Move out YGValue

Summary:
@public

Creates a single header file for `YGValue`. This is in preparation of a more compact representation of `YGValue` within `YGStyle`.

Also fixes the incorrect definition of NAN.

Reviewed By: SidharthGuglani

Differential Revision: D13172444

fbshipit-source-id: 4250dbcf8fe15ec3ecdee3913360a73bab633ce3

yoga/YGValue.h

@@ -0,0 +1,40 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the LICENSE
+ * file in the root directory of this source tree.
+ */
+#pragma once
+
+#include <math.h>
+#include "YGEnums.h"
+#include "YGMacros.h"
+
+YG_EXTERN_C_BEGIN
+
+// Not defined in MSVC++
+#ifndef NAN
+static const uint32_t __nan = 0x7fc00000;
+#define NAN (*(const float*)__nan)
+#endif
+
+#define YGUndefined NAN
+
+typedef struct YGValue {
+  float value;
+  YGUnit unit;
+} YGValue;
+
+extern const YGValue YGValueAuto;
+extern const YGValue YGValueUndefined;
+extern const YGValue YGValueZero;
+
+YG_EXTERN_C_END
+
+#ifdef __cplusplus
+
+bool operator==(const YGValue& lhs, const YGValue& rhs);
+
+bool operator!=(const YGValue& lhs, const YGValue& rhs);
+
+#endif