Sky support for flexbox justify content

sky/sdk/lib/framework/rendering/flex.dart

Index: sky/sdk/lib/framework/rendering/flex.dart
diff --git a/sky/sdk/lib/framework/rendering/flex.dart b/sky/sdk/lib/framework/rendering/flex.dart
index 1a0a1f61d1a0760f3a0749ae573f818e02d81366..0f5dc51072fdb697ba9ec82a82b093e8d8a7aaa0 100644
--- a/sky/sdk/lib/framework/rendering/flex.dart
+++ b/sky/sdk/lib/framework/rendering/flex.dart
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:math' as math;
 import 'box.dart';
 import 'object.dart';
 
@@ -16,14 +17,22 @@ class FlexBoxParentData extends BoxParentData with ContainerParentDataMixin<Rend
 }
 
 enum FlexDirection { horizontal, vertical }
+enum FlexJustifyContent {
+  flexStart,
+  flexEnd,
+  center,
+  spaceBetween,
+  spaceAround,
+}
 
 class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, FlexBoxParentData>,
                                         RenderBoxContainerDefaultsMixin<RenderBox, FlexBoxParentData> {
   // lays out RenderBox children using flexible layout
 
   RenderFlex({
-    FlexDirection direction: FlexDirection.horizontal
-  }) : _direction = direction;
+    FlexDirection direction: FlexDirection.horizontal,
+    FlexJustifyContent justifyContent: FlexJustifyContent.flexStart
+  }) : _direction = direction, _justifyContent = justifyContent;
 
   FlexDirection _direction;
   FlexDirection get direction => _direction;
@@ -34,6 +43,15 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
     }
   }
 
+  FlexJustifyContent _justifyContent;
+  FlexJustifyContent get justifyContent => _justifyContent;
+  void set justifyContent (FlexJustifyContent value) {
+    if (_justifyContent != value) {
+      _justifyContent = value;
+      markNeedsLayout();
+    }
+  }
+
   void setParentData(RenderBox child) {
     if (child.parentData is! FlexBoxParentData)
       child.parentData = new FlexBoxParentData();
@@ -55,10 +73,12 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
     // Based on http://www.w3.org/TR/css-flexbox-1/ Section 9.7 Resolving Flexible Lengths
     // Steps 1-3. Determine used flex factor, size inflexible items, calculate free space
     int totalFlex = 0;
+    int totalChildren = 0;
     assert(constraints != null);
     double freeSpace = (_direction == FlexDirection.horizontal) ? constraints.maxWidth : constraints.maxHeight;
     RenderBox child = firstChild;
     while (child != null) {
+      totalChildren++;
       int flex = _getFlex(child);
       if (flex > 0) {
         totalFlex += child.parentData.flex;
@@ -93,19 +113,54 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
             break;
         }
         child.layout(innerConstraints, parentUsesSize: true);
+        usedSpace += _direction == FlexDirection.horizontal ? child.size.width : child.size.height;
       }
+      child = child.parentData.nextSibling;
+    }
+
+    // Section 8.2: Axis Alignment using the justify-content property
+    double remainingSpace = math.max(0.0, freeSpace - usedSpace);
+    double leadingSpace;
+    double betweenSpace;
+    child = firstChild;
+    switch (_justifyContent) {
+      case FlexJustifyContent.flexStart:
+        leadingSpace = 0.0;
+        betweenSpace = 0.0;
+        break;
+      case FlexJustifyContent.flexEnd:
+        leadingSpace = remainingSpace;
+        betweenSpace = 0.0;
+        break;
+      case FlexJustifyContent.center:
+        leadingSpace = remainingSpace / 2.0;
+        betweenSpace = 0.0;
+        break;
+      case FlexJustifyContent.spaceBetween:
+        leadingSpace = 0.0;
+        betweenSpace = totalChildren > 1 ? remainingSpace / (totalChildren - 1) : 0.0;
+        break;
+      case FlexJustifyContent.spaceAround:
+        betweenSpace = totalChildren > 0 ? remainingSpace / totalChildren : 0.0;
+        leadingSpace = betweenSpace / 2.0;
+        break;
+    }
 
-      // For now, center the flex items in the cross direction
+    // Position elements. For now, center the flex items in the cross direction
+    double mainDimPosition = leadingSpace;
+    child = firstChild;
+    while (child != null) {
       switch (_direction) {
         case FlexDirection.horizontal:
-          child.parentData.position = new Point(usedSpace, size.height / 2.0 - child.size.height / 2.0);
-          usedSpace += child.size.width;
+          child.parentData.position = new Point(mainDimPosition, size.height / 2.0 - child.size.height / 2.0);
+          mainDimPosition += child.size.width;
           break;
         case FlexDirection.vertical:
-          child.parentData.position = new Point(size.width / 2.0 - child.size.width / 2.0, usedSpace);
-          usedSpace += child.size.height;
+          child.parentData.position = new Point(size.width / 2.0 - child.size.width / 2.0, mainDimPosition);
+          mainDimPosition += child.size.height;
           break;
       }
+      mainDimPosition += betweenSpace;
       child = child.parentData.nextSibling;
     }
   }