c# - ComboBox 触发器在更改 DataContext 时触发旧 DataContext

Question

因此，我在我的 View 模型中保留了一个对象 NewMyItem 作为负责在列表中添加新项目的控件的 DataContext。每当执行 AddCommand 时，我都会重置该对象，以便它可以添加另一个项目。

我在这里面临的问题是，一旦对象在 Add 方法中被重置，组合框的 SelectionChanged 触发器就会不必要地引发 < strong>刚刚添加的项目。它不应该首先被触发，但即使它被触发，为什么它会为前面的 DataContext 触发？

如何避免这种情况，因为我需要在触发器的命令中放置一些我无法承受运行两次的业务逻辑？

这是一个演示我面临的问题的简单示例:

XAML:

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:local="clr-namespace:WpfApplication2"
        Title="MainWindow" Height="350" Width="525"
        DataContext="{Binding RelativeSource={RelativeSource Self}}">

    <Window.Resources>
        <local:ChangeTypeConverter x:Key="changeTypeConverter" />

        <local:MyItems x:Key="myItems">
            <local:MyItem Name="Item 1" Type="1" />
            <local:MyItem Name="Item 2" Type="2" />
            <local:MyItem Name="Item 3" Type="3" />
        </local:MyItems>
    </Window.Resources>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <Grid Grid.Row="0" DataContext="{Binding DataContext.NewMyItem, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition Width="Auto" />
                <ColumnDefinition Width="Auto" />
            </Grid.ColumnDefinitions>

            <TextBox Grid.Column="0" Width="100" Text="{Binding Name, Mode=TwoWay}" />
            <ComboBox Grid.Column="1" Margin="10,0,0,0" Width="40" SelectedValue="{Binding Type, Mode=OneWay}"
                      ItemsSource="{Binding DataContext.Types, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="SelectionChanged">
                        <i:InvokeCommandAction Command="{Binding DataContext.ChangeTypeCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">
                            <i:InvokeCommandAction.CommandParameter>
                                <MultiBinding Converter="{StaticResource changeTypeConverter}">
                                    <Binding />
                                    <Binding Path="SelectedValue" RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType={x:Type ComboBox}}" />
                                </MultiBinding>
                            </i:InvokeCommandAction.CommandParameter>
                        </i:InvokeCommandAction>
                    </i:EventTrigger>
                </i:Interaction.Triggers>
            </ComboBox>

            <Button Grid.Column="2" Margin="10,0,0,0"
                    Command="{Binding DataContext.AddCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">Add</Button>
        </Grid>

        <ListBox Grid.Row="1" ItemsSource="{StaticResource myItems}">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Grid.Column="0" Width="100" Text="{Binding Name}" Foreground="Black" />
                        <TextBlock Grid.Column="1" Margin="10,0,0,0" Text="{Binding Type, StringFormat='Type {0}'}" Foreground="Black" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
    </Grid>
</Window>

代码隐藏:

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication2
{
  /// <summary>
  /// Interaction logic for MainWindow.xaml
  /// </summary>
  public partial class MainWindow : Window
  {
    public ICommand AddCommand { get; private set; }
    public ICommand ChangeTypeCommand { get; private set; }
    public IEnumerable<int> Types { get; private set; }

    public static readonly System.Windows.DependencyProperty NewMyItemProperty = System.Windows.DependencyProperty.Register( "NewMyItem", typeof( MyItem ), typeof( MainWindow ) );
    public MyItem NewMyItem { get { return (MyItem) GetValue( NewMyItemProperty ); } protected set { SetValue( NewMyItemProperty, value ); } }

    public MainWindow()
    {
      InitializeComponent();
      Types = new List<int> { 1, 2, 3 };
      NewMyItem = new MyItem();
      AddCommand = new MyCommand( Add );
      ChangeTypeCommand = new MyCommand<Tuple<MyItem, int>>( ChangeType );
    }

    private void Add()
    {
      MyItems myItems = Resources[ "myItems" ] as MyItems;
      myItems.Add( NewMyItem );
      NewMyItem = new MyItem();
    }

    private void ChangeType( Tuple<MyItem, int> tuple )
    {
      MyItem myItem = tuple.Item1;
      int type = tuple.Item2;

      myItem.Type = type;

      // TODO : some business checks
      // if(myItem.Type == 1)
      // if(myItem.Type == 2)
      // ...
    }
  }

  public class ChangeTypeConverter : IMultiValueConverter
  {
    public object Convert( object[] values, Type targetType, object parameter, CultureInfo culture )
    {
      if( values != null && values.Length > 1 && values[ 0 ] is MyItem && values[ 1 ] is int )
        return new Tuple<MyItem, int>( (MyItem) values[ 0 ], (int) values[ 1 ] );

      return values;
    }

    public object[] ConvertBack( object value, Type[] targetTypes, object parameter, CultureInfo culture )
    {
      throw new NotSupportedException();
    }
  }

  public class MyItem : DependencyObject
  {
    public static readonly DependencyProperty NameProperty = DependencyProperty.Register( "Name", typeof( string ), typeof( MyItem ) );
    public string Name { get { return (string) GetValue( NameProperty ); } set { SetValue( NameProperty, value ); } }

    public static readonly DependencyProperty TypeProperty = DependencyProperty.Register( "Type", typeof( int ), typeof( MyItem ) );
    public int Type { get { return (int) GetValue( TypeProperty ); } set { SetValue( TypeProperty, value ); } }
  }

  public class MyItems : ObservableCollection<MyItem>
  {

  }

  public class MyCommand : ICommand
  {
    private readonly Action executeMethod = null;
    private readonly Func<bool> canExecuteMethod = null;

    public MyCommand( Action execute )
      : this( execute, null )
    {
    }

    public MyCommand( Action execute, Func<bool> canExecute )
    {
      executeMethod = execute;
      canExecuteMethod = canExecute;
    }

    public event EventHandler CanExecuteChanged;

    public void NotifyCanExecuteChanged( object sender )
    {
      if( CanExecuteChanged != null )
        CanExecuteChanged( sender, EventArgs.Empty );
    }

    public bool CanExecute( object parameter )
    {
      return canExecuteMethod != null ? canExecuteMethod() : true;
    }

    public void Execute( object parameter )
    {
      if( executeMethod != null )
        executeMethod();
    }
  }

  public class MyCommand<T> : ICommand
  {
    private readonly Action<T> executeMethod = null;
    private readonly Predicate<T> canExecuteMethod = null;

    public MyCommand( Action<T> execute )
      : this( execute, null )
    {
    }

    public MyCommand( Action<T> execute, Predicate<T> canExecute )
    {
      executeMethod = execute;
      canExecuteMethod = canExecute;
    }

    public event EventHandler CanExecuteChanged;

    public void NotifyCanExecuteChanged( object sender )
    {
      if( CanExecuteChanged != null )
        CanExecuteChanged( sender, EventArgs.Empty );
    }

    public bool CanExecute( object parameter )
    {
      return canExecuteMethod != null && parameter is T ? canExecuteMethod( (T) parameter ) : true;
    }

    public void Execute( object parameter )
    {
      if( executeMethod != null && parameter is T )
        executeMethod( (T) parameter );
    }
  }
}

如果在 ChangeType 方法中放置一个断点，您会注意到当行 NewMyItem = new MyItem(); 时，它不必要地为刚刚添加的项目运行在 Add 方法中执行。

Answer 1

您可以使用 ComboBox.DropDownClosed 而不是使用 ComboBox.SelectionChanged 事件事件:

Occurs when the drop-down list of the ComboBox closes.

例子:

<ComboBox Name="MyComboBox" Grid.Column="1" Margin="10,0,0,0" Width="40" SelectedValue="{Binding Type, Mode=OneWay}"                     
            ItemsSource="{Binding DataContext.Types, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">

    <i:Interaction.Triggers>
        <i:EventTrigger EventName="DropDownClosed"
                        SourceObject="{Binding ElementName=MyComboBox}">

            <i:InvokeCommandAction Command="{Binding DataContext.ChangeTypeCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MainWindow}}}">
                <i:InvokeCommandAction.CommandParameter>
                    <MultiBinding Converter="{StaticResource changeTypeConverter}">
                        <Binding />
                        <Binding Path="SelectedValue" RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType={x:Type ComboBox}}" />
                    </MultiBinding>
                </i:InvokeCommandAction.CommandParameter>
            </i:InvokeCommandAction>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</ComboBox>

在这种情况下，ChangeType 命令只会被调用一次。