Confusion regarding MVVM pattern and dynamic loading of XAML in GUI.

Question

Well this question relates to MVVM pattern and i could good and fast answers on this forum so I thought to ask and clear the confusions i had about the pattern.

I am quite new to MVVM approach. I appreciate the pattern and understand the principals behind it. Maybe I have not worked that much with the pattern that’s why there are a few confusions.

If there is a scenario in which I want to load few parts of my WPF page dynamically with XAML and still want to be compliant with MVVM approach.

The confusion is:

1. Where the logic of loading a view dynamically with XAML reside.
2. Whether I should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes.
3. What if I had to build control tree displayed on the GUI using C# code in the codebehind itself. For the controls created using code should I do the commandbindings in the codebehind of the view itself.

Answer 1

I am also quite new to mvvm, but i will try to answer :

1. Where the logic of loading a view dynamically with XAML reside

If you mean "how can i show the view associated with my business object?", IMHO, you don't have to care about this. Usually, your VMs have corresponding views. With dataTemplate, you use only VM in the code but Views are displayed automatically.

2 Whether i should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes

It seems you have a top-bottom approach. I see the mvvm more as bottom-up : models (business objects) -> ViewModels -> Views. Every model should have its own ViewModel and view. So you can't have a whole WPF page in a viewModel unless you model represents a page.

3 What if i had to build control tree displayed on the GUI using C# code in the codebehind itself. For the controls created using code should i do the commandbindings in the codebehind of the view itself.

Don't understand. I think you may take a look at dataTemplate, it might be helpfull.

Answer 2

Where the logic for loading goes is something not really addressed by the pattern itself. There's an interesting blog post about this by Ward Bell. There's any number of ways to skin this cat, and they're all compatible with MVVM. Not really the answer you're looking for, I know, but it's honest :). Check out Ward's blog post... you'll get a much more in depth discussion of this topic.

As for whether or not to have a single VM for the page, or one for each control, that just depends. Generally, I have one for the page. If there's some portion that reusable elsewhere, I break it out into a user control with it's own VM, which means we have a VM within a VM. I don't agree with rockeye on this one. There isn't a one-to-one relationship between V-VM-M. You're Models are designed according to business needs, with NO regard to presentation at all. You're ViewModels are designed according to your presentation needs, and may encapsulate more than one Model. In fact, it's very common for them to encapsulate many models.

Like rockeye, I don't understand your last question.