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Calling child's viewmodel method from main Viewmodel

I see in many posts that communications between viewmodels should be done using a messenger bus.
What I wonder is, is it considered bad to simply execute a child vm method from the main vm?
Suppose having a view with a TabControl, where each TabItem is a child viewmodel.
When I close my application, I need to store my settings calling a SaveSettings() child vm method.
I need to know if it's a bad code, and in case why.
main view
-> closingEvent
-> call MainVM SaveAllSettings()
-> for (ChildVM childVM in allTabs)
--> childVM.SaveSettings() (public method)

If your objects are already coupled to each other, i.e. if a parent view model already has a strong reference a child view model, there is no need to use a messenger or an event aggregator to communicate between them. Then you can (or rather should) call any methods of the child directly.
There are certainly cases where these kind of parent/child relationsships exist and it's not necessarily a bad thing or a sign of poor design. It all depends on the relationsships of your objects really.

Navigation within Page using Template10

I have problem how to implement sub-page navigation in UWP. The page is in RootFrame, which I can use on navigation. But i want to use something like this:
<Page>
<Grid>
<Frame x:Name="MyFrame"/>
</Grid>
</Page>
What I want is, use Navigate method of control MyFrame in ViewModel. I can call the method from code-behind, but I'm developing my app using MVVM. I'm not sure, if Template10 can work with sub-frames.
I appreciate any advice.
EDIT:
More details:
I have pivot control which is in page. the pivot has 2 tabs (pivotitems). The content of the pivotitem must be navigable. What I mean: I pivotitem 1, I need to have one Frame and use it for navigation in the pivotitem. My problem is, how to use or how to call the frame in pivotitem from ViewModel, especially I need to call Navigate method. Now I'm using Template10's navigation service and it's working with rootframe. I don't know, how to use it for other let's say sub-frames.

You can always do this.
var nav = Bootstrapper.NavigationServiceFactory(BackButton.Attach, ExistingContent.Exclude, this.Frame);
This will give you a navigation service for the frame in your page. You can then use session state, if you like.
Bootstapper.SessionState["MyNav"] = nav;
From here your view-model can access the service and navigate. You can repeat this for as many frames as you have. And you can then handle navigation in your view-model without consideration of "where" the frame is, just that your logic requires it to nav.
Does this make sense?

I don't know how you are going to trigger the navigation change so I'll assume it will start from a button click. I am also assuming the button's Command property is already bound to an ICommand in the viewmodel (the same concepts can be applied to different kinds of views).
All we have to do now is to make the ICommand implementation call our custom NavigationService to perform the content switch. This NavigationService class will be nothing but a simple proxy to the window global frame. Its main navigation method can be as simples as:
public void Switch()
{
var rootFrame = Window.Current.Content as Frame;
if ((rootFrame.Content as ParentPage) != null)
{
rootFrame.Navigate(typeof(ChildPage));
}
}

So you have tagged this with Template10 but it seems to be a more general question for UWP as a whole. I wonder if you have considered all of the inherent complexities with this approach - specifically related to suspension and resume. For each frame you have, you would need to save and restore navigation state, which isn't straight-forward when you have nested frames. Have you also considered how global navigation would work?
Template 10 does support the concept of multiple NavigationServices and, therefore, multiple frames, but only from the perspective of you can create them. Template10 does not inherently understand how such frames may be related to each other, so cannot perform automatic back propagation where you have something like:
FrameA[Main->Page1->Page1:Pivot1.FrameB[View1->View2->View3]]
Here we have two frames - FrameA and FrameB. FrameA has navigated from
Main to Page1. Page1 has a Pivot that hosts FrameB in PivotItem1 and
FrameB has navigated from View1 to View 2 and from View2 to View 3.
Global navigation (i.e. the shell back, etc.) would be automatically wired to FrameA, so you would need to intercept that action, and then handle you own navigation activity for FrameB.
Take a look at the BackButtonBehavior to see how it is possible to intercept the global back and then put in place your own action.

I don't know if you can do something like that..
One possible workaround is to use a Messenger that sends a message from your viewmodel to the view's code behind.. I'm not a fan of this solution though, because as I said before you have to use the page's code behind..

Xamarin Forms: How notify my previous viewmodel than is observable collection need to be modified?

I use an observable collection with a selecteditem (name + detail) to push a new contentpage in my navigation and in this new page i modify the name of this selected item but in an other list.
I would like to refresh the data in the observable collection with this other list (saved in an internal storage)
So, can i use an event to notify the previous viewmodel than i push the back button and if it possible which event can i use?

Xamarin.Forms.Page
//
// Summary:
// When overridden, allows application developers to customize behavior immediately
// prior to the Xamarin.Forms.Page becoming visible.
//
// Remarks:
// To be added.
protected virtual void OnAppearing();
This may be helpfull you'll need to override it in your page code.

Overriding the OnAppearing method seems the simplest way to achieve this. This way you'll keep the logic to refresh the data in the page that belongs with the viewmodel that needs to be refreshed. When the page re-appears, it can trigger some logic in the ViewModel to refresh the ObservableCollection.
Another option is to use the MessagingCenter that comes with Xamarin.Forms: https://developer.xamarin.com/guides/xamarin-forms/messaging-center/ This allows pub/sub style messaging between components while staying loosely coupled. You could have the class that manages the internal storage publish messages whenever the list in the gets updated and broadcast this to ViewModels that have subscribed to these updates.
In this particular case, overriding OnAppearing seems the simplest solution though.

ViewModel calling a method of a user control (web browser)

I am trying to program in MVVM and I have the following use case:
A TextBox's text is bound to a property in the VM
A Button is command bound to a relay command
When the user presses the Button, the web browser's Navigate(url) method is called with the URL being the text in the TextBox
Above is the use case I want to create, but 1 and 2 is possible using the MVVM design pattern, but I could not find an adequate way to invoke the browser's Navigate() method. First of all, is it possible to call a method of a control from VM (please let me know if there is a way)? And in the above use case, what would be the appropriate way to structure the program if it is not possible?
Thanks

You could do the following:
Add a property MyUrl to your ViewModel
Bind MyUrl to your WebBrower's Source property
Make sure the property implements INotifyPropertyChanged. Then your Xaml:
<WebBrowser Source="{Binding MyUrl}" />
What if you REALLY wanted to call a UI method from the ViewModel?
If you ever do run into a situation where you absolutely need to call a method on a UI control for instance, you can hook up events on the ViewModel and then your UI registers to this event and does something UI specific...
VM code...
//... some VM logic
EpicNavigateEvent(url) // raise event, allowing UI to handle how
In your code-behind on your view (this is the part where some MVVM purests freak), you could register the event:
myVm.Navigate += doSomeNavigation;
...
public void doSomeNavigation(string url)
{
// call Navigate
}
I've successfully used this approach for applications where we have a single ViewModel layer and multiple technologies hooked up the views (WinForms, WPF and Asp.Net).
If you're looking for something more elegant, have a look at the User Interaction Patterns on MSDN.
The concept is the same though: Call something on the VM and the View is handles it appropriately.
Common scenarios for this type of approach is want to show a message to the user from the VM. Your VM should raise an event saying: ShowMyMessage("You're awesome"), then your UI is notified and handles it: MessageBox.Show(msg) or whatever.
As long as you stick to there rules you should be golden:
ViewModels should NOT be concerned about UI code
Views must ONLY handle the presentation of the data provided by your ViewModels.
Don't overcomplicate it. KISS...

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.

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));
}