Reaction if the file exists

I am writing the R code in R shiny to split a file. If program detects there is a excel file in the folder, it will split the file into two separate excel files automatically.

I want the function to check the folder non-stop until it finds a file.

I am now using "observe" and "invalidateLater". However, if there is no file in the folder, it will appear error message. Also, even if the program finds a file in the folder, the program will keep running and do the split function without stopping.