Q:

Caliburn.Micro screen transition via conductor

I have a Caliburn.Micro shell (i.e., an empty XAML view to contain other views) rendered by a Conductor ViewModel. From there I open a Screen via:
ActivateItem(...)

Usually from the newly displayed dialog the user can perform some operations and click buttons (OK, Cancel, Build....) which should each transition to another screen (in the shell).
public MyDialog : Screen
{
    public void Ok()
    {
        // TODO: Somehow tell the conductor or called of this class about this action.
    }
}

What are good ways to achieve these kind of dialog action/message screen transitions?

Simple .NET events are possible -- Wouldn't that be a bad idea?
CM IEventAggregator should also work by changing the view
Checking from the shell Conductor the ViewModel result once it has been closed via TryClose() -- Should be possible, just don't know how to achieve this in CM.
Reference the shell Conductor instance from that screen (via IoC or directly) -- That seems strong coupling.

Could you please advise.

A:

My preferred approach is to use the EventAggregator to facilitate messaging between VMs. 
This works especially well when you have multiple windows which are listening for a certain type of event (e.g. a Visual Studio style interface with multiple tool windows which may show context sensitive properties), however it sounds a little overkill for this implementation. Of course the advantages are still a good loose coupling between VMs and a lack of events (which is a good thing!)
It sounds like you want a modal dialog to popup and present an option, and then activate another screen once the first one has returned.
You can attach an event handler to the Deactivated event in the child VM which will fire when an item deactivates. It also passes a boolean in the arguments to notify if the item which deactivated was closed - you can check for this and activate the corresponding screen in your conductor.
e.g.
this.Deactivated += new EventHandler<DeactivationEventArgs>(WorkspaceViewModel_Deactivated);

void WorkspaceViewModel_Deactivated(object sender, DeactivationEventArgs e)
{
    if(e.WasClosed) // raise some event
}

Then pass an event up to the conductor, I wouldn't really go the event route for this. This couples the VMs one-way so it may not be the most flexible solution
The alternative is to fire a message via the event aggregator to tell the conductor it needs to open a different window when the child VM closes. The same method can be used but it's decoupled
this.Deactivated += new EventHandler<DeactivationEventArgs>(WorkspaceViewModel_Deactivated);

void WorkspaceViewModel_Deactivated(object sender, DeactivationEventArgs e)
{
    if(e.WasClosed) MainConductor.EventAggregator.Publish(new ActivateWindowMessage(typeof(SomeVM));
}

