more comments

Author: leotianlizhan

firebase-perf/src/main/java/com/google/firebase/perf/application/FrameMetricsRecorder.java

@@ -108,14 +108,17 @@ public Optional<PerfFrameMetrics> stop() {
     }
     Optional<PerfFrameMetrics> data = this.snapshot();
     try {
+      // No reliable way to check for hardware-acceleration, so we must catch retroactively (#2736).
       frameMetricsAggregator.remove(activity);
     } catch (IllegalArgumentException | NullPointerException ex) {
       // Both of these exceptions result from android.view.View.addFrameMetricsListener silently
       // failing when the view is not hardware-accelerated. Successful addFrameMetricsListener
       // stores an observer in a list, and initializes the list if it was uninitialized. Invoking
       // View.removeFrameMetricsListener(listener) throws IAE if it doesn't exist in the list, or
-      // throws NPE if the list itself was never initialized. This NPE is fixed in API 29.
+      // throws NPE if the list itself was never initialized (#4184).
       if (ex instanceof NullPointerException && Build.VERSION.SDK_INT > Build.VERSION_CODES.P) {
+        // Re-throw above API 28, since the NPE is fixed in API 29:
+        // https://android.googlesource.com/platform/frameworks/base/+/140ff5ea8e2d99edc3fbe63a43239e459334c76b
         throw ex;
       }
       logger.warn(