Add scroll anchor for Block

This patch teaches block how to anchor its scrolling to the end rather than the
start.

Fixes #136

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

@@ -14,6 +14,33 @@ enum ViewportAnchor {
   end,
 }
 
+class ViewportDimensions {
+  const ViewportDimensions({
+    this.contentSize: Size.zero,
+    this.containerSize: Size.zero
+  });
+
+  static const ViewportDimensions zero = const ViewportDimensions();
+
+  final Size contentSize;
+  final Size containerSize;
+
+  bool get _debugHasAtLeastOneCommonDimension {
+    return contentSize.width == containerSize.width
+        || contentSize.height == containerSize.height;
+  }
+
+  Offset getAbsolutePaintOffset({ Offset paintOffset, ViewportAnchor anchor }) {
+    assert(_debugHasAtLeastOneCommonDimension);
+    switch (anchor) {
+      case ViewportAnchor.start:
+        return paintOffset;
+      case ViewportAnchor.end:
+        return paintOffset + (containerSize - contentSize);
+    }
+  }
+}
+
 abstract class HasScrollDirection {
   Axis get scrollDirection;
 }
@@ -27,9 +54,11 @@ class RenderViewportBase extends RenderBox implements HasScrollDirection {
   RenderViewportBase(
     Offset paintOffset,
     Axis scrollDirection,
+    ViewportAnchor scrollAnchor,
     Painter overlayPainter
   ) : _paintOffset = paintOffset,
       _scrollDirection = scrollDirection,
+      _scrollAnchor = scrollAnchor,
       _overlayPainter = overlayPainter {
     assert(paintOffset != null);
     assert(scrollDirection != null);
@@ -76,6 +105,17 @@ class RenderViewportBase extends RenderBox implements HasScrollDirection {
     markNeedsLayout();
   }
 
+  ViewportAnchor get scrollAnchor => _scrollAnchor;
+  ViewportAnchor _scrollAnchor;
+  void set scrollAnchor(ViewportAnchor value) {
+    assert(value != null);
+    if (value == _scrollAnchor)
+      return;
+    _scrollAnchor = value;
+    markNeedsPaint();
+    markNeedsSemanticsUpdate();
+  }
+
   Painter get overlayPainter => _overlayPainter;
   Painter _overlayPainter;
   void set overlayPainter(Painter value) {
@@ -99,16 +139,25 @@ class RenderViewportBase extends RenderBox implements HasScrollDirection {
     _overlayPainter?.detach();
   }
 
-  Offset get _paintOffsetRoundedToIntegerDevicePixels {
+  ViewportDimensions get dimensions => _dimensions;
+  ViewportDimensions _dimensions = ViewportDimensions.zero;
+  void set dimensions(ViewportDimensions value) {
+    assert(debugDoingThisLayout);
+    _dimensions = value;
+  }
+
+  Offset get _effectivePaintOffset {
     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);
+    return _dimensions.getAbsolutePaintOffset(
+      paintOffset: new Offset(dxInDevicePixels / devicePixelRatio, dyInDevicePixels / devicePixelRatio),
+      anchor: _scrollAnchor
+    );
   }
 
   void applyPaintTransform(RenderBox child, Matrix4 transform) {
-    final Offset effectivePaintOffset = _paintOffsetRoundedToIntegerDevicePixels;
+    final Offset effectivePaintOffset = _effectivePaintOffset;
     super.applyPaintTransform(child, transform.translate(effectivePaintOffset.dx, effectivePaintOffset.dy));
   }
 
@@ -126,8 +175,9 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
     RenderBox child,
     Offset paintOffset: Offset.zero,
     Axis scrollDirection: Axis.vertical,
+    ViewportAnchor scrollAnchor: ViewportAnchor.start,
     Painter overlayPainter
-  }) : super(paintOffset, scrollDirection, overlayPainter) {
+  }) : super(paintOffset, scrollDirection, scrollAnchor, overlayPainter) {
     this.child = child;
   }
 
@@ -183,8 +233,10 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
       size = constraints.constrain(child.size);
       final BoxParentData childParentData = child.parentData;
       childParentData.offset = Offset.zero;
+      dimensions = new ViewportDimensions(containerSize: size, contentSize: child.size);
     } else {
       performResize();
+      dimensions = new ViewportDimensions(containerSize: size);
     }
   }
 
@@ -195,7 +247,7 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
 
   void paint(PaintingContext context, Offset offset) {
     if (child != null) {
-      final Offset effectivePaintOffset = _paintOffsetRoundedToIntegerDevicePixels;
+      final Offset effectivePaintOffset = _effectivePaintOffset;
 
       void paintContents(PaintingContext context, Offset offset) {
         context.paintChild(child, offset + effectivePaintOffset);
@@ -211,7 +263,7 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
   }
 
   Rect describeApproximatePaintClip(RenderObject child) {
-    if (child != null && _shouldClipAtPaintOffset(_paintOffsetRoundedToIntegerDevicePixels))
+    if (child != null && _shouldClipAtPaintOffset(_effectivePaintOffset))
       return Point.origin & size;
     return null;
   }
@@ -219,7 +271,7 @@ class RenderViewport extends RenderViewportBase with RenderObjectWithChildMixin<
   bool hitTestChildren(HitTestResult result, { Point position }) {
     if (child != null) {
       assert(child.parentData is BoxParentData);
-      Point transformed = position + -_paintOffsetRoundedToIntegerDevicePixels;
+      Point transformed = position + -_effectivePaintOffset;
       return child.hitTest(result, position: transformed);
     }
     return false;
@@ -234,10 +286,11 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
     LayoutCallback callback,
     Offset paintOffset: Offset.zero,
     Axis scrollDirection: Axis.vertical,
+    ViewportAnchor scrollAnchor: ViewportAnchor.start,
     Painter overlayPainter
   }) : _virtualChildCount = virtualChildCount,
        _callback = callback,
-       super(paintOffset, scrollDirection, overlayPainter);
+       super(paintOffset, scrollDirection, scrollAnchor, overlayPainter);
 
   int get virtualChildCount => _virtualChildCount;
   int _virtualChildCount;
@@ -262,11 +315,11 @@ abstract class RenderVirtualViewport<T extends ContainerBoxParentDataMixin<Rende
   }
 
   bool hitTestChildren(HitTestResult result, { Point position }) {
-    return defaultHitTestChildren(result, position: position + -_paintOffsetRoundedToIntegerDevicePixels);
+    return defaultHitTestChildren(result, position: position + -_effectivePaintOffset);
   }
 
   void _paintContents(PaintingContext context, Offset offset) {
-    defaultPaint(context, offset + _paintOffsetRoundedToIntegerDevicePixels);
+    defaultPaint(context, offset + _effectivePaintOffset);
     _overlayPainter?.paint(context, offset);
   }
 

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

@@ -65,6 +65,7 @@ export 'package:flutter/rendering.dart' show
     TextStyle,
     TransferMode,
     ValueChanged,
+    ViewportAnchor,
     VoidCallback;
 
 
@@ -796,8 +797,9 @@ class Baseline extends OneChildRenderObjectWidget {
 class Viewport extends OneChildRenderObjectWidget {
   Viewport({
     Key key,
-    this.scrollDirection: Axis.vertical,
     this.paintOffset: Offset.zero,
+    this.scrollDirection: Axis.vertical,
+    this.scrollAnchor: ViewportAnchor.start,
     this.overlayPainter,
     Widget child
   }) : super(key: key, child: child) {
@@ -805,6 +807,11 @@ class Viewport extends OneChildRenderObjectWidget {
     assert(paintOffset != null);
   }
 
+  /// The offset at which to paint the child.
+  ///
+  /// The offset can be non-zero only in the [scrollDirection].
+  final Offset paintOffset;
+
   /// The direction in which the child is permitted to be larger than the viewport
   ///
   /// If the viewport is scrollable in a particular direction (e.g., vertically),
@@ -812,26 +819,27 @@ class Viewport extends OneChildRenderObjectWidget {
   /// that direction (e.g., the child can be as tall as it wants).
   final Axis scrollDirection;
 
-  /// The offset at which to paint the child.
-  ///
-  /// The offset can be non-zero only in the [scrollDirection].
-  final Offset paintOffset;
+  final ViewportAnchor scrollAnchor;
 
   /// 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
-  );
+  RenderViewport createRenderObject() {
+    return new RenderViewport(
+      paintOffset: paintOffset,
+      scrollDirection: scrollDirection,
+      scrollAnchor: scrollAnchor,
+      overlayPainter: overlayPainter
+    );
+  }
 
   void updateRenderObject(RenderViewport renderObject, Viewport oldWidget) {
     // Order dependency: RenderViewport validates scrollOffset based on scrollDirection.
     renderObject
       ..scrollDirection = scrollDirection
+      ..scrollAnchor = scrollAnchor
       ..paintOffset = paintOffset
       ..overlayPainter = overlayPainter;
   }

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

@@ -520,8 +520,9 @@ class _ScrollableViewportState extends ScrollableState<ScrollableViewport> {
     return new SizeObserver(
       onSizeChanged: _handleViewportSizeChanged,
       child: new Viewport(
-        scrollDirection: config.scrollDirection,
         paintOffset: scrollOffsetToPixelDelta(scrollOffset),
+        scrollDirection: config.scrollDirection,
+        scrollAnchor: config.scrollAnchor,
         child: new SizeObserver(
           onSizeChanged: _handleChildSizeChanged,
           child: config.child
@@ -541,6 +542,7 @@ class Block extends StatelessComponent {
     this.padding,
     this.initialScrollOffset,
     this.scrollDirection: Axis.vertical,
+    this.scrollAnchor: ViewportAnchor.start,
     this.onScroll,
     this.scrollableKey
   }) : super(key: key) {
@@ -552,6 +554,7 @@ class Block extends StatelessComponent {
   final EdgeDims padding;
   final double initialScrollOffset;
   final Axis scrollDirection;
+  final ViewportAnchor scrollAnchor;
   final ScrollListener onScroll;
   final Key scrollableKey;
 
@@ -563,6 +566,7 @@ class Block extends StatelessComponent {
       key: scrollableKey,
       initialScrollOffset: initialScrollOffset,
       scrollDirection: scrollDirection,
+      scrollAnchor: scrollAnchor,
       onScroll: onScroll,
       child: contents
     );

packages/flutter/test/widget/block_test.dart

@@ -66,4 +66,53 @@ void main() {
       tester.dispatchEvent(pointer.up(), target);
     });
   });
+
+  test('Scroll anchor', () {
+    testWidgets((WidgetTester tester) {
+      int first = 0;
+      int second = 0;
+
+      Widget buildBlock(ViewportAnchor scrollAnchor) {
+        return new Block(
+          key: new UniqueKey(),
+          scrollAnchor: scrollAnchor,
+          children: <Widget>[
+            new GestureDetector(
+              onTap: () { ++first; },
+              child: new Container(
+                height: 2000.0, // more than 600, the height of the test area
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFF00FF00)
+                )
+              )
+            ),
+            new GestureDetector(
+              onTap: () { ++second; },
+              child: new Container(
+                height: 2000.0, // more than 600, the height of the test area
+                decoration: new BoxDecoration(
+                  backgroundColor: new Color(0xFF0000FF)
+                )
+              )
+            )
+          ]
+        );
+      }
+
+      tester.pumpWidget(buildBlock(ViewportAnchor.end));
+      tester.pump(); // for SizeObservers
+
+      Point target = const Point(200.0, 200.0);
+      tester.tapAt(target);
+      expect(first, equals(0));
+      expect(second, equals(1));
+
+      tester.pumpWidget(buildBlock(ViewportAnchor.start));
+      tester.pump(); // for SizeObservers
+
+      tester.tapAt(target);
+      expect(first, equals(1));
+      expect(second, equals(1));
+    });
+  });
 }