Summary:
We'll be using a native CallInvoker to dispatch sync and async method calls to ObjC NativeModules. This native CallInvoker will hold a reference to the ObjC NativeModule's method queue.
**Why is the native CallInvoker required for ObjC NativeModules?**
In the case where the ObjC NativeModule neither provides nor requests a method queue, we must create a method queue for it. When we go to invoke a method from JS, for these NativeModules specifically, there is no way to access this method queue. A native CallInvoker is a convenient abstraction that holds on to that method queue. For async calls, we'll just call `CallInvoker::invokeAsync`, and for sync calls, we'll just call `CallInvoker::invokeSync`.
**Why do we need sync call support for native `CallInvoker`?**
In ObjC, sync NativeModule method calls block the JS thread, then execute synchronously on the NativeModule's method queue, and then unblock the JS thread. This is what'll be implemented by `CallInvoker::invokeSync`.
Changelog: [Internal]
Reviewed By: fkgozali
Differential Revision: D20829955
fbshipit-source-id: efb9d5408a1ade81069a943c865f232d4d10acfe
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