因此,我在我的视图模型中保持一个名为NewMyItem
的对象作为负责向列表中添加新项的控件的DataContext
。每当执行AddCommand
时,我会重置该对象,使其准备好添加另一个项。
我面临的问题是,一旦在Add
方法内部重置了对象,组合框的SelectionChanged
触发器就不必要地为刚刚添加的项目触发。它本不应该被触发,但即使它被触发了,为什么也会为先前的DataContext
触发呢?
如何避免这种情况,因为我需要将某些业务逻辑放在触发器的命令中,而我无法承受运行两次的风险?
以下是演示我所面临问题的简单示例:
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>
代码后台:
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 );
}
}
}
如果您在ChangeType
方法内设置断点,您会注意到当在Add
方法内执行NewMyItem = new MyItem();
这行代码时,它会不必要地为刚添加的项目运行。