fix(react-native-host): notify when the runtime is ready (#3604)

Author: tido64

.changeset/calm-dingos-swim.md

@@ -0,0 +1,5 @@
+---
+"@rnx-kit/react-native-host": patch
+---
+
+Post a notification when the React runtime is ready

packages/react-native-host/cocoa/RNXTurboModuleAdapter.mm

@@ -53,12 +53,26 @@
 @implementation RNXTurboModuleAdapter {
 #if USE_FABRIC
     RCTTurboModuleManager *_turboModuleManager;
+    std::weak_ptr<facebook::react::CallInvoker> _jsInvoker;
 #endif  // USE_FABRIC
 #if USE_RUNTIME_SCHEDULER
     std::shared_ptr<facebook::react::RuntimeScheduler> _runtimeScheduler;
 #endif  // USE_RUNTIME_SCHEDULER
 }
 
+#if USE_FABRIC
+- (instancetype)init
+{
+    if (self = [super init]) {
+        [NSNotificationCenter.defaultCenter addObserver:self
+                                               selector:@selector(onRuntimeReady:)
+                                                   name:@"RCTInstanceDidLoadBundle"
+                                                 object:nil];
+    }
+    return self;
+}
+#endif  // USE_FABRIC
+
 - (std::unique_ptr<facebook::react::JSExecutorFactory>)jsExecutorFactoryForBridge:
     (RCTBridge *)bridge
 {
@@ -90,6 +104,7 @@ - (Class)getModuleClassFromName:(char const *)name
          jsInvoker:(std::shared_ptr<facebook::react::CallInvoker>)jsInvoker
 {
 #if __has_include(<react/nativemodule/defaults/DefaultTurboModules.h>)  // >= 0.75
+    _jsInvoker = jsInvoker;
     return facebook::react::DefaultTurboModules::getTurboModule(name, jsInvoker);
 #else
     return nullptr;
@@ -127,6 +142,18 @@ - (Class)getModuleClassFromName:(char const *)name
 #endif  // USE_RUNTIME_SCHEDULER
 }
 
+- (void)onRuntimeReady:(NSNotification *)note
+{
+    if (auto jsInvoker = _jsInvoker.lock()) {
+        jsInvoker->invokeAsync([](facebook::jsi::Runtime &runtime) {
+            NSDictionary *userInfo = @{@"runtime": [NSValue valueWithPointer:&runtime]};
+            [NSNotificationCenter.defaultCenter postNotificationName:@"ReactAppRuntimeReady"
+                                                              object:nil
+                                                            userInfo:userInfo];
+        });
+    }
+}
+
 #endif  // USE_FABRIC
 
 @end