MVVM - navigating between views

Question

I have a ListBox that is bound to a ViewModel that exposes a parameter of type ObservableCollection. I have setup an ICommand that gets fired when one of the rows in the ListBox is selected. (using method 3 in this post - it works great by the way).

Now my question (which has nothing to do with method 3 described above or the ListBox) is when my ICommand is fired and what I want to do is navigate to a different page (eg: details page), where is the logic stored (or how do I do it?).

The reason I ask is that I am not sure how to setup the command method in the ViewModel class such that it remains testable.

ps: I am using Prism and was also wondering if it provides any classes/patterns for Navigation.

score 6 · Answer 1 · answered Apr 02 '11 at 18:49

Just to elaborate on the use of IEventAggregator - it gives you a simple Pub/Sub model for sending arbitrary messages between decoupled (ie neither needs to know anything about the other) parts of the application. So we can get a reference to the IEventAggregator in our ViewModel constructor (this is automatically resolved for you by the framework) ie:

private IEventAggregator eventAggregator;

public PublisherViewModel(IEventAggregator eventAggregator)
{
    this.eventAggregator = eventAggregator;

Then in our selection changed handler, we can publish the event:

var changedEvt = eventAggregator.GetEvent<MyListboxChangedEvent>();
changedEvt.Publish(selectedItemId);

This relies on our custom event class MyListboxChangedEvent:

public class MyListboxChangedEvent : CompositePresentationEvent<int> {}

So finally, in the ViewModel which responds to this action, we set up a subscription to the event, and corresponding handler method:

public SubscriberViewModel(IEventAggregator eventAggregator)
{
    var changedEvt = eventAggregator.GetEvent<MyListboxChangedEvent>();
    changedEvt.Subscribe(OnListBoxChanged, ThreadOption.UIThread);
}

public void OnListBoxChanged(int selectionId)
{
    // do whatever we need
}

Seems like a lot of glue, but it becomes a logical method for wiring the different parts of the UI together, and it becomes second nature pretty quickly.

score 3 · Answer 2 · answered Apr 02 '11 at 17:37

3

Have you considered using the EventAggregator to send the message that you want to show a different view. The StockTrader application included in the PRISM distribution will provide a good example of the use.

answered Apr 02 '11 at 17:37

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MVVM - navigating between views

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