ListView ItemsPanelTemplate with horizontal orientation, how to Identify ListViewItems on first row?
First of all I am working with MVVM / WPF / .Net Framework 4.6.1
I have a ListView configured with ItemsPanelTemplate in horizontal orientation that displays items from a DataTemplate. This setup allows me to fit as many items inside the Width of the ListView (the witdth size is the same from the Window), and behaves responsively when I resize the window.
So far everything is fine, now I just want to Identify what items are positioned on the first row, including when the window get resized and items inside the first row increase or decrease.
I merely want to accomplish this behavior because I would like to apply a different template style for those items (let's say a I bigger image or different text color).
Here below the XAML definition for the ListView:
<ListView x:Name="lv"
ItemsSource="{Binding Path = ItemsSource}"
SelectedItem="{Binding Path = SelectedItem}">
<ListView.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel Orientation="Horizontal"></WrapPanel>
</ItemsPanelTemplate>
</ListView.ItemsPanel>
<ListView.ItemTemplate>
<DataTemplate>
<Grid Width="180" Height="35">
<Grid.RowDefinitions>
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Ellipse Grid.Column="0" Grid.Row="0" Height="32" Width="32"
VerticalAlignment="Top" HorizontalAlignment="Left">
<Ellipse.Fill>
<ImageBrush ImageSource="{Binding IconPathName}" />
</Ellipse.Fill>
</Ellipse>
<TextBlock Grid.Column="1" Grid.Row="0" TextWrapping="WrapWithOverflow"
HorizontalAlignment="Left" VerticalAlignment="Top"
Text="{Binding Name}" />
</Grid>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
BTW: I already did a work around where I am getting the Index from each ListViewItem and calculating against the Width of the Grid inside the DataTemplate that is a fixed value of 180, but unfortunately it did not work as I expected since I had to use a DependencyProperty to bind the ActualWidth of the of the ListView to my ViewModel and did not responded very well when I resized the window.
I know I am looking for a very particular behavior, but if anyone has any suggestions about how to deal with this I would really appreciate. Any thoughts are welcome even if you think I should be using a different control, please detail.
Thanks in advance!
You shouldn't handle the layout in any view model. If you didn't extend ListView consider to use an attached behavior (raw example):
ListBox.cs
public class ListBox : DependencyObject
{
#region IsAlternateFirstRowTemplateEnabled attached property
public static readonly DependencyProperty IsAlternateFirstRowTemplateEnabledProperty = DependencyProperty.RegisterAttached(
"IsAlternateFirstRowTemplateEnabled",
typeof(bool), typeof(ListView),
new PropertyMetadata(default(bool), ListBox.OnIsEnabledChanged));
public static void SetIsAlternateFirstRowTemplateEnabled(DependencyObject attachingElement, bool value) => attachingElement.SetValue(ListBox.IsAlternateFirstRowTemplateEnabledProperty, value);
public static bool GetIsAlternateFirstRowTemplateEnabled(DependencyObject attachingElement) => (bool)attachingElement.GetValue(ListBox.IsAlternateFirstRowTemplateEnabledProperty);
#endregion
private static void OnIsEnabledChanged(DependencyObject attachingElement, DependencyPropertyChangedEventArgs e)
{
if (!(attachingElement is System.Windows.Controls.ListBox listBox))
{
return;
}
if ((bool)e.NewValue)
{
listBox.Loaded += ListBox.Initialize;
}
else
{
listBox.SizeChanged -= ListBox.OnListBoxSizeChanged;
}
}
private static void Initialize(object sender, RoutedEventArgs e)
{
var listBox = sender as System.Windows.Controls.ListBox;
listBox.Loaded -= ListBox.Initialize;
// Check if items panel is WrapPanel
if (!listBox.TryFindVisualChildElement(out WrapPanel panel))
{
return;
}
listBox.SizeChanged += ListBox.OnListBoxSizeChanged;
ListBox.ApplyFirstRowDataTemplate(listBox);
}
private static void OnListBoxSizeChanged(object sender, SizeChangedEventArgs e)
{
if (!e.WidthChanged)
{
return;
}
var listBox = sender as System.Windows.Controls.ListBox;
ListBox.ApplyFirstRowDataTemplate(listBox);
}
private static void ApplyFirstRowDataTemplate(System.Windows.Controls.ListBox listBox)
{
double calculatedFirstRowWidth = 0;
var firstRowDataTemplate = listBox.Resources["FirstRowDataTemplate"] as DataTemplate;
foreach (FrameworkElement itemContainer in listBox.ItemContainerGenerator.Items
.Select(listBox.ItemContainerGenerator.ContainerFromItem).Cast<FrameworkElement>())
{
calculatedFirstRowWidth += itemContainer.ActualWidth;
if (itemContainer.TryFindVisualChildElement(out ContentPresenter contentPresenter))
{
if (calculatedFirstRowWidth > listBox.ActualWidth - listBox.Padding.Right - listBox.Padding.Left)
{
if (contentPresenter.ContentTemplate == firstRowDataTemplate)
{
// Restore the default template of previous first row items
contentPresenter.ContentTemplate = listBox.ItemTemplate;
continue;
}
break;
}
contentPresenter.ContentTemplate = firstRowDataTemplate;
}
}
}
}
Helper Extension Method
/// <summary>
/// Traverses the visual tree towards the leafs until an element with a matching element type is found.
/// </summary>
/// <typeparam name="TChild">The type the visual child must match.</typeparam>
/// <param name="parent"></param>
/// <param name="resultElement"></param>
/// <returns></returns>
public static bool TryFindVisualChildElement<TChild>(this DependencyObject parent, out TChild resultElement)
where TChild : DependencyObject
{
resultElement = null;
if (parent is Popup popup)
{
parent = popup.Child;
if (parent == null)
{
return false;
}
}
for (var childIndex = 0; childIndex < VisualTreeHelper.GetChildrenCount(parent); childIndex++)
{
DependencyObject childElement = VisualTreeHelper.GetChild(parent, childIndex);
if (childElement is TChild child)
{
resultElement = child;
return true;
}
if (childElement.TryFindVisualChildElement(out resultElement))
{
return true;
}
}
return false;
}
Usage
<ListView x:Name="lv"
ListBox.IsAlternateFirstRowTemplateEnabled="True"
ItemsSource="{Binding Path = ItemsSource}"
SelectedItem="{Binding Path = SelectedItem}">
<ListView.ItemsPanel>
<ItemsPanelTemplate>
<WrapPanel Orientation="Horizontal" />
</ItemsPanelTemplate>
</ListView.ItemsPanel>
<ListView.Resources>
<DataTemplate x:Key="FirstRowDataTemplate">
<!-- Draw a red border around first row items -->
<Border BorderThickness="2" BorderBrush="Red">
<Grid Width="180" Height="35">
<Grid.RowDefinitions>
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Ellipse Grid.Column="0" Grid.Row="0" Height="32" Width="32"
VerticalAlignment="Top" HorizontalAlignment="Left">
<Ellipse.Fill>
<ImageBrush ImageSource="{Binding IconPathName}" />
</Ellipse.Fill>
</Ellipse>
<TextBlock Grid.Column="1" Grid.Row="0" TextWrapping="WrapWithOverflow"
HorizontalAlignment="Left" VerticalAlignment="Top"
Text="{Binding Name}" />
</Grid>
</Border>
</DataTemplate>
</ListView.Resources>
<ListView.ItemTemplate>
<DataTemplate>
<Grid Width="180" Height="35">
<Grid.RowDefinitions>
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Ellipse Grid.Column="0" Grid.Row="0" Height="32" Width="32"
VerticalAlignment="Top" HorizontalAlignment="Left">
<Ellipse.Fill>
<ImageBrush ImageSource="{Binding IconPathName}" />
</Ellipse.Fill>
</Ellipse>
<TextBlock Grid.Column="1" Grid.Row="0" TextWrapping="WrapWithOverflow"
HorizontalAlignment="Left" VerticalAlignment="Top"
Text="{Binding Name}" />
</Grid>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
Remarks
If the visual tree itself will not change for the first row, consider to add a second attached property to the ListBox class (e.g., IsFirstRowItem) which you would set on the ListBoxItems. You can then use a DataTrigger to modify the control properties to change the appearance. This will very likely increase the performance too.