WPF automatically generate shapes fill wrappanel

//customize control
//xaml
<UserControl x:Class="WpfApp309.RectTbk"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WpfApp309"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid>
        <Rectangle Fill="{Binding RecBrush,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>
        <TextBlock Text="{Binding RecStr,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                   HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="30"/>
    </Grid>
</UserControl>


//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp309
{
    /// <summary>
    /// Interaction logic for RectTbk.xaml
    /// </summary>
    public partial class RectTbk : UserControl
    {
        public RectTbk()
        {
            InitializeComponent();
            this.DataContext = this;
        }



        public SolidColorBrush RecBrush
        {
            get { return (SolidColorBrush)GetValue(RecBrushProperty); }
            set { SetValue(RecBrushProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RecBrush.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RecBrushProperty =
            DependencyProperty.Register("RecBrush", typeof(SolidColorBrush),
                typeof(RectTbk), new PropertyMetadata(new SolidColorBrush(Colors.Red)));



        public string RecStr
        {
            get { return (string)GetValue(RecStrProperty); }
            set { SetValue(RecStrProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RecStr.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RecStrProperty =
            DependencyProperty.Register("RecStr", typeof(string), 
                typeof(RectTbk), new PropertyMetadata(""));

    }
}

<Window x:Class="WpfApp309.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp309"
        mc:Ignorable="d" WindowState="Maximized"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <ScrollViewer x:Name="scrollViewer">
            <WrapPanel ItemHeight="300" ItemWidth="200" x:Name="wrapPanel"
            ScrollViewer.HorizontalScrollBarVisibility="Visible"
            ScrollViewer.VerticalScrollBarVisibility="Visible"/>
        </ScrollViewer> 
    </Grid>
</Window>


//cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApp309
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Random rnd { get; set; }
        public MainWindow()
        {
            InitializeComponent();
            rnd = new Random();
            
            this.Loaded += MainWindow_Loaded;
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            wrapPanel.ItemWidth = (this.ActualWidth-40) / 5;
            for (int i=0;i<1000;i++)
            {
                DrawRectsInWrapPanel(i);
            }
            
        }

        private void DrawRectsInWrapPanel(int idx)
        {
            RectTbk rec = new  RectTbk();
            rec.MouseDoubleClick += Rec_MouseDoubleClick;
            Color cl = new Color();
            cl.A = 255;
            cl.R = (byte)rnd.Next(255);
            cl.G = (byte)rnd.Next(255);
            cl.B = (byte)rnd.Next(255);
            SolidColorBrush brush = new SolidColorBrush(cl);
            rec.RecBrush = brush;
            rec.RecStr = $"{idx+1}";
            if(wrapPanel!=null && !wrapPanel.Children.Contains(rec))
            {
                wrapPanel.Children.Add(rec);
            }
        }

        private void Rec_MouseDoubleClick(object sender, MouseButtonEventArgs e)
        {
            var recTbk = sender as RectTbk;
            if(recTbk!=null)
            {
                MessageBox.Show(recTbk.RecStr);
            }
        }
    }
}