在ItemsControl中绑定上下文菜单?

时间:2014-02-04 15:30:28

标签: c# wpf contextmenu

如何将ContextMenu添加到ItemsControl,其中:

  • ItemsControl的ItemsSource位于UserControl的ViewModel中,持有ItemsControl
  • Context Menu的CommandParameter是绑定到ItemsControl中Item的ViewModel。

我跟着this approach。但是,我有一个Command,它从绑定到ItemsControl的ObservableCollection中删除项目。发生这种情况时,会在RelayCommand中抛出异常。在我看来,ContextMenu不是“隐藏”,所以它试图为其命令评估“CanExecute”,但是当项目被删除时,它不能在RelayCommand的CanExecute方法中将参数强制转换为“T”类。

我想知道如何以正确的方式完成我需要的工作。


到目前为止我的实施:

MainViewModel

public class MainViewModel
{
    public ObservableCollection<MyContextMenuClass> ContextMenuItems{ get;set; }
    public ObservableCollection<MyItemClass> MyItems{ get;set; }

    public void AddItem(MyItemClass item)
    {
        MyItems.Add(item);
    }

    public void AddContextMenuItem(MyContextMenuClass item)
    {
        ContextMenuItems.Add(item);
    }

    public MainViewModel(IList<MyItemClass> myItems, IList<MyContextMenuClass> myContextualMenuItems)
    {
        MyItems.AddRange(myItems);
        ContextMenuItems.AddRange(myContextualMenuItems);
    }

    public MainViewModel()
    {}
}

MyItemClass

public class MyItemClass
{
    public string MyText{get;set;}
}

MyContextMenuClass

public class MyContextMenuClass 
{
    public RecentContextMenuItem()
    {}
    public string Caption{get;set;}

    public RelayCommand<MyItemClass> Command{get;set;}        
}

我的UserControl (DataContext = MainViewModel)

<UserControl x:Class="MyNamespace.MyUserControl"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"                         
         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
<UserControl.Resources>        
     <Style x:Key="CommandMenuItemStyle" TargetType="{x:Type MenuItem}" BasedOn="{StaticResource {x:Type MenuItem}}">
                    <Setter Property="MenuItem.Header" Value="{Binding Caption}" />
                    <Setter Property="MenuItem.Command" Value="{Binding Command}" />
                    <Setter Property="MenuItem.CommandParameter" Value="{Binding PlacementTarget.DataContext, 
                        RelativeSource={RelativeSource AncestorType={x:Type ContextMenu}}}" />                        
                </Style>

                <ContextMenu x:Key="ItemContextMenu" ItemsSource="{Binding ContextMenuItems}" 
                 ItemContainerStyle="{StaticResource CommandMenuItemStyle}"
                 DataContext="{Binding DataContext, RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/>

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

    <TextBlock Grid.Row="0" Text="{Binding Title}" Margin="20,5,0,5" Foreground="#FF5D5858" FontFamily="Courier" FontSize="15" Grid.ColumnSpan="2" FontWeight="SemiBold"></TextBlock>

    <ScrollViewer VerticalScrollBarVisibility="Auto" Grid.Row="2" Padding="5,0,0,0">
        <ItemsControl x:Name="myItems" ItemsSource="{Binding MyItems}" >
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding MyText}" /> <!--Simplied this for the example-->                  
                </DataTemplate>                    
            </ItemsControl.ItemTemplate>
            <ItemsControl.ItemContainerStyle>
                <Style TargetType="{x:Type ContentPresenter}">
                    <EventSetter Event="ContextMenu.ContextMenuOpening" Handler="Item_ContextMenuOpening"></EventSetter>
                </Style>
            </ItemsControl.ItemContainerStyle>
        </ItemsControl>
    </ScrollViewer>
</Grid>

我的UserControl Codebehind

public partial class MyUserControl : UserControl
{
    /// <summary>
    /// Initializes a new instance of the <see cref="RecentView"/> class.
    /// </summary>
    public MyUserControl()
    {
        InitializeComponent();
    }

    private void Item_ContextMenuOpening(object sender, ContextMenuEventArgs e)
    {
        var contentPresenter = sender as ContentPresenter;
        if (contentPresenter != null)
        {
            this.Dispatcher.BeginInvoke(new Action<ContentPresenter>(ShowItemContextMenu), new object[] { contentPresenter });
        }
    }

    private void ShowItemContextMenu(ContentPresenter sourceContentPresenter)
    {
        if (sourceContentPresenter != null)
        {
            var ctxMenu = (ContextMenu)this.FindResource("ItemContextMenu");
            ctxMenu.DataContext = this.DataContext;

            if (ctxMenu.Items.Count == 0)
            {
                sourceContentPresenter.ContextMenu = null;
            }
            else
            {
                ctxMenu.PlacementTarget = sourceContentPresenter;
                ctxMenu.IsOpen = true;
            }
        }
    }
}

RemoveItemCommand我添加到MainViewModel

new RelayCommand<MyItemClass>(RemoveItem, (param) => true);

    private void RemoveItem(MyItemClassitemToRemove)
    {
        MyItems.Remove(itemToRemove);
    }

RelayCommand的CanExecute方法

public bool CanExecute(object parameter)
{
    if (_canExecute == null)
    {
        return true;
    }

    if (parameter == null)
    {
        return _canExecute.Invoke(default(T));
    }

    T value;

    try
    {
        value = (T)parameter;                
    }
    catch(Exception exception)
    {
        Trace.TraceError(exception.ToString());
        return _canExecute.Invoke(default(T));
    }

    return _canExecute.Invoke(value);
}

我在 value =(T)参数; 行中收到错误,因为参数已断开连接且无法将其强制转换为 T

我得到例外

  

MyProgram.vshost.exe错误:0:System.InvalidCastException:无法将类型为“MS.Internal.NamedObject”的对象强制转换为“MyItemClass”。      at MyNamespace.RelayCommand`1.CanExecute(Object parameter)in c:\ MyPath \ RelayCommand.cs:line xxx

如果我检查参数,它是一个NamedObject:

  
      
  • 参数{DisconnectedItem} object {MS.Internal.NamedObject}
  •   
  • 非公职人员
          _name“{DisconnectedItem}”string
  •   

问题不在于Exception,它是通过DisconnectedItem到达这一点的事实。这被多次评估。就像ContextMenu在视觉树中永远“永远”一样。

2 个答案:

答案 0 :(得分:8)

首先,只需检查null的参数值:

return parameter == null ? false : _canExecuteMethod((T)parameter); 

其次,这是旧的ContextMenu.DataContext问题:ContextMenu显示在与UI其余部分不同的可视树中。因此,它无法从主UI可视树中访问DataContext。因此,我们必须使用一个小技巧将其传递给另一个可视化树。我们之间的联系是ContextMenu.PlacementTarget property

从链接页面,此属性

  

获取或设置ContextMenu在打开时所处的UIElement。

我们可以使用Tag对象的ContextMenu.PlacementTarget属性来传递DataContext。基本上,只需在要设置Tag的对象上设置ContextMenu属性即可。尝试这样的事情:

<ItemsControl x:Name="myItems" ItemsSource="{Binding MyItems}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding MyText}" Tag="{Binding DataContext, 
                RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}" 
                ContextMenu="{StaticResource ItemContextMenu}" />
        </DataTemplate>                    
    </ItemsControl.ItemTemplate>
</ItemsControl>

...

<ContextMenu x:Key="ItemContextMenu" ItemsSource="{Binding ContextMenuItems}" 
    ItemContainerStyle="{StaticResource CommandMenuItemStyle}"
    DataContext="{Binding PlacementTarget.Tag, RelativeSource={RelativeSource Self}}"/>

就是这样。现在,ContextMenu中声明的UI元素将可以访问数据绑定到Tag属性的任何对象。 EventSetter完全没有必要使用ContextMenu ...如果你知道怎么做就很简单

答案 1 :(得分:-1)