Synchronous single file download - is it the right approach in a GUI Qt application?

Question

I'm developing an updater for my application in Qt, primarily to get to know the framework (I realize there are multiple ready-made solutions available, that's not relevant here). It is a basic GUI application using a QMainWindow subclass for its main window and an MyAppUpdater class to perform the actual program logic.

The update information (version, changelog, files to be downloaded) is stored on my server as an XML file. The first thing the updater should do after it sets up the UI is query that server, get the XML file, parse it and display info to the user. Here's where I have a problem though; coming from a procedural/C background, I'd initiate a synchronous download, set a timeout of maybe 3 seconds, then see what happens - if I manage to download the file correctly, I'll parse it and carry on, otherwise display an error.

However, seeing how inconvenient something like that is to implement in Qt, I've come to believe that its network classes are designed in a different way, with a different approach in mind.

I was thinking about initiating an asynchronous download in, say, InitVersionInfoDownload, and then connecting QNetworkReply's finished signal to a slot called VersionInfoDownloadComplete, or something along these lines. I'd also need a timer somewhere to implement timeout checks - if the slot is not invoked after say 3 seconds, the update should be aborted. However, this approach seems overly complicated and in general inadequate to the situation; I cannot proceed without retrieving this file from the server, or indeed do anything while waiting for it to be downloaded, so an asynchronous approach seems inappropriate in general. Am I mistaken about that, or is there a better way?

Answer 1

TL;DR: It's the wrong approach in any GUI application.

how inconvenient something like that is to implement in Qt

It's not meant to be convenient, since whenever I see a shipping product that behaves that way, I have an urge to have a stern talk with the developers. Blocking the GUI is a usability nightmare. You never want to code that way.

coming from a procedural/C background, I'd initiate a synchronous download, set a timeout of maybe 3 seconds, then see what happens

If you write any sort of machine or interface control code in C, you probably don't want it to be synchronous either. You'd set up a state machine and process everything asynchronously. When coding embedded C applications, state machines make hard things downright trivial. There are several solutions out there, QP/C would be a first class example.

was thinking about initiating an asynchronous download in, say, InitVersionInfoDownload, and then connecting QNetworkReply's finished signal to a slot called VersionInfoDownloadComplete, or something along these lines. I'd also need a timer somewhere to implement timeout checks - if the slot is not invoked after say 3 seconds, the update should be aborted. However, this approach seems overly complicated

It is trivial. You can't discuss such things without showing your code: perhaps you've implemented it in some horribly verbose manner. When done correctly, it's supposed to look lean and sweet. For some inspiration, see this answer.

I cannot proceed without retrieving this file from the server, or indeed do anything while waiting for it to be downloaded

That's patently false. Your user might wish to cancel the update and exit your application, or resize its window, or minimize/maximize it, or check the existing version, or the OS might require a window repaint, or ...

Remember: Your user and the environment are in control. An application unresponsive by design is not only horrible user experience, but also makes your code harder to comprehend and test. Pseudo-synchronous spaghetti gets out of hand real quick. With async design, it's trivial to use signal spy or other products to introspect what the application is doing, where it's stuck, etc.