Implement MainAxisAlignment.spaceEvenly (#3550)

Fixes #3289

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

@@ -29,33 +29,45 @@ enum FlexDirection {
   vertical
 }
 
-/// How the children should be placed along the main axis in a flex layout
+/// How the children should be placed along the main axis in a flex layout.
 enum MainAxisAlignment {
-  /// Place the children as close to the start of the main axis as possible
+  /// Place the children as close to the start of the main axis as possible.
   start,
-  /// Place the children as close to the end of the main axis as possible
+
+  /// Place the children as close to the end of the main axis as possible.
   end,
-  /// Place the children as close to the middle of the main axis as possible
+
+  /// Place the children as close to the middle of the main axis as possible.
   center,
-  /// Place the free space evenly between the children
+
+  /// Place the free space evenly between the children.
   spaceBetween,
-  /// Place the free space evenly between the children as well as before and after the first and last child
+
+  /// Place the free space evenly between the children as well as half of that space before and after the first and last child.
   spaceAround,
+
+  /// Place the free space evenly between the children as well as before and after the first and last child.
+  spaceEvenly,
+
   /// Do not expand to fill the free space. None of the children may specify a flex factor.
   collapse,
 }
 
-/// How the children should be placed along the cross axis in a flex layout
+/// How the children should be placed along the cross axis in a flex layout.
 enum CrossAxisAlignment {
-  /// Place the children as close to the start of the cross axis as possible
+  /// Place the children as close to the start of the cross axis as possible.
   start,
-  /// Place the children as close to the end of the cross axis as possible
+
+  /// Place the children as close to the end of the cross axis as possible.
   end,
-  /// Place the children as close to the middle of the cross axis as possible
+
+  /// Place the children as close to the middle of the cross axis as possible.
   center,
-  /// Require the children to fill the cross axis
+
+  /// Require the children to fill the cross axis.
   stretch,
-  /// Place the children along the cross axis such that their baselines match
+
+  /// Place the children along the cross axis such that their baselines match.
   baseline,
 }
 
@@ -562,6 +574,10 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
         betweenSpace = totalChildren > 0 ? remainingSpace / totalChildren : 0.0;
         leadingSpace = betweenSpace / 2.0;
         break;
+      case MainAxisAlignment.spaceEvenly:
+        betweenSpace = totalChildren > 0 ? remainingSpace / (totalChildren + 1) : 0.0;
+        leadingSpace = betweenSpace;
+        break;
     }
 
     // Position elements

packages/flutter/test/rendering/flex_test.dart

@@ -136,4 +136,34 @@ void main() {
     expect(box2.size.width, equals(100.0));
     expect(box2.size.height, equals(100.0));
   });
+
+  test('Space evenly', () {
+    RenderConstrainedBox box1 = new RenderConstrainedBox(additionalConstraints: new BoxConstraints.tightFor(width: 100.0, height: 100.0));
+    RenderConstrainedBox box2 = new RenderConstrainedBox(additionalConstraints: new BoxConstraints.tightFor(width: 100.0, height: 100.0));
+    RenderConstrainedBox box3 = new RenderConstrainedBox(additionalConstraints: new BoxConstraints.tightFor(width: 100.0, height: 100.0));
+    RenderFlex flex = new RenderFlex(mainAxisAlignment: MainAxisAlignment.spaceEvenly);
+    flex.addAll(<RenderBox>[box1, box2, box3]);
+    layout(flex, constraints: const BoxConstraints(
+      minWidth: 0.0, maxWidth: 500.0, minHeight: 0.0, maxHeight: 400.0)
+    );
+    Offset getOffset(RenderBox box) {
+      FlexParentData parentData = box.parentData;
+      return parentData.offset;
+    }
+    expect(getOffset(box1).dx, equals(50.0));
+    expect(box1.size.width, equals(100.0));
+    expect(getOffset(box2).dx, equals(200.0));
+    expect(box2.size.width, equals(100.0));
+    expect(getOffset(box3).dx, equals(350.0));
+    expect(box3.size.width, equals(100.0));
+
+    flex.direction = FlexDirection.vertical;
+    pumpFrame();
+    expect(getOffset(box1).dy, equals(25.0));
+    expect(box1.size.height, equals(100.0));
+    expect(getOffset(box2).dy, equals(150.0));
+    expect(box2.size.height, equals(100.0));
+    expect(getOffset(box3).dy, equals(275.0));
+    expect(box3.size.height, equals(100.0));
+  });
 }