我如何将此事件处理程序转换为命令？

Question

private void GridSplitter_PreviewMouseDown(object sender, MouseButtonEventArgs e)
{
    vm.positionX = e.GetPosition((sender as GridSplitter).Parent as UIElement).X;
}

the Parent is a VirtualizingStackpanel. I kind of understand the idea behind commanding, but am still having trouble understanding how to implement it. This event handler is taking the X coordinate of the Mouse and sending it to the ViewModel's positionX. If I made this into a command, where would I get the information for sender and e?

Answer 1

Normally, Commands are used for Button-like controls, that trigger, when the user interacts with them (clicking the button, tapping it, etc.).

A Command , depending on your application design and needs, has a CanExecute and an Executed handler or event. Those allow you to set, if the Command can execute at all, and what should happen, when it executes.

However, you're referring to a PreviewMouseDown event. You cannot prevent the system from firing this event. So the CanExecute of the new Command is more or less useless in this scenario.

To still achieve your goal, you could use the EventToCommand mechanic, that Ganesh already pointed out in the comments. This article and the example below should give you a hint, how it will work:

<ResourceDictionary>
  <DataTemplate x:Key="PreviewMouseDownCommandTemplate">
    <StackPanel>
      <i:Interaction.Triggers>
        <i:EventTrigger EventName="PreviewMouseDown">
          <command:EventToCommand Command="{Binding MyCommandProvider.PreviewMouseDownCommand,
                                            Mode=OneWay,
                                            Source={StaticResource Locator}}"
                                  CommandParameter="{Binding Mode=OneWay}" />
        </i:EventTrigger>
      </i:Interaction.Triggers>

      <!-- Content -->

    </StackPanel>
  </DataTemplate>
</ResourceDictionary>