How to find visible elements inside of a ScrollViewer and WrapPanel - wpf

Question

I've a WrapPanel surrounded with a ScrollViewer, I want to find the visible elements on the screen when I click a button.
My code like:
<ScrollViewer>
<WrapPanel>
<Label Width="500" Height="500"/>
<Label Width="500" Height="500"/>
<Label Width="500" Height="500"/>
....
....
....
<Label Width="500" Height="500"/>
<Label Width="500" Height="500"/>
<Label Width="500" Height="500"/>
</WrapPanel>
</ScrollPanel>
How can I find the visible Label elements when ScrollViewer scrolled to some offset.

Answer 1

Hopefully this will get you started (you may want to change 'svViewportBounds.IntersectsWith' with 'Contains'.
public partial class MainWindow : Window
{
public string VisibleItems
{
get { return (string)GetValue(VisibleItemsProperty); }
set { SetValue(VisibleItemsProperty, value); }
}
public static readonly DependencyProperty VisibleItemsProperty =
DependencyProperty.Register("VisibleItems", typeof(string), typeof(MainWindow), new PropertyMetadata("??"));
public List<string> Items { get; private set; }
public MainWindow()
{
Items = new List<string>();
for (int i = 0; i < 25; ++i )
{
Items.Add("item_" + i);
}
DataContext = this;
InitializeComponent();
}
void OnScrollViewerScrollChanged(object sender, ScrollChangedEventArgs e)
{
ScrollViewer sv = (ScrollViewer)sender;
var visibleItems = new List<int>();
Rect svViewportBounds = new Rect(sv.HorizontalOffset, sv.VerticalOffset, sv.ViewportWidth, sv.ViewportHeight);
for(int i = 0; i < Items.Count; ++i)
{
var container = itemsHost.ItemContainerGenerator.ContainerFromIndex(i) as FrameworkElement;
if(container != null)
{
var offset = VisualTreeHelper.GetOffset(container);
var bounds = new Rect(offset.X, offset.Y, container.ActualWidth, container.ActualHeight);
if (svViewportBounds.IntersectsWith(bounds))
{
visibleItems.Add(i);
}
}
}
VisibleItems = string.Join(", ", visibleItems.ToArray());
}
}
<Window x:Class="WpfApplication59.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow"
WindowStartupLocation="CenterScreen"
Width="400"
Height="400">
<DockPanel>
<Border BorderThickness="1"
BorderBrush="Black"
DockPanel.Dock="Top">
<TextBlock Text="{Binding VisibleItems}"
Margin="5" />
</Border>
<ItemsControl ItemsSource="{Binding Items}" Name="itemsHost">
<ItemsControl.Template>
<ControlTemplate>
<ScrollViewer HorizontalScrollBarVisibility="Disabled"
ScrollChanged="OnScrollViewerScrollChanged">
<WrapPanel IsItemsHost="True" />
</ScrollViewer>
</ControlTemplate>
</ItemsControl.Template>
<ItemsControl.ItemTemplate>
<DataTemplate>
<Border BorderThickness="1"
BorderBrush="Black"
Width="100"
Height="100">
<Label Content="{Binding}" />
</Border>
</DataTemplate>
</ItemsControl.ItemTemplate>
<ItemsControl.ItemContainerStyle>
<Style TargetType="{x:Type ContentPresenter}">
<Setter Property="Margin"
Value="4" />
</Style>
</ItemsControl.ItemContainerStyle>
</ItemsControl>
</DockPanel>