MVVM Correct using of the pattern

Question

I just want to know if i'm doing it right. I have a mainview (MainView) with it's viewmodel (MainWindowViewModel). In the MainView there is a button to call another view (SubView). The SubView hast also a ViewModel (SubViewModel). After the SubView is closed through it's viewmodel i want to access a property in the subviewmodel from the mainviewmodel. The code to call the subview from the mainviewmodel and access the property looks like:

private void SubViewExecute(object parameter)
{
    SubView sub = new SubView();
    bool? result = sub .ShowDialog();
    if (!result.HasValue || !result.Value) return;
    if (sub.DataContext is SubViewModel)
    {
        SubViewModel subViewModel = (sub.DataContext as SubViewModel);
        string property = subViewModel.Property;
    }
}

Am I doing the mvvm-pattern correct, or is there a better way to achieve want i want?

Important: always remember MVVM is a set of Guidelines, not Rules, whatever suits best for you and mantains code clean and does not violate DRY and SOLID, is OK.. just my opinion, though — Fede
– Fede, Commented Jan 29, 2013 at 20:17
What I did specifically was to create an enum (Dialog, Window, Default, etc) as part of the ViewModels and then the WindowManager utilizes this value to determine whether or not this particular ViewModel should be opened in a new Dialog, and so on. — Fede
– Fede, Commented Jan 29, 2013 at 20:19
Aha! It's the View's job to determine when to open new Dialogues, not the ViewModel. =) — EtherDragon
– EtherDragon, Commented Jan 29, 2013 at 20:43

EtherDragon · Accepted Answer · 2013-01-29 20:41:41Z

To your core question: "Am I doing the mvvm-pattern correct, or is there a better way to achieve want i want?"

No, you aren't adherring to the core principle of MVVM correctly, and there is a better way to achieve what you want (if I understand what you want correctly).

First, MVVM comes from a need to make all layers testable without requiring knowledge of the layer "above." For instance, your application should be able to technically do everything it's supposed to through just the Model; it should be ableo to retrieve, update, and create data as needed - even if this data is not presented in a user intuitive manner, yet.

Second, your applicaiton should then be able to technically do everything that the User would want it to do through the View-Model, but without any kind of UI. So you should be able to "look" at your data and perform the various program functions, like Saving.

Then, when you throw your view on top, all you need is data-binding and event handling, and you are good to go! (mostly)...

Mainly, it is the View's responsibility to correctly manage it's own DataContext from the ViewModel; it is not the ViewModel's job to push a datacontext onto a particular View. Another way to look at it is, the View accesses methods and properties in the ViewModel to cary out the work requested by the user in the user interface.

So, I would start by flipping your code around, so that the View controls which views are active at any given time, and that each view is ware of it's own data context, and methods to utilize them.

(Now, before the SO community jumps on me about not saying anything about the VM first approach - here it is. You could try a VM first approach, but it is more difficult to understand at first, and you are going to want to use a framework to help you, like Caliburn.Micro or MVVMLite or something)

So, for View First, what you want to do is have the MainView know how to populate itself with SubViews. It's the job of the MainView to ensure that it's data context is the correct MainViewModel, as each SubView is created in the MainView, the MainView will ensure that each SubView has the correct SubViewModel instance set as it's data context.

This should be logically easy to approach because your MainViewModel already contains a set of SubViewModels (of various kinds) inside.

Hope that helps get you going, if you have more specific code questions (with sample code) we can help you futher.

Reed Copsey · Accepted Answer · 2013-01-29 20:06:21Z

3

It's not entirely clear what you want here - but this is definitely violating MVVM in a purist sense.

Your MainViewModel, in this example, needs direct knowledge of the View layer (SubView), which is typically avoided whenever possible.

However, the best approach to avoid this depends a lot on whether you're using a framework (many frameworks have tooling for matching a View to a ViewModel, and displaying dialogs, etc), which framework, and whether you're working View-first or ViewModel-first.

answered Jan 29, 2013 at 20:06

Reed Copsey

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