c# - 刷新 ListCollectionView 将 ComboBox 中所选项目的值设置为 null

Question

我有一个带有 ListBox 和两个 ComboBox 的 View 。当我在 ListBox 中选择一个项目时，ComboBoxes 的内容/值会根据所选项目的属性值进行刷新。在我的场景中，ListBox 包含一个客户列表，第一个 ComboBox 包含一个国家/地区列表。所选项目是客户的原籍国。第二个 ComboBox 包含一个城市列表。所选城市是客户的原籍城​​市。

第二个 ComboBox 的 ItemsSource 属性绑定(bind)到一个 ListViewCollection，基于一个 ObservableCollection 所有 使用过滤器的城市。当国家 ListBox 中的选择发生变化时，我刷新过滤器以仅显示属于所选国家/地区的城市。

假设客户 A 来自新西兰奥克兰，客户 B 来自加拿大多伦多。当我选择 A 时，一切正常。第二个 ComboBox 仅填充新西兰城市并选择奥克兰。现在我选择 B，所选国家现在是加拿大，城市列表只包含加拿大城市，多伦多被选中。如果现在我回到 A，在国家/地区中选择了新西兰，城市列表仅包含来自新西兰的城市，但未选择奥克兰。

当我调试这个场景时，我注意到当我选择 B 时，对 ListCollectionView.Refresh() 的调用将最初选择的客户端 A 上的城市值设置为 null(在对 Refresh 的调用处放置一个断点，在模型的城市 setter 上放置另一个断点，请参见下面的代码)。

我猜测 - 虽然我不是 100% 肯定 - 它正在发生是因为我有一个 TwoWay 绑定(bind)到 SelectedItem城市 ComboBox，当过滤器将列表更新为加拿大城市时，奥克兰消失，此信息被发送回属性，然后更新为 null。这在某种程度上是有道理的。

我的问题是:如何避免这种情况发生？当 ItemsSource 仅刷新时，如何防止我的模型上的属性被更新？

下面是我的代码(它有点长，虽然我试图让它尽可能少地重现问题):

public class Country
{
    public string Name { get; set; }
    public IEnumerable<City> Cities { get; set; }
}

public class City
{
    public string Name { get; set; }
    public Country Country { get; set; }
}

public class ClientModel : NotifyPropertyChanged
{
    #region Fields
    private string name;
    private Country country;
    private City city;
    #endregion

    #region Properties
    public string Name
    {
        get
        {
            return this.name;
        }

        set
        {
            this.name = value;
            this.OnPropertyChange("Name");
        }
    }

    public Country Country
    {
        get
        {
            return this.country;
        }

        set
        {
            this.country = value;
            this.OnPropertyChange("Country");
        }
    }

    public City City
    {
        get
        {
            return this.city;
        }

        set
        {
            this.city = value;
            this.OnPropertyChange("City");
        }
    }
    #endregion
}

public class ViewModel : NotifyPropertyChanged
{
    #region Fields
    private ObservableCollection<ClientModel> models;
    private ObservableCollection<Country> countries;
    private ObservableCollection<City> cities;
    private ListCollectionView citiesView;

    private ClientModel selectedClient;
    #endregion

    #region Constructors
    public ViewModel(IEnumerable<ClientModel> models, IEnumerable<Country> countries, IEnumerable<City> cities)
    {
        this.Models = new ObservableCollection<ClientModel>(models);
        this.Countries = new ObservableCollection<Country>(countries);
        this.Cities = new ObservableCollection<City>(cities);
        this.citiesView = (ListCollectionView)CollectionViewSource.GetDefaultView(this.cities);
        this.citiesView.Filter = city => ((City)city).Country.Name == (this.SelectedClient != null ? this.SelectedClient.Country.Name : string.Empty);

        this.CountryChangedCommand = new DelegateCommand(this.OnCountryChanged);
    }
    #endregion

    #region Properties
    public ObservableCollection<ClientModel> Models
    {
        get
        {
            return this.models;
        }

        set
        {
            this.models = value;
            this.OnPropertyChange("Models");
        }
    }

    public ObservableCollection<Country> Countries
    {
        get
        {
            return this.countries;
        }

        set
        {
            this.countries = value;
            this.OnPropertyChange("Countries");
        }
    }

    public ObservableCollection<City> Cities
    {
        get
        {
            return this.cities;
        }

        set
        {
            this.cities = value;
            this.OnPropertyChange("Cities");
        }
    }

    public ListCollectionView CitiesView
    {
        get
        {
            return this.citiesView;
        }
    }

    public ClientModel SelectedClient
    {
        get
        {
            return this.selectedClient;
        }

        set
        {
            this.selectedClient = value;
            this.OnPropertyChange("SelectedClient");
        }
    }

    public ICommand CountryChangedCommand { get; private set; }

    #endregion

    #region Methods
    private void OnCountryChanged(object obj)
    {
        this.CitiesView.Refresh();
    }
    #endregion
}

现在是 XAML:

    <Grid Grid.Column="0" DataContext="{Binding SelectedClient}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="25"/>
            <RowDefinition Height="25"/>
        </Grid.RowDefinitions>

        <TextBlock Grid.Column="0" Grid.Row="0" Text="Country"/>
        <local:ComboBox Grid.Column="1" Grid.Row="0" SelectedItem="{Binding Country}"
                        Command="{Binding DataContext.CountryChangedCommand, RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}"
                        ItemsSource="{Binding DataContext.Countries, RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}">
            <local:ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
            </local:ComboBox.ItemTemplate>
        </local:ComboBox>

        <TextBlock Grid.Column="0" Grid.Row="1" Text="City"/>
        <ComboBox Grid.Column="1" Grid.Row="1" SelectedItem="{Binding City}"
                  ItemsSource="{Binding DataContext.CitiesView, RelativeSource={RelativeSource AncestorType={x:Type local:MainWindow}}}">
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Name}"/>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>
    </Grid>

    <ListBox Grid.Column="1" ItemsSource="{Binding Models}" SelectedItem="{Binding SelectedClient}">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Name}"/>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
</Grid>

如果有任何帮助，这里还有我的自定义 ComboBox 代码，用于处理国家选择更改的通知。

public class ComboBox : System.Windows.Controls.ComboBox, ICommandSource
{
    #region Fields
    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register(
        "Command",
        typeof(ICommand),
        typeof(ComboBox));

    public static readonly DependencyProperty CommandParameterProperty = DependencyProperty.Register(
        "CommandParameter",
        typeof(object),
        typeof(ComboBox));

    public static readonly DependencyProperty CommandTargetProperty = DependencyProperty.Register(
        "CommandTarget",
        typeof(IInputElement),
        typeof(ComboBox));
    #endregion

    #region Properties
    public ICommand Command
    {
        get { return (ICommand)this.GetValue(CommandProperty); }
        set { this.SetValue(CommandProperty, value); }
    }

    public object CommandParameter
    {
        get { return this.GetValue(CommandParameterProperty); }
        set { this.SetValue(CommandParameterProperty, value); }
    }

    public IInputElement CommandTarget
    {
        get { return (IInputElement)this.GetValue(CommandTargetProperty); }
        set { this.SetValue(CommandTargetProperty, value); }
    }
    #endregion

    #region Methods

    protected override void OnSelectionChanged(System.Windows.Controls.SelectionChangedEventArgs e)
    {
        base.OnSelectionChanged(e);

        var command = this.Command;
        var parameter = this.CommandParameter;
        var target = this.CommandTarget;

        var routedCommand = command as RoutedCommand;
        if (routedCommand != null && routedCommand.CanExecute(parameter, target))
        {
            routedCommand.Execute(parameter, target);
        }
        else if (command != null && command.CanExecute(parameter))
        {
            command.Execute(parameter);
        }
    }
    #endregion
}

对于这个简化的示例，我在 Window 的构造函数中创建并填充 View 模型，此处:

public MainWindow()
{
    InitializeComponent();

    Country canada = new Country() { Name = "Canada" };
    Country germany = new Country() { Name = "Germany" };
    Country vietnam = new Country() { Name = "Vietnam" };
    Country newZealand = new Country() { Name = "New Zealand" };

    List<City> canadianCities = new List<City>
    {
        new City { Country = canada, Name = "Montréal" },
        new City { Country = canada, Name = "Toronto" },
        new City { Country = canada, Name = "Vancouver" }
    };
    canada.Cities = canadianCities;

    List<City> germanCities = new List<City>
    {
        new City { Country = germany, Name = "Frankfurt" },
        new City { Country = germany, Name = "Hamburg" },
        new City { Country = germany, Name = "Düsseldorf" }
    };
    germany.Cities = germanCities;

    List<City> vietnameseCities = new List<City>
    {
        new City { Country = vietnam, Name = "Ho Chi Minh City" },
        new City { Country = vietnam, Name = "Da Nang" },
        new City { Country = vietnam, Name = "Hue" }
    };
    vietnam.Cities = vietnameseCities;

    List<City> newZealandCities = new List<City>
    {
        new City { Country = newZealand, Name = "Auckland" },
        new City { Country = newZealand, Name = "Christchurch" },
        new City { Country = newZealand, Name = "Invercargill" }
    };
    newZealand.Cities = newZealandCities;

    ObservableCollection<ClientModel> models = new ObservableCollection<ClientModel>
    {
        new ClientModel { Name = "Bob", Country = newZealand, City = newZealandCities[0] },
        new ClientModel { Name = "John", Country = canada, City = canadianCities[1] }
    };

    List<Country> countries = new List<Country>
    {
        canada, newZealand, vietnam, germany
    };

    List<City> cities = new List<City>();
    cities.AddRange(canadianCities);
    cities.AddRange(germanCities);
    cities.AddRange(vietnameseCities);
    cities.AddRange(newZealandCities);

    ViewModel vm = new ViewModel(models, countries, cities);

    this.DataContext = vm;
}

应该可以通过简单地复制/粘贴上述所有代码来重现该问题。我正在使用 .NET 4.0。

最后，我阅读了this article (和其他一些人)并尝试将给定的建议适应/应用到我的案例中，但没有成功。我想我做错了:

我还读了this question但是如果我的 ListBox 变大，我可能最终不得不明确地跟踪数百个项目，如果可能的话我不想这样做。

Answer 1

您的模型有点多余。你有国家列表，每个国家都有城市列表。然后，您撰写了整个城市列表，您在选择更改时进行更新。如果您要更改城市 ComboBox 的数据源，您将获得所需的行为:

    <ComboBox Grid.Column="1" Grid.Row="1" SelectedItem="{Binding City}"
              ItemsSource="{Binding Country.Cities}">
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Name}"/>
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>

您猜对了城市设置为 null 的原因。

但是如果你想像上面描述的那样保留你的模型——你应该改变方法调用的顺序。为此，您应该使用 Application.Current.Dispatcher属性(并且您不需要更改上面提到的 ComboBox):

private void OnCountryChanged()
{
    var uiDispatcher = System.Windows.Application.Current.Dispatcher;
    uiDispatcher.BeginInvoke(new Action(this.CitiesView.Refresh));
}