Messenger Class - Can AppBar Button us it to update screen - mvvm-light

Question

I have a button on my AppBar to "Sync All". This call a webservice for each provider and updates their data that is has been cached. Once updated I need to update the selected providers data on the screen, so how do I do this with mvvm-light.
1) When I try to access the data in the click even of the button I am not sure how to access the currently loaded view models so I can refresh the data behind them. Is there a way to access the view model from a click event and refresh the underlying data.
2) I am wondering that is what the messenger class is used for and if so are there any good examples I can look at to use this feature in when the user clicks the "Sync All" Button.
3) If the messenger class is not the way to do this and there is no way to access the current view-model, what other options do I have.
Any help will be appreciated.

Answer 1

You've got two ways of doing this:
Using messenger
Using standard events
Option 1 works pretty much regardless of your design and goes something like this:
User clicks your AppBar button, which calls a command
The command calls a service that, internally calls your webservice (the nice way) or the command just calls your webservice (the not so nice way). I'm expecting this method to be an async method.
Once you've got your new data you call Messenger.Default.Send() to broadcast to all listening viewmodels that they should refresh their data.
Code:
ViewModel:
public class ViewModel
{
public ViewModel()
{
Messenger.Default.Register<DataRefreshEvent>(this,ReceiveDataRefreshEvent);
}
private void ReceiveDataRefreshEvent(DataRefreshEvent obj)
{
//do what you need to do
}
}
Service:
public class Service
{
public async void RefreshData()
{
await _webService.RefreshDataAsync();
Messenger.Default.Send(new ReceiveDataRefreshEvent());
}
}
Option 2 works like option 1 except it expects you to have a central data service wrapping your webservice. This service would be injected into all of your viewmodels.
User clicks your AppBar button, which calls a command
The command calls a service that, internally calls your webservice (the nice way) or the command just calls your webservice (the not so nice way). I'm expecting this method to be an async method.
The service raises a standard event to let the subscribed viewmodels know that they need to refresh their data. So basically same as option 1 except that each viewmodel is actives subscribing to an event on the service rather than a Messaging event.
Code:
ViewModel:
public class ViewModel
{
public ViewModel(IService service)
{
service.DataChanged+=ReceiveDataRefreshedEvent;
}
private void ReceiveDataRefreshEvent(sender obj,EventArgs args)
{
//do what you need to do
}
}
Service:
public class Service:IService
{
public event EventHandler ReceiveDataRefreshedEvent;
public async void RefreshData()
{
await _webService.RefreshDataAsync();
if(ReceiveDataRefreshedEvent!=null)
ReceiveDataRefreshedEvent(this,EventArgs.Empty);
}
}
Option 1 is nice because you don't have to hand over a reference to the service to the ViewModel. It's nicely de-coupled.
If you are unsure about messaging use option 2.