Run background function in MainWindow while modal Dialog is open

Run background function in MainWindow while modal Dialog is open

In my project, I have a "save as" button. However, the saving process for
what I have to do takes a minute or so. I would like to open a modal
dialog with a progress bar and some text displaying what is going on.
Easy enough, but how can I send "updates" from the main window to the
modal dialog. Basically, run both windows at once, where all the saving is
being done from the main window, but the progress is being shown in the
dialog. I do not want the dialog to be modeless, since I don't want the
user to interact with the main window during saving. I also don't want the
saving process to take place from the dialog.
Currently, my program just saves everything from the main window and no
dialog is shown.
Thanks for your time. :)