Add GraphNode example for Cxx TMs (#41766)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41766

Changelog: [Internal]

Adds a simple example showing a recursive node, stored inside a collection in a Cxx TM.

Currently we can't auto-generate [the necessary C++ Types](https://reactnative.dev/docs/next/the-new-architecture/cxx-custom-types#struct-generator) - but we can add it later if this scenarios becomes really common.

Reviewed By: rshest

Differential Revision: D51783974

fbshipit-source-id: 7352db1a354cd7da32febc650f7cc5e10dd16d2d

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.h

@@ -90,6 +90,42 @@ struct CustomHostObjectRef {
 
 using CustomHostObject = HostObjectWrapper<CustomHostObjectRef>;
 
+#pragma mark - recursive objects
+struct GraphNode {
+  std::string label;
+  std::optional<std::vector<GraphNode>> neighbors;
+};
+
+template <>
+struct Bridging<GraphNode> {
+  static GraphNode fromJs(
+      jsi::Runtime& rt,
+      const jsi::Object& value,
+      const std::shared_ptr<CallInvoker>& jsInvoker) {
+    GraphNode result{
+        bridging::fromJs<std::string>(
+            rt, value.getProperty(rt, "label"), jsInvoker),
+        bridging::fromJs<std::optional<std::vector<GraphNode>>>(
+            rt, value.getProperty(rt, "neighbors"), jsInvoker)};
+    return result;
+  }
+
+  static jsi::Object toJs(
+      jsi::Runtime& rt,
+      const GraphNode value,
+      const std::shared_ptr<CallInvoker>& jsInvoker) {
+    auto result = facebook::jsi::Object(rt);
+    result.setProperty(rt, "label", bridging::toJs(rt, value.label, jsInvoker));
+    if (value.neighbors) {
+      result.setProperty(
+          rt,
+          "neighbors",
+          bridging::toJs(rt, value.neighbors.value(), jsInvoker));
+    }
+    return result;
+  }
+};
+
 #pragma mark - implementation
 class NativeCxxModuleExample
     : public NativeCxxModuleExampleCxxSpec<NativeCxxModuleExample> {
@@ -120,6 +156,8 @@ class NativeCxxModuleExample
       jsi::Runtime& rt,
       std::shared_ptr<CustomHostObject> arg);
 
+  GraphNode getGraphNode(jsi::Runtime& rt, GraphNode arg);
+
   NativeCxxModuleExampleCxxEnumFloat getNumEnum(
       jsi::Runtime& rt,
       NativeCxxModuleExampleCxxEnumInt arg);