Childwindows in MVVM

Question

Hi

I'm having a problem understanding something about MVVM. My application relies on dialogs for certain things. The question is, where should these childwindows originate from? According to MVVM, viewmodels should contain only businesslogic and have zero actual knowledge about UI. However, what other place should I call my childwindows from, considering they're UI elements?

Doesn't this create tight coupling between elements?

Answer 1

See Handling Dialogs in WPF with MVVM

Answer 2

Since you tagged the question with Prism, I'll suggest the way I've done it in the past using Prism. Have the IEventAggregator injected into your ViewModel, and then when you want to pop open the dialog, publish a "ShowDialogEvent" or something like that. Then, have another Module called "DialogModule" or whatever, which upon initialization subscribes to that event, and shows the dialog. Furthermore, if you want to pass data back to the original ViewModel, have the ViewModel of the dialog publish a "DialogCloseEvent" or something like that with a payload of the data you need. You can then subscribe to that event back in your main ViewModel.

Answer 3

In the past, I have accomplished this by using Unity to resolve a custom interface that has a Show() method and a completed event. Then in the ViewModel I would call IScreen screen = container.Resolve<IScreen>(Resources.EditorWindowKey); and then just call screen.Show();.

The big advantage of this is that I can then just simply change my Unity configuration to remove the view when I'm testing my VM's.

Answer 4

The primary route I've been using to do this is to create a command inside your View layer. That command object accepts a parameter that is the ViewModel object that you want to display. The command then finds the appropriate ChildWindow, creates it and displays it with the parameter set as the content or however you will set it up. This way you can just bind a button's command property to that command, and its commandparameter to the object you want to show in the popup and your ViewModel objects never have to care how it's being displayed.

Prompting for user input (like saving a dirty file or something) doesn't work in this scheme. But for simple popups where you manipulate some data and then move on, this works very well.

Answer 5

The ViewModel sample application of the WPF Application Framework (WAF) demonstrates how to show a Modal Dialog.