C# Baseline function

Summary:
same as Measure function, based on #317
Closes https://github.com/facebook/yoga/pull/321

Reviewed By: emilsjolander

Differential Revision: D4385336

Pulled By: splhack

fbshipit-source-id: b583ec79861d2e9abba31a72503e2f706bfda8e8

csharp/Facebook.Yoga/YogaNode.cs

@@ -21,6 +21,8 @@ namespace Facebook.Yoga
         private List<YogaNode> _children;
         private MeasureFunction _measureFunction;
         private YogaMeasureFunc _ygMeasureFunc;
+        private BaselineFunction _baselineFunction;
+        private YogaBaselineFunc _ygBaselineFunc;
         private object _data;
 
         public YogaNode()
@@ -37,6 +39,7 @@ namespace Facebook.Yoga
         public void Reset()
         {
             _measureFunction = null;
+            _baselineFunction = null;
             _data = null;
 
             Native.YGNodeReset(_ygNode);
@@ -84,6 +87,14 @@ namespace Facebook.Yoga
             }
         }
 
+        public bool IsBaselineDefined
+        {
+            get
+            {
+                return _baselineFunction != null;
+            }
+        }
+
         public void CopyStyle(YogaNode srcNode)
         {
             Native.YGNodeCopyStyle(_ygNode, srcNode._ygNode);
@@ -585,6 +596,13 @@ namespace Facebook.Yoga
             Native.YGNodeSetMeasureFunc(_ygNode, _ygMeasureFunc);
         }
 
+        public void SetBaselineFunction(BaselineFunction baselineFunction)
+        {
+            _baselineFunction = baselineFunction;
+            _ygBaselineFunc = baselineFunction != null ? BaselineInternal : (YogaBaselineFunc)null;
+            Native.YGNodeSetBaselineFunc(_ygNode, _ygBaselineFunc);
+        }
+
         public void CalculateLayout()
         {
             Native.YGNodeCalculateLayout(
@@ -609,6 +627,16 @@ namespace Facebook.Yoga
             return _measureFunction(this, width, widthMode, height, heightMode);
         }
 
+        private float BaselineInternal(IntPtr node, float width, float height)
+        {
+            if (_baselineFunction == null)
+            {
+                throw new InvalidOperationException("Baseline function is not defined.");
+            }
+
+            return _baselineFunction(this, width, height);
+        }
+
         public string Print(YogaPrintOptions options =
             YogaPrintOptions.Layout|YogaPrintOptions.Style|YogaPrintOptions.Children)
         {