Issue with "Thumb.DragStarted" event with MVVMLight - wpf

Question

I'm trying to trigger Slider Thumb.DragStarted event by using MVVMLight EventToCommand but it is not working. The same thing is working perfectly for Slider Event ValueChanged.
Below is my code:
<Slider
Width="150"
AutoToolTipPlacement="BottomRight"
AutoToolTipPrecision="2"
IsSnapToTickEnabled="True"
Maximum="{Binding SilderMaxValue}"
Minimum="0"
Value="{Binding SliderValue}">
<i:Interaction.Triggers>
<i:EventTrigger EventName="ValueChanged">
<cmd:EventToCommand
Command="{Binding SliderValueChangedCommand}"
PassEventArgsToCommand="True" />
</i:EventTrigger>
<i:EventTrigger EventName="Thumb.DragStarted">
<cmd:EventToCommand
Command="{Binding SliderDragStartedCommand}"
PassEventArgsToCommand="True" />
</i:EventTrigger>
</Slider>
Thanks..

Answer 1

I saw your post while I was trying to do something similar (albeit with Thumb.DragCompleted). In any case, I used an attached property. I'll post my solution in case it's of use to anyone.
SliderDragBehavoirs.cs:
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Controls.Primitives;
using System.Windows.Input;
using System.Windows.Media;
namespace WpfApplication1
{
public static class SliderDragBehaviors
{
public static readonly DependencyProperty DragCompletedCommandProperty =
DependencyProperty.RegisterAttached("DragCompletedCommand", typeof(ICommand), typeof(SliderDragBehaviors),
new FrameworkPropertyMetadata(new PropertyChangedCallback(DragCompleted)));
private static void DragCompleted(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var slider = (Slider)d;
var thumb = GetThumbFromSlider(slider);
thumb.DragCompleted += thumb_DragCompleted;
}
private static void thumb_DragCompleted(object sender, DragCompletedEventArgs e)
{
FrameworkElement element = (FrameworkElement)sender;
element.Dispatcher.Invoke(() =>
{
var command = GetDragCompletedCommand(element);
var slider = FindParentControl<Slider>(element) as Slider;
command.Execute(slider.Value);
});
}
public static void SetDragCompletedCommand(UIElement element, ICommand value)
{
element.SetValue(DragCompletedCommandProperty, value);
}
public static ICommand GetDragCompletedCommand(FrameworkElement element)
{
var slider = FindParentControl<Slider>(element);
return (ICommand)slider.GetValue(DragCompletedCommandProperty);
}
private static Thumb GetThumbFromSlider(Slider slider)
{
var track = slider.Template.FindName("PART_Track", slider) as Track;
return track == null ? null : track.Thumb;
}
private static DependencyObject FindParentControl<T>(DependencyObject control)
{
var parent = VisualTreeHelper.GetParent(control);
while (parent != null && !(parent is T))
{
parent = VisualTreeHelper.GetParent(parent);
}
return parent;
}
}
}
There's a couple of things worth noting here. Because the command is hooked up to the Slider, but the event is fired on the Thumb, it is necessary to be able to look up/down the visual tree in order to get one from the other.
Example XAML:
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:behaviors="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525">
<Grid x:Name="grid">
<Slider behaviors:SliderDragBehaviors.DragCompletedCommand="{Binding Path=DragCompletedCommand}"/>
</Grid>
</Window>
Hope that's of some use :)

Answer 2

I had a problem with the code from Tom Allen because the slider template was not available at the time I wanted to bind it with command. Basically, all i needed to do is wait for the slider control to load and try again.
Here are the changes that i needed to make in order for it to work:
private static void DragCompleted(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
//the Template of the slider is not available now
//we have to wait for the slider to load completely in order to do this
var slider = (Slider)d;
slider.Loaded += slider_Loaded;
}
static void slider_Loaded(object sender, RoutedEventArgs e)
{
var slider = (Slider)sender;
var thumb = GetThumbFromSlider(slider);
thumb.DragCompleted += thumb_DragCompleted;
}
Hope it helps!
Regards