ViewModel as member of ViewModel

I have been using the MVVM Light Toolkit to help learn the MVVM pattern. However, I have not been able to solve the problem of usercontrols within controls scenario.

For example, in a Timesheet application, lets say we have a control called NewUnitOfWork. When it first loads, a panel with a ListBox with a list of projects is loaded as the Content of the NewUnitOfWork. The user clicks on one. A new panel is swapped in with a ListBox containing the possible tasks for that project. A task is selected and a new panel is loaded which will contain controls to input data for the chosen task of the chosen project.

So, we have the selected item in one usercontrol being passed to the other two user controls, which are, in turn swapped in as the Content of the NewUnitOfWork control (or window).

If each control has its own ViewModel, we need to pass the selected value from one ViewModel to the next etc.

I have got it working in a single user situation using global variables (via a "service"). However, there are concurrency issues with that and it is not a good solution. It's sub-par.

I have seen many times the suggestion on this forum to have on ViewModel as a member of another ViewModel. Whilst this solves the problem at hand, I believe it is a violation of the MVVM pattern. Another ViewModel is not UI-related functionality that the Vie开发者_如何学JAVAwModel shoule be directly.

So. Has anyone found a clean MVVM-complying way to do this sort of thing?

Cheers

Please always keep in mind that MVVM is just a pattern and it is designed to help you separate your UI and logic. Do not be afraid to “violate the pattern” if it helps to increase testability or maintainability of the application.

Having a master ViewModel with several child ViewModels is very handy if you have a complex UI. The main ViewModel may be responsible for handling the top level UI controls and for coordination of the child VMs, while other ViewModels are responsible for communication with the sub regions of your UI.

Moreover, if you have a really complex UI with the multiple nesting UI layers, you can implement an infrastructure to automatically cascade all the events from master to child VMs. And of cause, you may try to use one of the more advanced MVVM frameworks. For example Catel implements pretty comprehensive model to resolve such situations with nested VMs.

I don't see a problem with ViewModels referencing other ViewModels (based on my experience with TreeViews). Have a look at any article about TreeView and MVVM. You will see that each node is a ViewModel, that references a collection of child nodes, which are ViewModels. Trying to do that without VM-VM references would be a nightmare.

Josh Smith http://www.codeproject.com/KB/WPF/TreeViewWithViewModel.aspx

I have been using the following setup:

A 'master' VM with a 'collection' VM and a 'details' VM as nested properties.

The master VM is tied to a View that is used as a master-detail form. This master-detail View is composed from two other Views.

I find it a very neat setup because it allows me to put search criteria in the master View(Model) and keeps the other View(Model)s clean.

I can't see how this would break the pattern.