Fabric: `RNWrapManagedObject` and `RNUnwrapManagedObject` helpers

Summary: We use this pattern already and seems we use it more. Those two functions introduce a "semantical" wrappers for this context, so now there is no need to think which exact `__bridge ***` qualifier we should use, so it's much less error-prone.

Reviewed By: JoshuaGross

Differential Revision: D14896800

fbshipit-source-id: 85b86bfcefdad5aff0375e7172769df86c001506

React/Fabric/RCTConversions.h

@@ -16,6 +16,27 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+/*
+ * `RNWrapManagedObject` and `RNUnwrapManagedObject` are wrapper functions that convert ARC-managed objects into
+ * `std::shared_ptr<void>` and vice-versa. It's a very useful mechanism when we need to pass Objective-C objects through
+ * pure C++ code, pass blocks into C++ lambdas, and so on.
+ *
+ * The idea behind this mechanism is quite simple but tricky: When we instantiate a C++ shared pointer for a managed
+ * object, we practically call `CFRetain` for it once and then we represent this single retaining operation as a counter
+ * inside the shared pointer; when the counter became zero, we call `CFRelease` on the object. In this model, one bump
+ * of ARC-managed counter is represented as multiple bumps of C++ counter, so we can have multiple counters for the same
+ * object that form some kind of counters tree.
+ */
+inline std::shared_ptr<void> RNWrapManagedObject(id object)
+{
+  return std::shared_ptr<void>((__bridge_retained void *)object, CFRelease);
+}
+
+inline id RNUnwrapManagedObject(std::shared_ptr<void> const &object)
+{
+  return (__bridge id)object.get();
+}
+
 inline NSString *RCTNSStringFromString(const std::string &string, const NSStringEncoding &encoding = NSUTF8StringEncoding) {
   return [NSString stringWithCString:string.c_str() encoding:encoding];
 }