Although this project is much larger and with longer code, it is actually pretty similar to the TCP networking projects we have done in the previous recipes. We have also made use of the signals and slots mechanism provided by Qt 5 to make our life easier. 

In the past, Qt used to support FTP directly through the QFtp class. However, the QFtp class has since been deprecated in the newer versions and so we have to implement our own by using networking classes such as QNetworkRequest, QNetworkReply, and so on.

Since we no longer have the QFtp class, there is no way to acquire the file list directly from FileZilla Server. Therefore, we develop a workaround to solve this problem, which is to use a text file for storing the file list.

The progress bar is very important for this project to show the upload and download progresses. This is especially useful for uploading or downloading large files.