Add native function to determine if systrace is enabled

Summary:
We're currently using `global.__RCTProfileIsProfiling` to determine if the app is currently profiling to forward calls to Systrace to native.

We're defining `global.__RCTProfileIsProfiling` when the React Native runtime is initialized (which, in most cases, is on startup or shortly after startup), which means that enabling profiling in the middle of the session doesn't set this value to `true`.

This diff adds a new method (in the global scope for now, as the rest of bindings for Systrace) to check if we're profiling in real time. This call should be very cheap as it's implemented as a JSI function that calls into a C++ API directly.

Changelog: [internal]

Reviewed By: rshest

Differential Revision: D40095840

fbshipit-source-id: e7030d6370ecb43d56478b03cb63806ffa0cd0cf

Libraries/Performance/Systrace.js

@@ -12,7 +12,6 @@ import * as Systrace from './Systrace';
 
 const TRACE_TAG_REACT_APPS = 1 << 17; // eslint-disable-line no-bitwise
 
-let _enabled = false;
 let _asyncCookie = 0;
 
 type EventName = string | (() => string);
@@ -32,7 +31,9 @@ type EventArgs = ?{[string]: string};
  * }
  */
 export function isEnabled(): boolean {
-  return _enabled;
+  return global.nativeTraceIsTracing
+    ? global.nativeTraceIsTracing(TRACE_TAG_REACT_APPS)
+    : Boolean(global.__RCTProfileIsProfiling);
 }
 
 /**
@@ -48,7 +49,7 @@ export function setEnabled(_doEnable: boolean): void {}
  * frame. The end of this event should be marked using the `endEvent` function.
  */
 export function beginEvent(eventName: EventName, args?: EventArgs): void {
-  if (_enabled) {
+  if (isEnabled()) {
     const eventNameString =
       typeof eventName === 'function' ? eventName() : eventName;
     global.nativeTraceBeginSection(TRACE_TAG_REACT_APPS, eventNameString, args);
@@ -59,7 +60,7 @@ export function beginEvent(eventName: EventName, args?: EventArgs): void {
  * Marks the end of a synchronous event started in the same stack frame.
  */
 export function endEvent(args?: EventArgs): void {
-  if (_enabled) {
+  if (isEnabled()) {
     global.nativeTraceEndSection(TRACE_TAG_REACT_APPS, args);
   }
 }
@@ -74,7 +75,7 @@ export function beginAsyncEvent(
   args?: EventArgs,
 ): number {
   const cookie = _asyncCookie;
-  if (_enabled) {
+  if (isEnabled()) {
     _asyncCookie++;
     const eventNameString =
       typeof eventName === 'function' ? eventName() : eventName;
@@ -97,7 +98,7 @@ export function endAsyncEvent(
   cookie: number,
   args?: EventArgs,
 ): void {
-  if (_enabled) {
+  if (isEnabled()) {
     const eventNameString =
       typeof eventName === 'function' ? eventName() : eventName;
     global.nativeTraceEndAsyncSection(
@@ -113,7 +114,7 @@ export function endAsyncEvent(
  * Registers a new value for a counter event.
  */
 export function counterEvent(eventName: EventName, value: number): void {
-  if (_enabled) {
+  if (isEnabled()) {
     const eventNameString =
       typeof eventName === 'function' ? eventName() : eventName;
     global.nativeTraceCounter &&
@@ -126,7 +127,3 @@ if (__DEV__) {
   // Ensure that `Systrace` is available in polyfill by exposing it globally.
   global[(global.__METRO_GLOBAL_PREFIX__ || '') + '__SYSTRACE'] = Systrace;
 }
-
-if (global.__RCTProfileIsProfiling) {
-  _enabled = true;
-}