Coming from XAML and WPF, there are two options to forward selection changes from the UI to the ViewModel (as I understand your question, you're not asking about the other way around - feedbacking changes in the ViewModel to the UI - here):
Command with payload
The ICommand
s Execute
method has a payload parameter. Executing a command without a payload can be done passing null:
SomeCommand.Execute(null);
In your case, it would make sense to pass the selected item as the parameter in the event handler:
vm.ItemSelectedCommand.Execute(eventArgs.SelectedItem);
or
vm.ItemSelectedCommand.Execute(myList.SelectedItem);
In the command's execution method, you can handle the parameter. Note that your ViewModel property SelectedItem
is not directly involved here. If you need the selected index explicitly (which is not the case, usually), I would check the selected item's index in the Items
collection.
Binding selected item of list to a ViewModel property
Option B is to 'bind' the selected item of the list to a distinct property on the ViewModel, in your case the SelectedItem property in the event handler of the list:
vm.SelectedItem = myList.SelectedItem;
The command is kind of redundant then, although you could invoke it without a payload after setting SelectedItem on the ViewModel. I would rather handle the change of the selected item in the set accessor of the property on the ViewModel.
Note: XAML and WPF come with quite a lot of infrastructure code out of the box. MVVM doesn't make sense without a proper framework to actually take care of binding UI and ViewModels in a loosely coupled way. You quickly end up with a lot of extra work and little benefit, because you're still maintaining tight dependencies. Bottom line: I recommend getting or writing a proper MVVM framework, before actually implementing it.