ViewModels and rendering

Question

In several sample projects, I\'ve seen ViewModels being used to convert data objects into strings, for use in the View.

The ViewModel will typically have a construct

Answer 1

The purpose of the ViewModel is to represent (a part of) the complex Domain Model decomposed as primitives that can be rendered in some form other other.

This decomposition must take place somewhere. It may involve some simple kind of logic, such as my favorite example: converting a discrete value (OK, warning, error) into colors (Green, Yellow, Red). This is the essence of what a ViewModel does, so my default approach would be to encapsulate this logic into the ViewModel itself.

Consider the alternative: If not implemented in the ViewModel, then where? If you put the logic somewhere else, you end up with a ViewModel that's basically just a structure without logic. Letting a ViewModel encapsulate the transformation/decomposition of a Domain Object fits well with the Single Responsibility Principle.

Although this is my default approach, I'm always aware that the logic may need to be reused across multiple ViewModels. In such cases, this may be an indication that the original ViewModel is really a complex ViewModel made up of several sub-views. In such cases, you can extract the common logic into a sub-ViewModel that encapsulates only that little part.