any marker or method can indicated the binlog sync has stared？

[lonely-root-number2]

story：
I want to cache some data from the database in the application. I need to wait until binlog synchronization has started before starting pulling data from the database so that updates are not lost. (If synchronization has not started when pulling data, some updates will be lost).
I noticed that there was a variable called running in syncer, but it was not exposed to the public. Judging by synedPosition doesn't seem very accurate.