Avoid closure allocation in RCTDeviceEventEmitter.emit when tracing is disabled

packages/react-native/Libraries/EventEmitter/RCTDeviceEventEmitter.js

@@ -10,7 +10,7 @@
 
 import type {IEventEmitter} from '../vendor/emitter/EventEmitter';
 
-import {trace} from '../Performance/Systrace';
+import {beginEvent, endEvent, isEnabled} from '../Performance/Systrace';
 import EventEmitter from '../vendor/emitter/EventEmitter';
 
 // FIXME: use typed events
@@ -24,17 +24,24 @@ type RCTDeviceEventDefinitions = {[name: string]: Array<any>};
  * NativeModules that emit events should instead subclass `NativeEventEmitter`.
  */
 class RCTDeviceEventEmitterImpl extends EventEmitter<RCTDeviceEventDefinitions> {
-  // Add systrace to RCTDeviceEventEmitter.emit method for debugging
+  // Add systrace to RCTDeviceEventEmitter.emit method for debugging.
+  // `beginEvent`/`endEvent` are used instead of `trace` so that no closures
+  // are allocated when tracing is disabled (the common case); `trace` always
+  // allocates a callback, which is costly on this hot path.
   emit<TEvent extends keyof RCTDeviceEventDefinitions>(
     eventType: TEvent,
     ...args: RCTDeviceEventDefinitions[TEvent]
   ): void {
-    trace(
-      () => `RCTDeviceEventEmitter.emit#${eventType}`,
-      () => {
+    if (isEnabled()) {
+      beginEvent(`RCTDeviceEventEmitter.emit#${eventType}`);
+      try {
         super.emit(eventType, ...args);
-      },
-    );
+      } finally {
+        endEvent();
+      }
+    } else {
+      super.emit(eventType, ...args);
+    }
   }
 }
 const RCTDeviceEventEmitter: IEventEmitter<RCTDeviceEventDefinitions> =

packages/react-native/Libraries/EventEmitter/__tests__/RCTDeviceEventEmitter-itest.js

@@ -0,0 +1,99 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
+
+import {DeviceEventEmitter} from 'react-native';
+
+const TRACE_TAG_REACT = 1 << 13; // eslint-disable-line no-bitwise
+
+function enableTracing() {
+  global.nativeTraceIsTracing = jest.fn(() => true);
+  global.nativeTraceBeginSection = jest.fn();
+  global.nativeTraceEndSection = jest.fn();
+}
+
+function disableTracing() {
+  delete global.nativeTraceIsTracing;
+  delete global.nativeTraceBeginSection;
+  delete global.nativeTraceEndSection;
+  delete global.__RCTProfileIsProfiling;
+}
+
+describe('DeviceEventEmitter', () => {
+  afterEach(() => {
+    DeviceEventEmitter.removeAllListeners();
+    disableTracing();
+  });
+
+  it('forwards events and arguments to listeners', () => {
+    const listener = jest.fn();
+    DeviceEventEmitter.addListener('event', listener);
+
+    DeviceEventEmitter.emit('event', 'one', 2);
+
+    expect(listener).toHaveBeenCalledTimes(1);
+    expect(listener).toHaveBeenCalledWith('one', 2);
+  });
+
+  it('does not call trace sections when tracing is disabled', () => {
+    const listener = jest.fn();
+    DeviceEventEmitter.addListener('event', listener);
+
+    DeviceEventEmitter.emit('event');
+
+    expect(listener).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceBeginSection).toBeUndefined();
+  });
+
+  it('wraps emit in a trace section when tracing is enabled', () => {
+    enableTracing();
+    const listener = jest.fn();
+    DeviceEventEmitter.addListener('event', listener);
+
+    DeviceEventEmitter.emit('event');
+
+    expect(global.nativeTraceBeginSection).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceBeginSection).toHaveBeenCalledWith(
+      TRACE_TAG_REACT,
+      'RCTDeviceEventEmitter.emit#event',
+      undefined,
+    );
+    expect(listener).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceEndSection).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceEndSection).toHaveBeenCalledWith(
+      TRACE_TAG_REACT,
+      undefined,
+    );
+  });
+
+  it('ends the trace section even when a listener throws', () => {
+    enableTracing();
+    DeviceEventEmitter.addListener('event', () => {
+      throw new Error('boom');
+    });
+
+    expect(() => DeviceEventEmitter.emit('event')).toThrow('boom');
+
+    expect(global.nativeTraceBeginSection).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceEndSection).toHaveBeenCalledTimes(1);
+  });
+
+  it('traces when __RCTProfileIsProfiling is set and nativeTraceIsTracing is absent', () => {
+    global.__RCTProfileIsProfiling = true;
+    global.nativeTraceBeginSection = jest.fn();
+    global.nativeTraceEndSection = jest.fn();
+
+    DeviceEventEmitter.emit('event');
+
+    expect(global.nativeTraceBeginSection).toHaveBeenCalledTimes(1);
+    expect(global.nativeTraceEndSection).toHaveBeenCalledTimes(1);
+  });
+});