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))
+        {
+            
+        }
+    }
 }

src/Ursa/Controls/DateTimePicker/DatePicker.cs

@@ -52,16 +52,7 @@ public class DatePicker: DatePickerBase, IClearControl
 
     private void OnSelectionChanged(AvaloniaPropertyChangedEventArgs<DateTime?> args)
     {
-        if (args.NewValue.Value is null)
+        SyncSelectedDateToText(args.NewValue.Value);
-        {
-            _calendar?.ClearSelection();
-            _textBox?.Clear();
-        }
-        else
-        {
-            _calendar?.MarkDates(startDate: args.NewValue.Value, endDate: args.NewValue.Value);
-            _textBox?.SetValue(TextBox.TextProperty, args.NewValue.Value.Value.ToString(DisplayFormat ?? "yyyy-MM-dd"));
-        }
     }
 
     protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
@@ -91,6 +82,7 @@ public class DatePicker: DatePickerBase, IClearControl
         {
             _calendar.DateSelected += OnDateSelected;
         }
+        SyncSelectedDateToText(SelectedDate);
     }
 
     private void OnDateSelected(object? sender, CalendarDayButtonEventArgs e)
@@ -121,6 +113,20 @@ public class DatePicker: DatePickerBase, IClearControl
     {
         SetSelectedDate(true);
     }
+    
+    private void SyncSelectedDateToText(DateTime? date)
+    {
+        if (date is null)
+        {
+            _textBox?.SetValue(TextBox.TextProperty, null);
+            _calendar?.ClearSelection();
+        }
+        else
+        {
+            _textBox?.SetValue(TextBox.TextProperty, date.Value.ToString(DisplayFormat ?? "yyyy-MM-dd"));
+            _calendar?.MarkDates(startDate: date.Value, endDate: date.Value);
+        }
+    }
 
     private void SetSelectedDate(bool fromText = false)
     {

src/Ursa/Controls/DateTimePicker/DateRangePicker.cs

@@ -165,6 +165,42 @@ public class DateRangePicker : DatePickerBase, IClearControl
             _endCalendar.DatePreviewed += OnDatePreviewed;
             _endCalendar.ContextDateChanged += OnContextDateChanged;
         }
+        SyncDateToText();
+    }
+
+    private void SyncDateToText()
+    {
+        if (SelectedStartDate is not null)
+        {
+            _startTextBox?.SetValue(TextBox.TextProperty, SelectedStartDate.Value.ToString(DisplayFormat ?? "yyyy-MM-dd"));
+        }
+        if (SelectedEndDate is not null)
+        {
+            _endTextBox?.SetValue(TextBox.TextProperty, SelectedEndDate.Value.ToString(DisplayFormat ?? "yyyy-MM-dd"));
+        }
+
+        if (SelectedStartDate is null)
+        {
+            _startCalendar?.ClearSelection();
+        }
+        if(SelectedEndDate is null)
+        {
+            _endCalendar?.ClearSelection();
+        }
+        if(SelectedStartDate is not null && SelectedEndDate is not null)
+        {
+            _startCalendar?.MarkDates(SelectedStartDate, SelectedEndDate);
+            _endCalendar?.MarkDates(SelectedStartDate, SelectedEndDate);
+        }
+        else if(SelectedStartDate is not null)
+        {
+            _startCalendar?.MarkDates(SelectedStartDate, SelectedStartDate);
+        }
+        else if(SelectedEndDate is not null)
+        {
+            _endCalendar?.MarkDates(SelectedEndDate, SelectedEndDate);
+        }
+        PseudoClasses.Set(PseudoClassName.PC_Empty, SelectedStartDate is null && SelectedEndDate is null);
     }
 
     private void OnTextBoxLostFocus(object? sender, RoutedEventArgs e)
@@ -381,4 +417,6 @@ public class DateRangePicker : DatePickerBase, IClearControl
 
         SetCurrentValue(IsDropdownOpenProperty, true);
     }
+    
+    
 }