Rationalize RenderViewport and RenderVirtualViewport

These classes now share more code and have feature parity.

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

@@ -771,7 +771,7 @@ 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
         )
       );

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/rendering/viewport.dart

@@ -18,26 +18,22 @@ abstract class HasScrollDirection {
   Axis get scrollDirection;
 }
 
-/// A render object that's bigger on the inside.
+/// A base class for render objects that are bigger on the inside.
 ///
-/// The child of a viewport can layout to a larger size than the viewport
-/// itself. If that happens, only a portion of the child will be visible through
-/// the viewport. The portion of the child that is visible is controlled by the
-/// scroll offset.
-///
-/// Viewport is the core scrolling primitive in the system, but it can be used
-/// in other situations.
-class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox>
-    implements HasScrollDirection {
-
-  RenderViewport({
-    RenderBox child,
-    Offset scrollOffset: Offset.zero,
-    Axis scrollDirection: Axis.vertical
-  }) : _scrollOffset = scrollOffset,
-       _scrollDirection = scrollDirection {
-    assert(_offsetIsSane(scrollOffset, scrollDirection));
-    this.child = child;
+/// This class holds the common fields for viewport render objects but does not
+/// have a child model. See [RenderViewport] for a viewport with a single child
+/// and [RenderVirtualViewport] for a viewport with multiple children.
+class RenderViewportBase extends RenderBox implements HasScrollDirection {
+  RenderViewportBase(
+    Offset paintOffset,
+    Axis scrollDirection,
+    Painter overlayPainter
+  ) : _paintOffset = paintOffset,
+      _scrollDirection = scrollDirection,
+      _overlayPainter = overlayPainter {
+    assert(paintOffset != null);
+    assert(scrollDirection != null);
+    assert(_offsetIsSane(_paintOffset, scrollDirection));
   }
 
   bool _offsetIsSane(Offset offset, Axis direction) {
@@ -52,13 +48,14 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
   /// The offset at which to paint the child.
   ///
   /// The offset can be non-zero only in the [scrollDirection].
-  Offset get scrollOffset => _scrollOffset;
-  Offset _scrollOffset;
-  void set scrollOffset(Offset value) {
-    if (value == _scrollOffset)
+  Offset get paintOffset => _paintOffset;
+  Offset _paintOffset;
+  void set paintOffset(Offset value) {
+    assert(value != null);
+    if (value == _paintOffset)
       return;
     assert(_offsetIsSane(value, scrollDirection));
-    _scrollOffset = value;
+    _paintOffset = value;
     markNeedsPaint();
     markNeedsSemanticsUpdate();
   }
@@ -71,13 +68,69 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
   Axis get scrollDirection => _scrollDirection;
   Axis _scrollDirection;
   void set scrollDirection(Axis value) {
+    assert(value != null);
     if (value == _scrollDirection)
       return;
-    assert(_offsetIsSane(scrollOffset, value));
+    assert(_offsetIsSane(_paintOffset, value));
     _scrollDirection = value;
     markNeedsLayout();
   }
 
+  Painter get overlayPainter => _overlayPainter;
+  Painter _overlayPainter;
+  void set overlayPainter(Painter value) {
+    if (_overlayPainter == value)
+      return;
+    if (attached)
+      _overlayPainter?.detach();
+    _overlayPainter = value;
+    if (attached)
+      _overlayPainter?.attach(this);
+    markNeedsPaint();
+  }
+
+  void attach() {
+    super.attach();
+    _overlayPainter?.attach(this);
+  }
+
+  void detach() {
+    super.detach();
+    _overlayPainter?.detach();
+  }
+
+  Offset get _paintOffsetRoundedToIntegerDevicePixels {
+    final double devicePixelRatio = ui.window.devicePixelRatio;
+    int dxInDevicePixels = (_paintOffset.dx * devicePixelRatio).round();
+    int dyInDevicePixels = (_paintOffset.dy * devicePixelRatio).round();
+    return new Offset(dxInDevicePixels / devicePixelRatio,
+                      dyInDevicePixels / devicePixelRatio);
+  }
+
+  void applyPaintTransform(RenderBox child, Matrix4 transform) {
+    final Offset effectivePaintOffset = _paintOffsetRoundedToIntegerDevicePixels;
+    super.applyPaintTransform(child, transform.translate(effectivePaintOffset.dx, effectivePaintOffset.dy));
+  }
+
+}
+
+/// A render object that's bigger on the inside.
+///
+/// The child of a viewport can layout to a larger size than the viewport
+/// itself. If that happens, only a portion of the child will be visible through
+/// the viewport. The portion of the child that is visible is controlled by the
+/// paint offset.
+class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<RenderBox> {
+
+  RenderViewport({
+    RenderBox child,
+    Offset paintOffset: Offset.zero,
+    Axis scrollDirection: Axis.vertical,
+    Painter overlayPainter
+  }) : super(paintOffset, scrollDirection, overlayPainter) {
+    this.child = child;
+  }
+
   BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
     BoxConstraints innerConstraints;
     switch (scrollDirection) {
@@ -135,41 +188,30 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
     }
   }
 
-  Offset get _scrollOffsetRoundedToIntegerDevicePixels {
-    double devicePixelRatio = ui.window.devicePixelRatio;
-    int dxInDevicePixels = (scrollOffset.dx * devicePixelRatio).round();
-    int dyInDevicePixels = (scrollOffset.dy * devicePixelRatio).round();
-    return new Offset(dxInDevicePixels / devicePixelRatio,
-                      dyInDevicePixels / devicePixelRatio);
-  }
-
-  bool _wouldNeedClipAtOffset(Offset offset) {
+  bool _shouldClipAtPaintOffset(Offset paintOffset) {
     assert(child != null);
-    return offset < Offset.zero || !(Offset.zero & size).contains(((Offset.zero - offset) & child.size).bottomRight);
+    return paintOffset < Offset.zero || !(Offset.zero & size).contains((paintOffset & child.size).bottomRight);
   }
 
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
-      Offset roundedScrollOffset = _scrollOffsetRoundedToIntegerDevicePixels;
-      bool _needsClip = _wouldNeedClipAtOffset(roundedScrollOffset);
-      if (_needsClip) {
-        context.pushClipRect(needsCompositing, offset, Point.origin & size, (PaintingContext context, Offset offset) {
-          context.paintChild(child, offset - roundedScrollOffset);
-        });
+      final Offset effectivePaintOffset = _paintOffsetRoundedToIntegerDevicePixels;
+
+      void paintContents(PaintingContext context, Offset offset) {
+        context.paintChild(child, offset + effectivePaintOffset);
+        _overlayPainter?.paint(context, offset);
+      }
+
+      if (_shouldClipAtPaintOffset(effectivePaintOffset)) {
+        context.pushClipRect(needsCompositing, offset, Point.origin & size, paintContents);
       } else {
-        context.paintChild(child, offset - roundedScrollOffset);
+        paintContents(context, offset);
       }
     }
   }
 
-  void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    transform.translate(-scrollOffset.dx, -scrollOffset.dy);
-    super.applyPaintTransform(child, transform);
-  }
-
   Rect describeApproximatePaintClip(RenderObject child) {
-    if (child != null &&
-        _wouldNeedClipAtOffset(_scrollOffsetRoundedToIntegerDevicePixels))
+    if (child != null && _shouldClipAtPaintOffset(_paintOffsetRoundedToIntegerDevicePixels))
       return Point.origin & size;
     return null;
   }
@@ -177,7 +219,7 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
   bool hitTestChildren(HitTestResult result, { Point position }) {
     if (child != null) {
       assert(child.parentData is BoxParentData);
-      Point transformed = position + _scrollOffsetRoundedToIntegerDevicePixels;
+      Point transformed = position + -_paintOffsetRoundedToIntegerDevicePixels;
       return child.hitTest(result, position: transformed);
     }
     return false;
@@ -185,17 +227,17 @@ class RenderViewport extends RenderBox with RenderObjectWithChildMixin<RenderBox
 }
 
 abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<RenderBox>>
-    extends RenderBox with ContainerRenderObjectMixin<RenderBox, T>,
-                           RenderBoxContainerDefaultsMixin<RenderBox, T> {
+    extends RenderViewportBase with ContainerRenderObjectMixin<RenderBox, T>,
+                                    RenderBoxContainerDefaultsMixin<RenderBox, T> {
   RenderVirtualViewport({
     int virtualChildCount,
-    Offset paintOffset,
     LayoutCallback callback,
+    Offset paintOffset: Offset.zero,
+    Axis scrollDirection: Axis.vertical,
     Painter overlayPainter
   }) : _virtualChildCount = virtualChildCount,
-       _paintOffset = paintOffset,
        _callback = callback,
-       _overlayPainter = overlayPainter;
+       super(paintOffset, scrollDirection, overlayPainter);
 
   int get virtualChildCount => _virtualChildCount;
   int _virtualChildCount;
@@ -206,21 +248,7 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
     markNeedsLayout();
   }
 
-  /// The offset at which to paint the first item.
-  ///
-  /// Note: you can modify this property from within [callback], if necessary.
-  Offset get paintOffset => _paintOffset;
-  Offset _paintOffset;
-  void set paintOffset(Offset value) {
-    assert(value != null);
-    if (value == _paintOffset)
-      return;
-    _paintOffset = value;
-    markNeedsPaint();
-    markNeedsSemanticsUpdate();
-  }
-
-  /// Called during [layout] to determine the grid's children.
+  /// Called during [layout] to determine the render object's children.
   ///
   /// Typically the callback will mutate the child list appropriately, for
   /// example so the child list contains only visible children.
@@ -233,39 +261,12 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
     markNeedsLayout();
   }
 
-  Painter get overlayPainter => _overlayPainter;
-  Painter _overlayPainter;
-  void set overlayPainter(Painter value) {
-    if (_overlayPainter == value)
-      return;
-    if (attached)
-      _overlayPainter?.detach();
-    _overlayPainter = value;
-    if (attached)
-      _overlayPainter?.attach(this);
-    markNeedsPaint();
-  }
-
-  void attach() {
-    super.attach();
-    _overlayPainter?.attach(this);
-  }
-
-  void detach() {
-    super.detach();
-    _overlayPainter?.detach();
-  }
-
-  void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    super.applyPaintTransform(child, transform.translate(paintOffset.dx, paintOffset.dy));
-  }
-
   bool hitTestChildren(HitTestResult result, { Point position }) {
-    return defaultHitTestChildren(result, position: position + -paintOffset);
+    return defaultHitTestChildren(result, position: position + -_paintOffsetRoundedToIntegerDevicePixels);
   }
 
   void _paintContents(PaintingContext context, Offset offset) {
-    defaultPaint(context, offset + paintOffset);
+    defaultPaint(context, offset + _paintOffsetRoundedToIntegerDevicePixels);
     _overlayPainter?.paint(context, offset);
   }
 

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/scrollable.dart

@@ -467,15 +467,17 @@ class ScrollNotification extends Notification {
 class ScrollableViewport extends Scrollable {
   ScrollableViewport({
     Key key,
-    this.child,
     double initialScrollOffset,
     Axis scrollDirection: Axis.vertical,
+    ViewportAnchor scrollAnchor: ViewportAnchor.start,
     ScrollListener onScrollStart,
     ScrollListener onScroll,
-    ScrollListener onScrollEnd
+    ScrollListener onScrollEnd,
+    this.child
   }) : super(
     key: key,
     scrollDirection: scrollDirection,
+    scrollAnchor: scrollAnchor,
     initialScrollOffset: initialScrollOffset,
     onScrollStart: onScrollStart,
     onScroll: onScroll,
@@ -514,18 +516,12 @@ class _ScrollableViewportState extends ScrollableState<ScrollableViewport> {
     ));
   }
 
-  Offset get _scrollOffsetVector {
-    if (config.scrollDirection == Axis.horizontal)
-      return new Offset(scrollOffset, 0.0);
-    return new Offset(0.0, scrollOffset);
-  }
-
   Widget buildContent(BuildContext context) {
     return new SizeObserver(
       onSizeChanged: _handleViewportSizeChanged,
       child: new Viewport(
-        scrollOffset: _scrollOffsetVector,
         scrollDirection: config.scrollDirection,
+        paintOffset: scrollOffsetToPixelDelta(scrollOffset),
         child: new SizeObserver(
           onSizeChanged: _handleChildSizeChanged,
           child: config.child

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_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'^$'),
     ];