Bing Maps MVVM in universal app - mvvm

Question

For a long time i´ve been trying to understand the correct way of dealing with bing Maps in n MVVM scenario.
I might create a map in my XAML-view like this:
<map:Map x:Name="MyMap"
Credentials="MySuperSecretCredentials"/>
I the code behind file I can easily interact with map for eaxmple like this:
private async void FindMe_Clicked()
{
_cts = new CancellationTokenSource();
CancellationToken token = _cts.Token;
// Get the location.
Geoposition pos = await _geolocator.GetGeopositionAsync().AsTask(token);
MyMap.SetView(new BasicGeoposition() { Latitude = pos.Coordinate.Latitude, Longitude = pos.Coordinate.Longitude }, 15);
}
Simply by referencing the MyMap we can do whatever we like with it in the code behind.
But how can I execute the same command my viewModel?
I guess I should start with replacing the FindMe_Clicked with a command calling a method on my viewModel? And have that method execute a method similar to the one in the code-behind. But How do I acess MyMap in the viewModel?
Maybe my VM looks like this:
public class MainViewModel
{
public RelayCommand GetLocation { get; private set; }
public MainViewModel()
{
this.GetLocation = new RelayCommand(this.FindMe());
}
public void FindMe()
{
_cts = new CancellationTokenSource();
CancellationToken token = _cts.Token;
// Get the location.
Geoposition pos = await _geolocator.GetGeopositionAsync().AsTask(token);
MyMap.SetView(new BasicGeoposition() { Latitude = pos.Coordinate.Latitude, Longitude = pos.Coordinate.Longitude }, 15);
}
}
If im not thinking about this problem all wrong what I need to do Is to pass the same instance of MyMap that exists in the view to my viewmodel somehow?
Help with this is appreciated, I would also love to see any examples of how to use bibg maps i portable class libraries or i an Mvvm-pattern if anyone has come across it somewhere. Thanks!

Answer 1

When following the MVVM pattern, you should never try to access elements of the View layer (e.g. the maps control) from within the Viewmodel. Instead, you'd (theoretically) create two public properties CenterLatitude and CenterLongitude and bind them to the maps control directly within XAML code:
<Map Credentials="MySuperSecretCredentials" ZoomLevel="15">
<Map.Center>
<Location Latitude="{Binding CenterLatitude}" Longitude="{Binding CenterLongitude}"/>
</Map.Center>
</Map>
It's ok to have a method FindMe within your Viewmodel, but instead of accessing the MyMapcontrol and calling SetView(...) from within there, you'd just update the two properties CenterLatitude and CenterLongitude and make sure that the PropertyChanged event is raised in order to inform the View about the changed data and to update itself.
BUT: Unfortunately, the bing map control's Center attribute does not support Data Binding - it seems that Microsoft turned this feature off intentionally for performance reasons. If you still want to keep the MVVM pattern, check out this article I've written some time ago that explains how to circumvent this restiction.
(In my experience, this workaround works quite well and does not affect performance. Just make sure that you don't update the Center property too often, meaning several times per second...)