Allow MeasureFunc to be set to NULL no matter how many children a nod…

…e has.

Summary: Within `UIView+CSSLayout`, we often nil out the measure function of a node because view hierarchy can often change. Unfortunately, this causes us to hit an assert which crashes the app. Instead, lets the measure func to be set to NULL, regardless of how many children a node might have.

Reviewed By: emilsjolander

Differential Revision: D4148727

fbshipit-source-id: 79a0f3ef1bf7b1dce9a14de96f870e35c042b78b

CSSLayout/CSSLayout.c

@@ -259,8 +259,13 @@ static void _CSSNodeMarkDirty(const CSSNodeRef node) {
 }
 
 void CSSNodeSetMeasureFunc(const CSSNodeRef node, CSSMeasureFunc measureFunc) {
-  CSS_ASSERT(CSSNodeChildCount(node) == 0, "Cannot set measure function: Nodes with measure functions cannot have children.");
-  node->measure = measureFunc;
+  // You can always NULLify the measure function of a node.
+  if (measureFunc == NULL) {
+    node->measure = NULL;
+  } else {
+    CSS_ASSERT(CSSNodeChildCount(node) == 0, "Cannot set measure function: Nodes with measure functions cannot have children.");
+    node->measure = measureFunc;
+  }
 }
 
 CSSMeasureFunc CSSNodeGetMeasureFunc(const CSSNodeRef node) {

tests/CSSLayoutMeasureTest.cpp

@@ -28,13 +28,25 @@ TEST(CSSLayoutTest, cannot_add_child_to_node_with_measure_func) {
 
   const CSSNodeRef root_child0 = CSSNodeNew();
   ASSERT_DEATH(CSSNodeInsertChild(root, root_child0, 0), "Cannot add child.*");
+  CSSNodeFreeRecursive(root);
 }
 
-TEST(CSSLayoutTest, cannot_add_measure_func_to_non_leaf_node) {
+TEST(CSSLayoutTest, cannot_add_nonnull_measure_func_to_non_leaf_node) {
   const CSSNodeRef root = CSSNodeNew();
   const CSSNodeRef root_child0 = CSSNodeNew();
   CSSNodeInsertChild(root, root_child0, 0);
 
   ASSERT_DEATH(CSSNodeSetMeasureFunc(root, _measure), "Cannot set measure function.*");
+  CSSNodeFreeRecursive(root);
 }
+
+TEST(CSSLayoutTest, can_nullify_measure_func_on_any_node) {
+  const CSSNodeRef root = CSSNodeNew();
+  CSSNodeInsertChild(root, CSSNodeNew(), 0);
+
+  CSSNodeSetMeasureFunc(root, NULL);
+  ASSERT_TRUE(CSSNodeGetMeasureFunc(root) == NULL);
+  CSSNodeFreeRecursive(root);
+}
+
 #endif