Question

I want to display a list of objects in a LongListSelector, and format them with a DataTemplate. To properly use MVVM I’d like to have a ViewModel in this DataTemplate.

Creating of this ViewModel is no problem, but how do I pass the Item to the ViewModel?

I’m using this code:

<Controls:LongListSelector 
     ItemsSource="{Binding MyItems}" Margin="0" HorizontalAlignment="Stretch" 
                           HorizontalContentAlignment="Stretch" >
    <Controls:LongListSelector.DataContext>
        <viewmodel:MyListOfItemsViewModel />
    </Controls:LongListSelector.DataContext>
 <Controls:LongListSelector.ItemTemplate>
        <DataTemplate>
            <StackPanel x:Name="CurTemplate">
                <Grid Margin="10" >
                    <Grid.DataContext>
                        <viewmodel:MyViewModel MyItem="{Binding Path=DataContext,ElementName=CurTemplate}" />
                    </Grid.DataContext>

But alas, the only thing that is set for MyItem is null, and this is never updated to the real value. I found out that later in the process (after the initial setting of MyItem CurTemplate does have a valid DataContext, but this is not sent to my ViewModel. Am I missing something here?

For completeness the code for MyViewModel:

public static DependencyProperty MyItemProperty = DependencyProperty.Register("MyItem", typeof(object), typeof(MyViewModel), new PropertyMetadata("asd", ItemChanged));

    private static void ItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
       System.Diagnostics.Debugger.Break(); // to set when something is set
       // called once, NewValue is null
    }

    public object MyItem
    {
        get
        {
            return (object)GetValue(MyItemProperty);
        }
        set
        {
            SetValue(MyItemProperty, value);
            RaisePropChangeEvent("MyItem");
        }
    }

I did a lot of searching and fiddling around, but I’m pretty sure this is just a minor thing that is missing here. I would be very glad if you could help me out here…

EDIT: Solved

I solved my problem by using {Binding Path=Content,RelativeSource={RelativeSource Mode=TemplatedParent}} as binding for the viewmodel. I have no idea why this works with Content, but not with DataContext…

Thanks for your help, robertftw, your linked post brought me to the right track!

Answer 1

Seems like the problem I had a few days ago:
Binding does not update usercontrol property correctly MVVM

public static DependencyProperty MyItemProperty = DependencyProperty.Register("MyItem", typeof(object), typeof(MyViewModel), new PropertyMetadata(string.Empty, ItemChanged));
 private static void ItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
        var obj = d as MyViewModel;
        obj.RaisePropChangeEvent("MyItem");
}

The problem I had is that the set of MyItem isn’t actually called.