Question

Let’s say we have an application that has a layered architecture. On the view we use a MVC or MVVM. The model is treated as the domain, it has a good part of the business logic.

Now let’s say we have, in the model, a method that takes some time. A complicated calculation or a treatment that has to be done to each items of an object for example.

In the UI, we would like to display a progress bar and a text that would display the current step of the calculation (for example a listbox with all the process history).

How would you do that? How to send from the model the information of the progress of the process and how to hook up the Controller or ViewModel so that it will update the progress?

Answer 1

I often implement this in the following manner. My business-layer process, which takes a long time to run, raises events every so often to indicate that it’s hitting specific “milestones”. You decide what milestones to signal through events and how many of them. If your time-consuming process is a plain loop, you may choose, for example, to raise the same event again and again every 10% of the items in the loop. If it is a process with distinct phases, you may choose to raise a different event as each phase is completed.

Now, your presentation layer subscribes to those events and acts in consequence, updating the progress bar, the text or whatever.

This mechanism is good because:

1. The business layer stays independent of what may go on up in the presentation layer.
2. It is easy to extend for bi-directional communication. You can easily alter events so that the presentation layer (or any other subscriber) can return a cancel flag so that the business layer knows that a long-running process must be cancelled.
3. It admits either synchronous or asynchronous work. That is, you can use it on blocking calls (i.e. your presentation and business layer share the same thread) or non-blocking calls (i.e. your business layer uses a background worker thread). The System.ComponentModel.BackgroundWorker class can be used in the latter case, but it’s not too good if you want to raise multiple types of events.

Hope this helps.