fix: Fix DatePicker initialization issue.

demo/Ursa.Demo/Pages/DatePickerDemo.axaml

@@ -3,6 +3,8 @@
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              xmlns:u="https://irihi.tech/ursa"
+             xmlns:viewModels="clr-namespace:Ursa.Demo.ViewModels"
+             x:DataType="viewModels:DatePickerDemoViewModel"
              mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
              x:Class="Ursa.Demo.Pages.DatePickerDemo">
     <StackPanel Margin="20" HorizontalAlignment="Left">
@@ -10,5 +12,9 @@
         <TextBlock Text="{Binding #singlePicker.SelectedDate}" ></TextBlock>
         <u:DatePicker Name="singlePicker" Width="200" Classes="ClearButton" />
         <u:DateRangePicker Width="300" DisplayFormat="yyyyMMdd" Classes="ClearButton" />
+        
+        <TextBlock Text="Binding"></TextBlock>
+        <u:DatePicker Width="200" SelectedDate="{Binding SelectedDate, Mode=TwoWay}"/>
+        <u:DateRangePicker Width="300" SelectedStartDate="{Binding StartDate}" SelectedEndDate="{Binding EndDate}"/>
     </StackPanel>
 </UserControl>

demo/Ursa.Demo/ViewModels/DatePickerDemoViewModel.cs

@@ -1,6 +1,28 @@
-namespace Ursa.Demo.ViewModels;
+using System;
+using System.ComponentModel;
+using CommunityToolkit.Mvvm.ComponentModel;
 
-public class DatePickerDemoViewModel
+namespace Ursa.Demo.ViewModels;
+
+public partial class DatePickerDemoViewModel: ObservableObject
 {
-    
+    [ObservableProperty] private DateTime? _selectedDate;
+    [ObservableProperty] private DateTime? _startDate;
+    [ObservableProperty] private DateTime? _endDate;
+
+    public DatePickerDemoViewModel()
+    {
+        SelectedDate = DateTime.Today;
+        StartDate = DateTime.Today;
+        EndDate = DateTime.Today.AddDays(7);
+    }
+
+    protected override void OnPropertyChanged(PropertyChangedEventArgs e)
+    {
+        base.OnPropertyChanged(e);
+        if (e.PropertyName == nameof(SelectedDate))
+        {
+            
+        }
+    }
 }