Macro that save the file base on cell value

Hi,

I have this macro which works but I like to extend it to if the file name already exits then it show the message to user and did not do anything, also if the cell a1 and b1 are empty it ask user to fill this out before it run.

here is my code

Public Sub SaveAsA1()
ThisFile = Range("A1").Value
ThisPath = Range("B1").Value

ActiveWorkbook.SaveAs Filename:=ThisPath & ThisFile
End Sub

Thanks,
RR