androidx.metrics module used: metrics-performance androidx.metrics version used: 1.0.0-alpha01

I was reading the sources for androidx.metrics.performance.DelegatingOnPreDrawListener#onPreDraw and noticed what seems like a potential bug pre API 24 (or at list a limitation):

androidx.metrics.performance.DelegatingOnPreDrawListener#onPreDraw posts an async message (great) but not at the front.

Pre API 24, the “end” time of a frame is approximated by posting an async message from onPreDraw.

This means there can be messages that were already posted and will execute before it runs. MessageQueue only looks for async messages when there’s a sync barrier, otherwise, they keep their posted order. So if there’s no sync barrier then any already enqueued message will execute right after the frame but before the message posted from onPreDraw runs, which means the frame end time will be off. If there is a sync barrier, this still applies if there are other async messages already posted.