     assert(scrollDirection != null);
-    assert(scrollOffset != null);
+    assert(paintOffset != null);
   }
 
   /// The direction in which the child is permitted to be larger than the viewport
@@ -814,14 +815,25 @@ class Viewport extends OneChildRenderObjectWidget {
   /// The offset at which to paint the child.
   ///
   /// The offset can be non-zero only in the [scrollDirection].
-  final Offset scrollOffset;
+  final Offset paintOffset;
 
-  RenderViewport createRenderObject() => new RenderViewport(scrollDirection: scrollDirection, scrollOffset: scrollOffset);
+  /// Paints an overlay over the viewport.
+  ///
+  /// Often used to paint scroll bars.
+  final Painter overlayPainter;
+
+  RenderViewport createRenderObject() => new RenderViewport(
+    scrollDirection: scrollDirection,
+    paintOffset: paintOffset,
+    overlayPainter: overlayPainter
+  );
 
   void updateRenderObject(RenderViewport renderObject, Viewport oldWidget) {
     // Order dependency: RenderViewport validates scrollOffset based on scrollDirection.
-    renderObject.scrollDirection = scrollDirection;
-    renderObject.scrollOffset = scrollOffset;
+    renderObject
+      ..scrollDirection = scrollDirection
+      ..paintOffset = paintOffset
+      ..overlayPainter = overlayPainter;
   }
 }
 

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/rendering/viewport_test.dart

@@ -24,7 +24,7 @@ void main() {
       ),
       child: size);
 
-    RenderViewport viewport = new RenderViewport(child: red, scrollOffset: new Offset(0.0, -10.0));
+    RenderViewport viewport = new RenderViewport(child: red, paintOffset: new Offset(0.0, 10.0));
     layout(viewport);
 
     HitTestResult result;

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));
       });

packages/flutter_tools/lib/src/commands/analyze.dart

@@ -298,6 +298,7 @@ class AnalyzeCommand extends FlutterCommand {
       new RegExp(r'\[lint\] Prefer using lowerCamelCase for constant names.'), // sometimes we have no choice (e.g. when matching other platforms)
       new RegExp(r'\[lint\] Avoid defining a one-member abstract class when a simple function will do.'), // too many false-positives; code review should catch real instances
       new RegExp(r'\[info\] TODO.+'),
+      new RegExp('\\[warning\\] Missing concrete implementation of \'RenderObject\\.applyPaintTransform\''), // https://github.com/dart-lang/sdk/issues/25232
       new RegExp(r'[0-9]+ (error|warning|hint|lint).+found\.'),
       new RegExp(r'^$'),
     ];