模型或ViewModel中的MVVM计算字段？

Question

I am trying to determine the correct pattern for handling Calculated fields or methods in MVVM. I have a number of calculated fields that will be required for both the presentation layer and some back end processing. Let say it is " CalculateTotal " which is based on summing a number of different values from the Data object and its sub objects.

I would like this calculation in one location. The first thought is that it belongs in the model (ie, not the viewModel) since the model object will be passed to other back end processing systems. What is then the best way to make this available to the ViewModel ?

Option 1, is that I statically load the viewModel when I create it based off the Model (eg, vwModel.Total = model.CalculateTotal() ) This suffers if I am needing to update the ViewModel dynamcally, for example I modify the underlying data and try to get the new total.

Option 2: More service oriented and each calculation calls a service to return the values. The issue I see with this is more performance based. Once I load my object once, I have all the data needed to perform the calculation. It seems each call would require the data object to be reloaded

Option 3: Have the ViewModel contain the data model and create methods to call the data model methods

Thoughts? Suggestions?

Answer 1

I would do all calculations in the model. The view model should subscribe to events (eg via INotifyPropertyChanged ) on the model, so when the back-end values change, the view model will be notified.

The view model can, of course, interrogate the model for calculations and doesn't have to get all information through events. The above is only for when the data changes come directly from the model or the underlying data layer.

Answer 2

I would go with something like option 2. Moving the calculation logic out of the model and view model will simplify things and make that logic more reusable by other classes. From your question it seems like you already have an idea of how to implement that. Keeping the model "dumb" and having services/utility/helper classes that know how to process the model will help you in the long run if the model is being passed around a lot. Just something to think about but keeping the responsibility of a class very limited makes for easier code maintenance down the road. You might end up with more classes but I personally find it easier to focus and work on two or three small classes than one very large class. I don't know what type of application you have or how heavy the calculations are but unless you are having measurable performance issues I wouldn't worry about trying to pre-optimize things.

Answer 3

I would like this calculation in one location.

If there is a dedicated Service Layer , then almost all calculations should be done in Service Layer .

Option 1

Instead of using CalculateTotal method on Model , it's better make a read only property Total and calculate it in a service layer method every time Model is requested. This is the easiest and the best option to use if there are no performance issues, since it drastically simplifies ViewModel.

Option 2

Calling a dedicated CalculateTotal service method from ViewModel is the only option available if calculation process is resource intensive and calculated total is not used every time Model is requested.