return result from async operation in mvvm - silverlight-4.0

Question

I have a viewModel named CarsList with main property
public ObservableCollection<Car> Cars
{
get
{
if (_cars.Count == 0)
{
IsBusy = true;
_ws.GetCarsCompleted += new EventHandler<GetCarsCompletedEventArgs>(GetCarsCompleted);
_ws.GetCarsAsync(_app.HandlerId);
}
return _cars;
}
set
{
if (_cars != value)
{
if (_cars != null)
{
Unsubscribe(_cars);
}
_cars = value;
if (_cars != null)
{
Subscribe(_cars);
}
RaisePropertyChanged("Cars");
}
}
}
private void GetCarsCompleted(object sender, GetCarsCompletedEventArgs e)
{
//_cars = e.Result;
IsBusy = false;
}
When view gets _cars and the list is empty I must wait to get collection of cars from wcf service, and there is a problem because it is async operation.
Or maybe if list is empty I should return null, and fire async operation, and in asynccompleted set _cars to result from the wcf service?

Answer 1

I can only guess that you are trying to set up a view binding and property change notification. If I am right I would change you code as follows:
public void GetCars(Int32 handlerId)
{
_ws.GetCarsCompleted += new EventHandler<GetCarsCompletedEventArgs>GetCarsCompleted);
IsBusy = true;
_ws.GetCarsAsync(handlerId);
}
public ObservableCollection<Car> Cars
{
get
{
return _cars;
}
set
{
if (_cars != value)
{
_cars = value;
RaisePropertyChanged("Cars");
}
}
private void GetCarsCompleted(object sender, GetCarsCompletedEventArgs e)
{
_ws.GetCarsCompleted -= new EventHandler<GetCarsCompletedEventArgs>GetCarsCompleted);
IsBusy = false;
if (e.Error != null)
{
//Error handler
}
else
{
Cars = e.Result;
}
}
And then the view binding (in the case of a DataGrid) would look something like this..
<DataGrid IsReadOnly="True"
ItemsSource="{Binding Cars}"
.........
........./>