Add isDirty property and make markDirty less crashy/more performant.

Summary: For some internal Instagram diffs, and some external ones (auto-detection of dirty nodes). We need to give the dirty properties a little love. This adds a getter to find out if your node is dirty, and makes sure that the leaf node is properly setup before it is marked dirty to prevent a crash.

Reviewed By: emilsjolander

Differential Revision: D4549241

fbshipit-source-id: 36eda6fdb4ea7f968d126aab6da67896f4a01d40

YogaKit/Source/YGLayout.m

@@ -120,11 +120,26 @@ YG_VALUE_EDGE_PROPERTY(lowercased_name, capitalized_name, capitalized_name, YGEd
   YGNodeFree(self.node);
 }
 
+- (BOOL)isDirty
+{
+  return YGNodeIsDirty(self.node);
+}
+
 - (void)markDirty
 {
-  if (self.isLeaf) {
-    YGNodeMarkDirty(self.node);
+  if (self.isDirty || !self.isLeaf) {
+    return;
   }
+
+  // Yoga is not happy if we try to mark a node as "dirty" before we have set
+  // the measure function. Since we already know that this is a leaf,
+  // this *should* be fine. Forgive me Hack Gods.
+  const YGNodeRef node = self.node;
+  if (YGNodeGetMeasureFunc(node) == NULL) {
+    YGNodeSetMeasureFunc(node, YGMeasureView);
+  }
+
+  YGNodeMarkDirty(node);
 }
 
 - (NSUInteger)numberOfChildren