问WinUI3 ComboBox如何取消所选项

EN

<Page
    x:Class="ComboBoxes.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ComboBoxes"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <StackPanel>
        <ComboBox
            x:Name="ComboBoxControl"
            MinWidth="120"
            IsEditable="True"
            PlaceholderText="please select">
            <ComboBoxItem Tapped="ComboBoxItem_Tapped">A</ComboBoxItem>
            <ComboBoxItem Tapped="ComboBoxItem_Tapped">B</ComboBoxItem>
            <ComboBoxItem Tapped="ComboBoxItem_Tapped">C</ComboBoxItem>
        </ComboBox>
    </StackPanel>
</Page>

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Controls.Primitives;
using Microsoft.UI.Xaml.Input;
using System.Linq;

namespace ComboBoxes;

public sealed partial class MainPage : Page
{
    public MainPage()
    {
        InitializeComponent();
        Loaded += MainPage_Loaded;
    }

    private object? SelectedItemWhenPopupOpened { get; set; }

    private void MainPage_Loaded(object sender, RoutedEventArgs e)
    {
        if (this.ComboBoxControl.FindChildrenOfType<Popup>()?.FirstOrDefault() is Popup popup)
        {
            popup.Opened += Popup_Opened;
        }
    }

    private void Popup_Opened(object? sender, object e)
    {
        SelectedItemWhenPopupOpened = this.ComboBoxControl.SelectedItem;
    }

    private void ComboBoxItem_Tapped(object sender, TappedRoutedEventArgs e)
    {
        if (sender.Equals(SelectedItemWhenPopupOpened) is true)
        {
            this.ComboBoxControl.SelectedIndex = -1;
            this.ComboBoxControl.Text = "";
        }
    }
}

using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Media;
using Microsoft.UI.Xaml;
using System.Collections.Generic;

namespace ComboBoxes;

public static class Extensions
{
    public static IEnumerable<T> FindChildrenOfType<T>(this DependencyObject parent) where T : DependencyObject
    {
        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
        {
            DependencyObject child = VisualTreeHelper.GetChild(parent, i);

            if (child is T childOfT)
            {
                yield return childOfT;
            }

            foreach (T grandChild in child.FindChildrenOfType<T>())
            {
                yield return grandChild;
            }
        }

        if (parent is ContentControl contentControl)
        {
            if (contentControl.Content is T contentOfT)
            {
                yield return contentOfT;
            }

            if (contentControl.Content is DependencyObject dependencyObjectContent)
            {
                foreach (T grandChild in dependencyObjectContent.FindChildrenOfType<T>())
                {
                    yield return grandChild;
                }
            }
        }
    }
}

using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Controls.Primitives;
using Microsoft.UI.Xaml.Input;
using System.Linq;

namespace ComboBoxes;

public sealed partial class MainPage : Page
{
    public MainPage()
    {
        InitializeComponent();
        Loaded += MainPage_Loaded;
    }

    private object? SelectedItemWhenPopupOpened { get; set; }

   private void Page_Loaded(object sender, Microsoft.UI.Xaml.RoutedEventArgs e)
    {
        if (this.ComboBoxControl.FindChildrenOfType<Popup>()?.FirstOrDefault() is Popup popup)
        {
            popup.Opened += Popup_Opened; 
        }
        if (this.ComboBoxControl.FindChildrenOfType<TextBox>()?.FirstOrDefault() is TextBox textBox)
        {
            textBox.TextChanged += TextBox_TextChanged;
        }
    }

    private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        if (string.IsNullOrWhiteSpace((sender as TextBox)?.Text))
        {
            this.ComboBoxControl.SelectedIndex = -1;
            this.ComboBoxControl.Text = "";
            this.ComboBoxControl.SelectedValue = null;
            this.ComboBoxControl.SelectedItem = null;
        }
    }

    private void Popup_Opened(object? sender, object e)
    {
        SelectedItemWhenPopupOpened = this.ComboBoxControl.SelectedItem;
    }

    private void ComboBoxItem_Tapped(object sender, TappedRoutedEventArgs e)
    {
        if (sender.Equals(SelectedItemWhenPopupOpened) is true)
        {
            this.ComboBoxControl.SelectedIndex = -1;
            this.ComboBoxControl.Text = "";
        }
    }
}