Cannot hide dialog until signal handler exits

[Pseudo Nym <pseudo nym ukgateway net>]

The signal handler for a button in my main window launches a file 
chooser dialog, then processes the selected files.
I try to hide the dialog during processing with 'gtk_widget_hide 
(dialog);' but this does not take effect until the signal handler exits. 
Is there a way to hide the dialog immediately to show the progress bar 
in the underlying window?

Thanks

Kevin