WPF automatically fill WrapPanel

//xaml
<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>
            <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)
        {
            for(int i=0;i<100;i++)
            {
                DrawRectsInWrapPanel();
            }
            
        }

        private void DrawRectsInWrapPanel()
        {
            Rectangle rec = new Rectangle();
            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.Fill = brush;
            if(wrapPanel!=null && !wrapPanel.Children.Contains(rec))
            {
                wrapPanel.Children.Add(rec);
            }
        }
    }
}