How can I invoke a command on the ViewModel on SelectionChanged of a ListView?

Question

In a MVVM/WPF environment, I want to invoke a command (ComputeCommand) on the ViewModel when the SelectionChanged event of a ListView is raised. How can this be done, either in XAML or in C#?

Here is my command class. I have tried MainViewModel.Instance.MyCommand.Execute(); in the codebehind, but it's doesn't accept that.

public class ComputeCommand : ICommand
{
    public ComputeCommand(Action updateReport)
    {
        _executeMethod = updateReport;
    }

    Action _executeMethod;

    public bool CanExecute(object parameter)
    {
        return true;
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        _executeMethod.Invoke();
    }
}

score 2 · Answer 1 · answered Mar 07 '13 at 09:24

2

To answwer your question - you are missing a parameter. THis call should work:

MainViewModel.Instance.MyCommand.Execute(null);

However, you dont need an ICommand for that, this interface serves different purpose.

What you need is to either handle SelectionChanged on view side

var vm = DataContext as YourViewModelType;
if (vm != null)
{
    vm.Compute(); //some public method, declared in your viewmodel
}

or to handle it on viewmodel side by binding to IsSelected property of item container

answered Mar 07 '13 at 09:24

Nikita B

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I have tried bothe of solutions but none worked :/ I'm using MVVM because I have some data to be update on the screen at the moment of the execution – gsmida Mar 07 '13 at 09:35
What do you mean by "none worked"? They failed to compile? Failed to execute? With exception? Or what? – Nikita B Mar 07 '13 at 10:00
You cant bind event to an ICommand. You should replace it with SelectionChanged="SelectionChangedHandler" and then implement this handler in code behind the way i described above – Nikita B Mar 07 '13 at 10:11

score 2 · Accepted Answer · answered Mar 07 '13 at 10:03

I really recommend the use of a Mvvm framework like MVVM Light, so you can do something like this:

XAML:

xmlns:MvvmLight_Command="clr-namespace:GalaSoft.MvvmLight.Command;assembly=GalaSoft.MvvmLight.Extras"
xmlns:Custom="clr-namespace:System.Windows.Interactivity;  assembly=System.Windows.Interactivity"

 <ListBox>
 ...
     <Custom:Interaction.Triggers>
          <Custom:EventTrigger EventName="SelectionChanged ">
             <MvvmLight_Command:EventToCommand PassEventArgsToCommand="False" Command="{Binding Path=ComputeCommand}"/>
          </Custom:EventTrigger>
     </Custom:Interaction.Triggers>

</Listbox>

ViewModel:

public RelayCommand ComputeCommand{ get; private set; }

This is IMO an elegant way to keep your events wiring clean and tidy.

score 1 · Answer 3 · answered Mar 07 '13 at 10:25

In general: To invoke a command when an event of a control is raised, you can use EventTriggers.

<ListView>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="SelectionChanged" >
            <i:InvokeCommandAction Command="{Binding CommandToBindTo}" CommandParameter="{Binding CommandParameterToBindTo}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</ListView>

To do this, you need to reference System.Windows.Interactivity.dll in your XAML:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

That being said, you should use a MVMM framework, for example MVVM to simplify the implementation of commands in general. It is not maintainable in the long run to have a single class for every command you need. A framework like MVVMLight or PRISM provides DelegateCommands which allow you to create new commands from delegates (methods on your ViewModel) directly.

score 0 · Answer 4 · answered Mar 07 '13 at 09:19

First you have to define a binding for Command, so the function that have to be executed on the invokation of the command.

This can be done in XAML, like:

<CommandBinding Command="name_of_the_namespace:ComputeCommand" Executed="ComputeCommandHandler" />

After, you can, for example in some class initialize a command, like:

 public class AppCommands {
    public static readonly ICommand ComputeCommand = 
             new   RoutedCommand("ComputeCommand", typeof(AppCommands));
 }

and after can use it like:

 AppCommands.ComputeCommand.Execute(sender);

When you're dealing with WPF, so MVVM pattern, you need to write the much more code the usual, but benefit from flexibility of it.

And where to add the ?? anywhere or inside ? – gsmida Mar 07 '13 at 09:28 — gsmida, Mar 07 '13 at 09:28

How can I invoke a command on the ViewModel on SelectionChanged of a ListView?

4 Answers4