Add a Widget wrapper around Grid and test RenderGrid

@abarth

packages/flutter/lib/rendering/grid.dart

@@ -10,11 +10,14 @@ class GridParentData extends BoxParentData with ContainerParentDataMixin<RenderB
 class GridMetrics {
   // Grid is width-in, height-out.  We fill the max width and adjust height
   // accordingly.
-  factory GridMetrics({ double width, int childCount, double childExtent }) {
+  factory GridMetrics({ double width, int childCount, double maxChildExtent }) {
     assert(width != null);
     assert(childCount != null);
-    assert(childExtent != null);
-    int childrenPerRow = (width / childExtent).floor() + 1;
+    assert(maxChildExtent != null);
+    double childExtent = maxChildExtent;
+    int childrenPerRow = (width / childExtent).floor();
+    // If the child extent divides evenly into the width use that, otherwise + 1
+    if (width / childExtent != childrenPerRow.toDouble()) childrenPerRow += 1;
     double totalPadding = 0.0;
     if (childrenPerRow * childExtent > width) {
       // TODO(eseidel): We should snap to pixel bounderies.
@@ -50,6 +53,14 @@ class RenderGrid extends RenderBox with ContainerRenderObjectMixin<RenderBox, Gr
   double _maxChildExtent;
   bool _hasVisualOverflow = false;
 
+  double get maxChildExtent => _maxChildExtent;
+  void set maxChildExtent (double value) {
+    if (_maxChildExtent != value) {
+      _maxChildExtent = value;
+      markNeedsLayout();
+    }
+  }
+
   void setupParentData(RenderBox child) {
     if (child.parentData is! GridParentData)
       child.parentData = new GridParentData();
@@ -93,7 +104,7 @@ class RenderGrid extends RenderBox with ContainerRenderObjectMixin<RenderBox, Gr
     return new GridMetrics(
       width: constraints.maxWidth,
       childCount: childCount,
-      childExtent: _maxChildExtent
+      maxChildExtent: _maxChildExtent
     );
   }
 

packages/flutter/lib/widgets/basic.dart

@@ -14,6 +14,7 @@ import 'package:sky/painting/paragraph_painter.dart';
 import 'package:sky/rendering/block.dart';
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
+import 'package:sky/rendering/grid.dart';
 import 'package:sky/rendering/image.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:sky/rendering/paragraph.dart';
@@ -454,6 +455,21 @@ class Stack extends MultiChildRenderObjectWrapper {
   RenderStack get renderObject => super.renderObject;
 }
 
+class Grid extends MultiChildRenderObjectWrapper {
+  Grid(List<Widget> children, { Key key, this.maxChildExtent })
+    : super(key: key, children: children);
+
+  final double maxChildExtent;
+
+  RenderGrid createNode() => new RenderGrid(maxChildExtent: maxChildExtent);
+  RenderGrid get renderObject => super.renderObject;
+
+  void syncRenderObject(Widget old) {
+    super.syncRenderObject(old);
+    renderObject.maxChildExtent = maxChildExtent;
+  }
+}
+
 class Positioned extends ParentDataNode {
   Positioned({
     Key key,
@@ -495,7 +511,6 @@ class Flex extends MultiChildRenderObjectWrapper {
     renderObject.alignItems = alignItems;
     renderObject.textBaseline = textBaseline;
   }
-
 }
 
 class Row extends Flex {

packages/unit/test/rendering/grid_test.dart

+import 'package:sky/rendering.dart';
+import 'package:test/test.dart';
+
+import 'rendering_tester.dart';
+
+void main() {
+  test('Basic grid layout test', () {
+    List<RenderBox> children = [
+      new RenderDecoratedBox(decoration: new BoxDecoration()),
+      new RenderDecoratedBox(decoration: new BoxDecoration()),
+      new RenderDecoratedBox(decoration: new BoxDecoration()),
+      new RenderDecoratedBox(decoration: new BoxDecoration())
+    ];
+
+    RenderBox grid = new RenderGrid(children: children, maxChildExtent: 100.0);
+    RenderingTester tester = layout(grid, constraints: const BoxConstraints(maxWidth: 200.0));
+
+    children.forEach((child) {
+      expect(child.size.width, equals(100.0), reason: "child width");
+      expect(child.size.height, equals(100.0), reason: "child height");
+    });
+
+    expect(grid.size.width, equals(200.0), reason: "grid width");
+    expect(grid.size.height, equals(200.0), reason: "grid height");
+
+    expect(grid.needsLayout, equals(false));
+    grid.maxChildExtent = 60.0;
+    expect(grid.needsLayout, equals(true));
+
+    tester.pumpFrame(phase: EnginePhase.layout);
+
+    children.forEach((child) {
+      expect(child.size.width, equals(50.0), reason: "child width");
+      expect(child.size.height, equals(50.0), reason: "child height");
+    });
+
+    expect(grid.size.width, equals(200.0), reason: "grid width");
+    expect(grid.size.height, equals(50.0), reason: "grid height");
+  });
+}