Add BigInt Support to C++ Turbo Module Example (#56017)

Summary:
Pull Request resolved: #56017

This diff adds BigInt method support to the C++ sample Turbo Module (NativeCxxModuleExample),
demonstrating how to use `facebook::react::BigInt` with the bridging layer.

Changes:
- Added getBigInt method to NativeCxxModuleExample.cpp/.h using
  `facebook::react::BigInt` as the C++ type
- Added getBigInt to NativeCxxModuleExample.js spec
- Added BigIntKind to TurboModule.h for the method dispatch infrastructure
- Updated RCTTurboModule.mm to handle BigIntKind in the iOS TurboModule bridge
- Fixed BigInt rendering in NativeCxxModuleExampleExample.js — JSON.stringify()
  cannot serialize BigInt, so bigint values are now rendered via .toString()

The `facebook::react::BigInt` wrapper class (introduced in the bridging diff)
stores values as `std::variant<int64_t, uint64_t>`, preserving signedness.
The `Bridging<BigInt>` specialization handles conversion to/from `jsi::BigInt`.

Example:
```cpp
BigInt NativeCxxModuleExample::getBigInt(jsi::Runtime& rt, BigInt arg) {
  return arg;
}
```

Changelog: [General][Added] - Add BigInt bridging support for Turbo Modules

Differential Revision: D95232265

Author: christophpurrer

packages/react-native/ReactCommon/react/nativemodule/core/ReactCommon/TurboModule.cpp

@@ -21,6 +21,8 @@ TurboModuleMethodValueKind getTurboModuleMethodValueKind(
     return NumberKind;
   } else if (value->isString()) {
     return StringKind;
+  } else if (value->isBigInt()) {
+    return BigIntKind;
   } else if (value->isObject()) {
     auto object = value->asObject(rt);
     if (object.isArray(rt)) {

packages/react-native/ReactCommon/react/nativemodule/core/ReactCommon/TurboModule.h

@@ -26,6 +26,7 @@ enum TurboModuleMethodValueKind {
   VoidKind,
   BooleanKind,
   NumberKind,
+  BigIntKind,
   StringKind,
   ObjectKind,
   ArrayKind,

packages/react-native/ReactCommon/react/nativemodule/core/platform/ios/ReactCommon/RCTTurboModule.mm

@@ -526,6 +526,10 @@ TraceSection s(
       throw jsi::JSError(runtime, "convertReturnIdToJSIValue: FunctionKind is not supported yet.");
     case PromiseKind:
       throw jsi::JSError(runtime, "convertReturnIdToJSIValue: PromiseKind wasn't handled properly.");
+    case BigIntKind: {
+      returnValue = jsi::BigInt::fromInt64(runtime, [(NSNumber *)result longLongValue]);
+      break;
+    }
   }
 
   return returnValue;
@@ -813,7 +817,8 @@ TraceSection s(
     case StringKind:
     case ObjectKind:
     case ArrayKind:
-    case FunctionKind: {
+    case FunctionKind:
+    case BigIntKind: {
       id result = performMethodInvocation(runtime, true, methodName, inv, retainedObjectsForInvocation);
       TurboModulePerfLogger::syncMethodCallReturnConversionStart(moduleName, methodName);
       returnValue = convertReturnIdToJSIValue(runtime, methodName, returnType, result);

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.cpp

@@ -7,6 +7,7 @@
 
 #include "NativeCxxModuleExample.h"
 #include <react/debug/react_native_assert.h>
+#include <cstdint>
 #include <iomanip>
 #include <ostream>
 #include <sstream>
@@ -265,4 +266,8 @@ AsyncPromise<> NativeCxxModuleExample::promiseAssert(jsi::Runtime& rt) {
   return promise;
 };
 
+BigInt NativeCxxModuleExample::getBigInt(jsi::Runtime& /*rt*/, BigInt arg) {
+  return arg;
+}
+
 } // namespace facebook::react

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.h

@@ -182,6 +182,8 @@ class NativeCxxModuleExample : public NativeCxxModuleExampleCxxSpec<NativeCxxMod
 
   AsyncPromise<> promiseAssert(jsi::Runtime &rt);
 
+  BigInt getBigInt(jsi::Runtime &rt, BigInt arg);
+
  private:
   std::optional<AsyncCallback<std::string>> valueCallback_;
 };

packages/rn-tester/NativeCxxModuleExample/NativeCxxModuleExample.js

@@ -114,6 +114,7 @@ export interface Spec extends TurboModule {
   +voidFuncAssert: () => void;
   +getObjectAssert: (arg: ObjectStruct) => ObjectStruct;
   +promiseAssert: () => Promise<void>;
+  +getBigInt: (arg: bigint) => bigint;
 }
 
 export default TurboModuleRegistry.get<Spec>(

packages/rn-tester/js/examples/TurboModule/NativeCxxModuleExampleExample.js

@@ -50,6 +50,7 @@ type Examples =
   | 'getStrEnum'
   | 'getMap'
   | 'getNumber'
+  | 'getBigInt'
   | 'getObject'
   | 'getSet'
   | 'getString'
@@ -122,7 +123,10 @@ class NativeCxxModuleExampleExample extends React.Component<{}, State> {
       }),
     getNumEnum: () => NativeCxxModuleExample?.getNumEnum(EnumInt.IB),
     getStrEnum: () => NativeCxxModuleExample?.getStrEnum(EnumNone.NB),
+    getMap: () => NativeCxxModuleExample?.getMap({one: 1, two: 2, three: null}),
     getNumber: () => NativeCxxModuleExample?.getNumber(99.95),
+    getBigInt: () =>
+      NativeCxxModuleExample?.getBigInt(BigInt('9223372036854775807')),
     getObject: () =>
       NativeCxxModuleExample?.getObject({a: 1, b: 'foo', c: null}),
     getSet: () => NativeCxxModuleExample?.getSet([1, 1.1, 1.1, 1.1, 2]),
@@ -259,7 +263,9 @@ class NativeCxxModuleExampleExample extends React.Component<{}, State> {
     return (
       <View style={styles.result}>
         <RNTesterText style={[styles.value]}>
-          {JSON.stringify(result.value)}
+          {typeof result.value === 'bigint'
+            ? result.value.toString()
+            : JSON.stringify(result.value)}
         </RNTesterText>
         <RNTesterText style={[styles.type]}>{result.type}</RNTesterText>
       </View>