fix: separate measurement lifecycle from polling interval

Changing the polling interval no longer restarts the measurement
and discards accumulated stats. The measurement lifecycle is now
keyed only on session/enabled, while the polling interval is
managed in a separate effect.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: msilivonik-sc

src/useLensFrameMetrics.test.ts

@@ -221,6 +221,30 @@ describe("useLensFrameMetrics", () => {
     expect(session.metrics.beginMeasurement).toHaveBeenCalledTimes(1);
   });
 
+  it("does not restart measurement when interval changes", () => {
+    const { measurement, session } = setupContext();
+
+    const { rerender } = renderHook(({ interval }) => useLensFrameMetrics({ interval }), {
+      initialProps: { interval: 500 },
+    });
+
+    expect(session.metrics.beginMeasurement).toHaveBeenCalledTimes(1);
+    measurement.end.mockClear();
+
+    rerender({ interval: 1000 });
+
+    // Should NOT end/restart the measurement
+    expect(measurement.end).not.toHaveBeenCalled();
+    expect(session.metrics.beginMeasurement).toHaveBeenCalledTimes(1);
+
+    // But should poll at the new interval
+    act(() => {
+      jest.advanceTimersByTime(1000);
+    });
+
+    expect(measurement.measure).toHaveBeenCalled();
+  });
+
   it("clears metrics state when session is lost", () => {
     const { measurement } = setupContext();
     measurement.measure.mockReturnValue(createMockMetrics({ avgFps: 60 }));

src/useLensFrameMetrics.ts

@@ -31,13 +31,13 @@ interface UseLensFrameMetricsOptions {
  * ```
  */
 export function useLensFrameMetrics(options: UseLensFrameMetricsOptions): ComputedFrameMetrics | undefined {
-  const { enabled = true } = options;
+  const { enabled = true, interval } = options;
   const { currentSession, lens } = useInternalCameraKit();
   const [metrics, setMetrics] = useState<ComputedFrameMetrics | undefined>(undefined);
-  const measurementRef = useRef<LensPerformanceMeasurement | null>(null);
+  const measurementRef = useRef<LensPerformanceMeasurement | undefined>(undefined);
   const isFirstLensRender = useRef(true);
 
-  // Begin/end measurement and set up polling interval
+  // Begin/end measurement lifecycle
   useEffect(() => {
     if (!currentSession || !enabled) {
       setMetrics(undefined);
@@ -47,17 +47,24 @@ export function useLensFrameMetrics(options: UseLensFrameMetricsOptions): Comput
     const measurement = currentSession.metrics.beginMeasurement();
     measurementRef.current = measurement;
 
-    const intervalId = setInterval(() => {
-      setMetrics(measurement.measure());
-    }, options.interval);
-
     return () => {
-      clearInterval(intervalId);
       measurement.end();
-      measurementRef.current = null;
+      measurementRef.current = undefined;
       isFirstLensRender.current = true;
     };
-  }, [currentSession, options.interval, enabled]);
+  }, [currentSession, enabled]);
+
+  // Poll measurement on interval (separate so changing interval doesn't restart measurement)
+  useEffect(() => {
+    if (!currentSession || !enabled) return;
+
+    const intervalId = setInterval(() => {
+      const result = measurementRef.current?.measure();
+      if (result) setMetrics(result);
+    }, interval);
+
+    return () => clearInterval(intervalId);
+  }, [currentSession, enabled, interval]);
 
   // Reset measurement when lens changes (skip on initial mount)
   useEffect(() => {