MainWindow.xaml

 <controls:MList Items="{Binding Photos}">
                    <controls:MList.ItemTemplate>
                        <DataTemplate>
                            <Grid>
                                <TextBlock Text="{Binding urls}"></TextBlock>
                            </Grid>
                        </DataTemplate>
                    </controls:MList.ItemTemplate>
                </controls:MList>

MList.cs

using System;
using Avalonia;
using Avalonia.Controls;
using Avalonia.Controls.Primitives;
using Avalonia.Interactivity;

namespace Switch_Wallparer.Views.Controls
{
    public class MList : ListBox
    {
        public MList()
        {
            foreach (var item in Items)
            {
                Console.WriteLine(item);
            }
        }
    }
}

MList.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:controls="using:Switch_Wallparer.Views.Controls">
    <Design.PreviewWith>
        <controls:MList />
    </Design.PreviewWith>

    <Style Selector="controls|MList">
        <!-- Set Defaults -->
        <Setter Property="Template">
            <ControlTemplate>
                <Grid></Grid>
            </ControlTemplate>
        </Setter>
    </Style>
</Styles>