Android Question Upload Progress in notification area even after closing the app

Hi,
I have seen an app (frankly, its readwhere ) which downloads the file in background even when the app is not activated. Also once you open the app, it shows the progress of download in the app. If you are initiating a download from the app, it continues showing the download progress in notification bar even after the app is closed using back button. How this continuous download behavior can be achieved in B4A?