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/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'^$'),
     ];