Cxx TurboModules > Add example to return a JS function from Cxx to JS (#41385)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41385

Changelog: Internal

Adding a Cxx TM example which adds a listener and returns a subscription to remove that listener from the TM.

You should be able to use this with React Hooks - https://legacy.reactjs.org/docs/hooks-reference.html

E.g.

```
useEffect(() => {
  const subscription =  NativeCxxModuleExample.setValueCallbackWithSubscription(
          callbackValue => // use it
        );
  return subscription;
});
```

Reviewed By: shwanton

Differential Revision: D50473063

fbshipit-source-id: 4e9b92aeccff1771eb4ffad6bdaa20ba7f18435f

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.cpp

@@ -20,6 +20,18 @@ void NativeCxxModuleExample::getValueWithCallback(
   callback({"value from callback!"});
 }
 
+std::function<void()> NativeCxxModuleExample::setValueCallbackWithSubscription(
+    jsi::Runtime& rt,
+    AsyncCallback<std::string> callback) {
+  valueCallback_ = std::make_optional(callback);
+  return [&]() {
+    if (valueCallback_.has_value()) {
+      valueCallback_.value()({"value from callback on clean up!"});
+      valueCallback_ = std::nullopt;
+    }
+  };
+}
+
 std::vector<std::optional<ObjectStruct>> NativeCxxModuleExample::getArray(
     jsi::Runtime& rt,
     std::vector<std::optional<ObjectStruct>> arg) {