Python Qt – How to Make a Multithreaded GUI Application

I generally go for the option of having a good service layer between any two dependencies unless it is not possible. I am a big believer in TDD, and it is a lot easier to test anything you can mock or fake out to better control the behavior for which you want to test. Trying to mock up SQL Server isn't as easy as mocking a service. Additionally, GUIs are notoriously hard to test with any kind of consistency as they are usually changing and are sometimes very non-deterministic (multiple events causing multiple outcomes, etc.). With a service, you have the ability to set up a mocked service that implements the interface/contract. If done right, you can then take your time developing the GUI and writing tests for it prior to having to work out the database schema and windows service. I would also look at maybe having the windows service update the database through a service layer of some sort as well. It doesn't have to be a "web" service per se. In some ways, I've had luck with these kinds of things being done through a message bus. We used TIBCO's EMS, but you could easily use any JMS implementation and a .NET connector. The advantage of this kind of setup is that you could write your file data to a queue, have a service that takes the data from the queue and updates the database AND notifies your client(s) GUI to refresh their data. A variant of this setup could be that the service actually sends the data to the clients via this topic saving you the added bandwidth of having multiple clients (if you have them) from having to hit the database with a query on every update.

MVC does not tell you how small to make the models.

For a spreadsheet, when you find that passing the whole data structure between a View and a Model is too slow, or makes the code too complex, you could make an MVC triad to represent something as small as a single cell.

The model part of the cell could then connect in some appropriate way to a larger business object that represents the entire sheet.

Similarly, the cell views will be embedded in another view to represent a sheet, or a window onto a sheet.

Update

The main aim of MVC is not to improve performance, but to make the code more understandable, and therefore easier to maintain.

If you make an MVC triad for each individual cell, this should be easier to understand and modify than if the triad represents a whole column, or the entire sheet. However, performance may suffer for changes that affect the entire sheet, as you may end up with many small objects sending messages to each other.

One alternative to MVC that you might also want to consider if you want to avoid passing data between Views and Models is direct manipulation. In direct manipulation, the object the user interacts with is the object that stores the business data (the cell value) and the business rules (how the cell value can change.)