feat(tracing): Add extend app start API for standalone app start

CHANGELOG.md

@@ -6,6 +6,12 @@
 > make sure you follow our [migration guide](https://docs.sentry.io/platforms/react-native/migration/) first.
 <!-- prettier-ignore-end -->
 
+## Unreleased
+
+### Features
+
+- Add experimental `extendAppStart`/`finishExtendedAppStart`/`getExtendedAppStartSpan` to extend the standalone app start window and instrument post-init work ([#6392](https://github.com/getsentry/sentry-react-native/pull/6392))
+
 ## 8.17.0
 
 ### Features

packages/core/etc/sentry-react-native.api.md

@@ -305,6 +305,9 @@ export const expoRouterIntegration: (options?: ExpoRouterIntegrationOptions) =>
 // @public
 export const expoUpdatesListenerIntegration: () => Integration;
 
+// @public
+export function extendAppStart(): void;
+
 export { extraErrorDataIntegration }
 
 export { FeatureFlagsIntegration }
@@ -353,6 +356,9 @@ export const feedbackIntegration: (initOptions?: Partial<FeedbackFormProps> & {
     enableShakeToReport?: boolean;
 }) => FeedbackIntegration;
 
+// @public
+export function finishExtendedAppStart(): Promise<void>;
+
 // @public
 export function flush(): Promise<boolean>;
 
@@ -384,6 +390,9 @@ export function getDataFromUri(uri: string): Promise<Uint8Array | null>;
 // @public
 export function getDefaultIdleNavigationSpanOptions(): StartSpanOptions;
 
+// @public
+export function getExtendedAppStartSpan(): Span;
+
 export { getGlobalScope }
 
 export { getIsolationScope }

packages/core/src/js/index.ts

@@ -109,6 +109,9 @@ export {
   withScope,
   crashedLastRun,
   appLoaded,
+  extendAppStart,
+  getExtendedAppStartSpan,
+  finishExtendedAppStart,
   pauseAppHangTracking,
   resumeAppHangTracking,
 } from './sdk';

packages/core/src/js/sdk.tsx

@@ -1,5 +1,5 @@
 /* oxlint-disable eslint(complexity) */
-import type { Breadcrumb, BreadcrumbHint, Integration, Scope } from '@sentry/core';
+import type { Breadcrumb, BreadcrumbHint, Integration, Scope, Span } from '@sentry/core';
 
 import {
   debug,
@@ -25,7 +25,12 @@ import { shouldEnableNativeNagger } from './options';
 import { enableSyncToNative } from './scopeSync';
 import { TouchEventBoundary } from './touchevents';
 import { ReactNativeProfiler } from './tracing';
-import { _appLoaded } from './tracing/integrations/appStart';
+import {
+  _appLoaded,
+  _extendAppStart,
+  _finishExtendedAppStart,
+  _getExtendedAppStartSpan,
+} from './tracing/integrations/appStart';
 import { useEncodePolyfill } from './transports/encodePolyfill';
 import { DEFAULT_BUFFER_SIZE, makeNativeTransportFactory } from './transports/native';
 import { getDefaultEnvironment, isExpoGo, isRunningInMetroDevServer, isWeb } from './utils/environment';
@@ -250,6 +255,64 @@ export function appLoaded(): void {
   _appLoaded();
 }
 
+/**
+ * Extends the app start window so work done after initialization (remote config, session restore,
+ * splash screen dismissal, etc.) is included in the app start measurement. Call
+ * {@link finishExtendedAppStart} when the app is ready, or attach child spans via
+ * {@link getExtendedAppStartSpan} to break the extended work down.
+ *
+ * Requires standalone app start tracing (`_experiments.enableStandaloneAppStartTracing`). No-ops if
+ * the app start transaction was already created, if extend was already called, or if called before
+ * `Sentry.init()`.
+ *
+ * @experimental This API is subject to change in future versions.
+ *
+ * @example
+ * ```ts
+ * Sentry.extendAppStart();
+ * await initializeRemoteConfig();
+ * Sentry.finishExtendedAppStart();
+ * ```
+ */
+export function extendAppStart(): void {
+  _extendAppStart();
+}
+
+/**
+ * Returns the extended app start span for attaching child spans, or a no-op span when there is no
+ * active extension. Only meaningful between {@link extendAppStart} and {@link finishExtendedAppStart}.
+ *
+ * @experimental This API is subject to change in future versions.
+ *
+ * @example
+ * ```ts
+ * Sentry.extendAppStart();
+ * const parentSpan = Sentry.getExtendedAppStartSpan();
+ * const child = Sentry.startInactiveSpan({ parentSpan, op: 'app.init', name: 'fetch remote config' });
+ * await loadRemoteConfig();
+ * child.end();
+ * Sentry.finishExtendedAppStart();
+ * ```
+ */
+export function getExtendedAppStartSpan(): Span {
+  return _getExtendedAppStartSpan();
+}
+
+/**
+ * Finishes the app start extension started with {@link extendAppStart}, finalizing the app start
+ * transaction (its duration is trimmed to the last child span). No-ops if there is no active
+ * extension.
+ *
+ * Returns a promise that resolves once the app start transaction has been captured. `await` it
+ * before {@link flush} (e.g. before a code-push/expo update) to make sure the app start data is
+ * queued.
+ *
+ * @experimental This API is subject to change in future versions.
+ */
+export function finishExtendedAppStart(): Promise<void> {
+  return _finishExtendedAppStart();
+}
+
 /**
  * Flushes all pending events in the queue to disk.
  * Use this before applying any realtime updates such as code-push or expo updates.