[CMake] CPACK_NSIS_EXTRA_PREINSTALL_COMMAND Not Working as Expected

[Danoush]

Hello,

I have been trying to use the CPACK_NSIS_EXTRA_PREINSTALL_COMMANDS 
variable to uninstall a package before a new installation. However I get 
some unexpected results. The message box and uninstaller in the 
following command are launched after all of the files in the package 
have been installed to the target system. From the documentation of 
CPACK_NSIS_EXTRA_PREINSTALL_COMMANDS, I was expecting the commands below 
to get executed before the files are installed on the target.

SET( CPACK_NSIS_EXTRA_PREINSTALL_COMMANDS "
         MessageBox MB_OK 'Starting preinstall command'
         ReadRegStr $0 HKLM 
\\\"Software\\\\Microsoft\\\\Windows\\\\CurrentVersion\\\\Uninstall\\\\${CPACK_PACKAGE_NAME}\\\" 
\\\"UninstallString\\\"
         ExecWait '$0 _?=$INSTDIR'
         ")

I would appreciate any insights.

Thanks and Regards,
Dan