introduce RCTMockDef

Summary:
here's a way we can mock C apis - however i am not sure if the flag i'm using is correct

used in D31949237

Changelog:
[General][Added] - add macros to be able to stub C functions in tests

Reviewed By: RSNara

Differential Revision: D31949238

fbshipit-source-id: 0f18a65f810f1b855dbc844f11f5a304c1e5ecea

BUCK

@@ -253,6 +253,7 @@ REACT_PUBLIC_HEADERS = {
     "React/RCTLayoutAnimationGroup.h": RCTMODULES_PATH + "RCTLayoutAnimationGroup.h",
     "React/RCTLog.h": RCTBASE_PATH + "RCTLog.h",
     "React/RCTManagedPointer.h": RCTBASE_PATH + "RCTManagedPointer.h",
+    "React/RCTMockDef.h": RCTBASE_PATH + "RCTMockDef.h",
     "React/RCTModalHostViewController.h": RCTVIEWS_PATH + "RCTModalHostViewController.h",
     "React/RCTModalHostViewManager.h": RCTVIEWS_PATH + "RCTModalHostViewManager.h",
     "React/RCTModalManager.h": RCTVIEWS_PATH + "RCTModalManager.h",

React/Base/RCTMockDef.h

@@ -0,0 +1,58 @@
+/*
+ * 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.
+ */
+
+#import <React/RCTDefines.h>
+
+/* These macros are used to stub C functions. Here's an example:
+ *
+ * Helpers.h
+ * ------
+ * boolean ReturnsTrueOrFalse(void);
+ *
+ * FileToBeTested.h
+ * ------
+ * RCT_MOCK_DEF(Testing, ReturnsTrueOrFalse);
+ * #define ReturnsTrueOrFalse RCT_MOCK_USE(Testing, ReturnsTrueOrFalse)
+ *
+ * int FunctionToBeTested(int input) {
+ *   return ReturnsTrueOrFalse() ? input + 1 : input - 1;
+ * }
+ *
+ * Test.h
+ * -----
+ * RCT_MOCK_GET(Testing, ReturnsTrueOrFalse);
+ *
+ * boolean _ReturnsTrue(void) { return true; }
+ * boolean _ReturnsFalse(void) { return false; }
+ *
+ * void TestFunctionTrue(void) {
+ *   RCT_MOCK_SET(Testing, ReturnsTrueOrFalse, _ReturnsTrue);
+ *   assert(FunctionToBeTested(5) == 6);
+ *   RCT_MOCK_RESET(Testing, ReturnsTrueOrFalse);
+ * }
+ *
+ * void TestFunctionFalse(void) {
+ *   RCT_MOCK_SET(Testing, ReturnsTrueOrFalse, _ReturnsFalse);
+ *   assert(FunctionToBeTested(5) == 4);
+ *   RCT_MOCK_RESET(Testing, ReturnsTrueOrFalse);
+ * }
+ *
+ */
+
+#ifdef RCT_DEV
+ #define RCT_MOCK_DEF(context, api) __typeof(__typeof(api) *) mockptr_ ## context ## _ ## api = &api;
+ #define RCT_MOCK_REF(context, api) extern __typeof(__typeof(api) *) mockptr_ ## context ## _ ## api;
+ #define RCT_MOCK_SET(context, api, mockapi) (mockptr_ ## context ## _ ## api = &mockapi)
+ #define RCT_MOCK_RESET(context, api) (mockptr_ ## context ## _ ## api = &api)
+ #define RCT_MOCK_USE(context, api) (*mockptr_ ## context ## _ ## api)
+#else
+ #define RCT_MOCK_DEF(context, api)
+ #define RCT_MOCK_REF(context, api)
+ #define RCT_MOCK_SET(context, api, mockapi)
+ #define RCT_MOCK_RESET(context, api)
+ #define RCT_MOCK_USE(context, api) api
+#endif