WPF MVVM: How do ViewModels communicate with each other?

Question

I have a View which has 2 sub views on it and a ViewModel is assigned to each view:

ViewA - ViewModelA
{ 
   ViewB - ViewModelB
   ViewC - ViewModelC
}

ViewB has a text box and ViewC has a combobox, both of which i need access from ViewModelA. Not the GUI control itself, but the bound value i.e. .Text of the textbox and .SelectedItem of the ComboBox. Currently i just have ViewModelB and ViewModelC as properties on ViewModelA but it feels wrong.

What's the standard way for view models to communicate with each other without breaking the MVVM pattern?

Answer 1

One way to have disconnected ViewModels communicate to each other is to use a publish / subscribe mechanism such as PRISMs EventAggregator. However, in a parent / child ViewModel relationship, I think it's fine for the parent to have direct knowledge and control over the child ViewModel.

Personally, I don't think composing a ViewModel out of other ViewModels is a bad practice. I do it all the time. I generally favor composition over inheritance in my ViewModels.

Answer 2

The ViewModels usually 'inherit' relationships from the Model. And it's not wrong to add a relation when it makes sense.

It's perfectly OK for a ViewModel to have a property referring to another ViewModel.