c# - UWP 反射(reflect)通过绑定(bind)从设置中加载的 ComboBox 选定项

Question

这是 an earlier question 的延续.

对于某些应用程序设置，我想使用 ComboBox 来选择一个选项。我可以将所选选项保存到(漫游)设置并再次加载。加载的选项正确显示在 TextBlock 中，但 ComboBox 显示空白。我怎样才能在 ComboBox 中反射(reflect)加载的当前选择的选项？

这是 XAML:

<Page
x:Class="ComboBoxTest.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:ComboBoxTest"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:converter="using:ComboBoxTest.Converter"
mc:Ignorable="d">

<Page.Resources>
    <converter:ComboBoxItemConvert x:Key="ComboBoxItemConvert" />
</Page.Resources>


<Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <StackPanel>
        <ComboBox 
            Name="ComboBox" 
            ItemsSource="{x:Bind ComboBoxOptions}" 
            SelectedItem="{x:Bind SelectedComboBoxOption, Mode=TwoWay, Converter={StaticResource ComboBoxItemConvert}}" 
            SelectedValuePath="ComboBoxOption" 
            DisplayMemberPath="ComboBoxHumanReadableOption"  
            Header="ComboBox" >
        </ComboBox>
        <TextBlock Name="BoundTextblock" Text="{x:Bind SelectedComboBoxOption.ComboBoxOption, Mode=OneWay}"/>
    </StackPanel>
</Grid>

这是背后的代码:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Xml.Serialization;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.Storage;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

// The Blank Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409

namespace ComboBoxTest
{
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page, INotifyPropertyChanged
{
    ApplicationDataContainer roamingSettings = null;

    private ObservableCollection<ComboBoxItem> ComboBoxOptions;

    public MainPage()
    {
        this.InitializeComponent();
        ComboBoxOptions = new ObservableCollection<ComboBoxItem>();
        ComboBoxOptionsManager.GetComboBoxList(ComboBoxOptions);


        roamingSettings = ApplicationData.Current.RoamingSettings;


        var value = (string)roamingSettings.Values["ComboBoxSelection"];
        if (value != null)
        {
            SelectedComboBoxOption = Deserialize<ComboBoxItem>(value); //loaded selection reflected in the textbox but not in the ComboBox
        }
        else
        {
            SelectedComboBoxOption = ComboBoxOptions[0];
        }

    }

    public class ComboBoxItem
    {
        public string ComboBoxOption { get; set; }
        public string ComboBoxHumanReadableOption { get; set; }
    }

    public class ComboBoxOptionsManager
    {
        public static void GetComboBoxList(ObservableCollection<ComboBoxItem> ComboBoxItems)
        {
            var allItems = getComboBoxItems();
            ComboBoxItems.Clear();
            allItems.ForEach(p => ComboBoxItems.Add(p));
        }

        private static List<ComboBoxItem> getComboBoxItems()
        {
            var items = new List<ComboBoxItem>();

            items.Add(new ComboBoxItem() { ComboBoxOption = "Option1", ComboBoxHumanReadableOption = "Option 1" });
            items.Add(new ComboBoxItem() { ComboBoxOption = "Option2", ComboBoxHumanReadableOption = "Option 2" });
            items.Add(new ComboBoxItem() { ComboBoxOption = "Option3", ComboBoxHumanReadableOption = "Option 3" });

            return items;
        }
    }

    private ComboBoxItem _SelectedComboBoxOption;

    public ComboBoxItem SelectedComboBoxOption
    {
        get
        {
            return _SelectedComboBoxOption;
        }
        set
        {
            if (_SelectedComboBoxOption != value)
            {
                _SelectedComboBoxOption = value;
                roamingSettings.Values["ComboBoxSelection"] = Serialize(value);
                RaisePropertyChanged("SelectedComboBoxOption");
            }
        }
    }




    public static string Serialize(object obj)
    {
        using (var sw = new StringWriter())
        {
            var serializer = new XmlSerializer(obj.GetType());
            serializer.Serialize(sw, obj);
            return sw.ToString();
        }
    }

    public static T Deserialize<T>(string xml)
    {
        using (var sw = new StringReader(xml))
        {
            var serializer = new XmlSerializer(typeof(T));
            return (T)serializer.Deserialize(sw);
        }
    }


    void RaisePropertyChanged(string prop)
    {
        if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(prop)); }
    }
    public event PropertyChangedEventHandler PropertyChanged;
 }
}

Answer 1

这是一个典型的项目案例，看起来相同，但由于它们有不同的引用(请阅读 C# reference types)。

您使用 3 个值加载 ComboBox，这 3 个值显示在下拉列表中。要在 ComboBox 关闭时查看所选项目，它必须是(=具有相同的引用)这 3 个值之一。当您的漫游设置中没有保存任何内容时，您可以选择第一个作为 SelectedItem。切换到另一个选定项时，您还将在 SelectedItem 属性中拥有一个有效引用。

但是，当您反序列化保存的 RoamingSettings 值时，您会创建一个具有不同引用的新对象。当您将此项目设置为 SelectedItem 时，ComboBox 控件将不会在其项目中找到它，因此不会选择项目。

要解决此问题，您必须在 ItemSource 集合中找到正确的项目:

var value = (string)roamingSettings.Values["ComboBoxSelection"];
if (value != null)
{
    var deserialized = Deserialize<ComboBoxItem>(value);
    // using ComboBoxOption as the primary key field of your object
    SelectedComboBoxOption = ComboBoxOptions.SingleOrDefault(c => 
                c.ComboBoxOption == deserialized.ComboBoxOption);
}
else
{
    SelectedComboBoxOption = ComboBoxOptions[0];
}