从ViewModel在DataGrid中使某些行无法选择

Question

I am building a WPF client following the MVVM pattern. The client uses a wpf DataGrid to display some data with some complex grouping requirements. Rather than try to find a way to display all this grouping within the grid, I simply generate synthetic entries for group headers and footers.

All this works ok but it leaves me with the problem that I have rows in my DataGrid that should not be selectable. I was hoping I could control the selection from the View Model but that doesn't seem to be working. I have created a sample project that illustrates the problem.

App.xaml

<Application x:Class="TestMVVM.App"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="clr-namespace:TestMVVM.ViewModel"
         StartupUri="MainWindow.xaml">
  <Application.Resources>
    <ResourceDictionary>
        <vm:MainViewModel x:Key="MainViewModel"/>
    </ResourceDictionary>
  </Application.Resources>
</Application>

MainWindow.xaml

<Window x:Class="TestMVVM.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525" DataContext="{StaticResource MainViewModel}">
  <Grid>
    <DataGrid ItemsSource="{Binding MyItems}" SelectedItem="{Binding MySelectedItem}" IsSynchronizedWithCurrentItem="True"
              AutoGenerateColumns="False" IsReadOnly="True" SelectionMode="Single" SelectionUnit="FullRow">
        <!-- In the real app I use Style triggers here to highlight title and total rows and make them unselectable from the mouse,
             but you can still select them via the keyboard and when the grid is initially displayed it's on an invalid row -->
        <DataGrid.Columns>
            <DataGridTextColumn Header="Name" Binding="{Binding Name}"/>
            <DataGridTextColumn Header="Count" Binding="{Binding Count}"/>
        </DataGrid.Columns>
    </DataGrid>
  </Grid>
</Window>

MyItem.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestMVVM.Model
{
  public class MyItem
  {
    public MyItem(string name, int count)
    {
        Name = name;
        Count = count;
    }

    public string Name { get; private set; }
    public int Count { get; private set; }
  }

  public class MyItemTitle
  {
    public MyItemTitle(string name)
    {
        Name = name;
    }

    public string Name { get; private set; }
  }

  public class MyItemTotal
  {
    public MyItemTotal(string name, int total)
    {
        Name = name;
        Count = total;
    }

    public string Name { get; private set; }
    public int Count { get; private set; }
  }
}

MainViewModel.cs

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Data;
using TestMVVM.Model;

namespace TestMVVM.ViewModel
{
  public class MainViewModel : INotifyPropertyChanged
  {
    private object _mySelectedItem;
    private int _lastIndex = -1;

    public MainViewModel()
    {
        var myItems = new List<object> {
            new MyItemTitle("Top Title"),
            new MyItemTitle("Subtitle"),
            new MyItem("Real Item", 1),
            new MyItem("Second Real Item", 4),
            new MyItemTotal("Subtitle totals", 5),
            new MyItem("Third Real Item", 11),
            new MyItemTotal("Top Title Totals", 16)
        };

        // Initially I used a List<> here but I thought maybe ObservableCollection
        // might make a difference.  As you can see, it does not.
        MyItems = new ObservableCollection<object>(myItems);
    }

    public IList<object> MyItems { get; private set; }

    public object MySelectedItem
    {
        get { return _mySelectedItem; }
        set
        {
            SetValidItem(value);
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void SetValidItem(object value)
    {
        var newIndex = MyItems.IndexOf(value);

        if (newIndex == _lastIndex) return;  // no change, doubt this can happen

        if (IsItemValid(value))
        {
            _mySelectedItem = value;
            _lastIndex = newIndex;

            return;                         // selection worked as DataGrid expected
        }

        if (newIndex > _lastIndex)          // selection going down the grid
        {
            for (var i = newIndex + 1; i < MyItems.Count; ++i)
            {
                if (IsItemValid(MyItems[i]))
                {
                    _mySelectedItem = MyItems[i];
                    _lastIndex = i;
                }
            }
        }
        else if (newIndex < _lastIndex)    // selection going up the grid
        {
            for (var i = newIndex - 1; i > -1; --i)
            {
                if (IsItemValid(MyItems[i]))
                {
                    _mySelectedItem = MyItems[i];
                    _lastIndex = i;
                }
            }
        }

        // three possible scenarios when we get here:
        //   1) selection went up higher than selected in grid
        //   2) selection went down lower than selected in grid
        //   3) no valid higher/lower item was found so we keep the previous selection
        // in any of those cases we need to raise an event so the grid knows
        // that SelectedItem has moved

        RaiseOnMySelectedItemChanged();

        // I checked in the debugger and the event fired correctly and the DataGrid
        // called the getter again with the correct value.  The grid seemed to ignore
        // it though so I thought I could force the issue with this.  This doesn't seem
        // to do anything either (I even tried collectionView.MoveCurrentToLast())
        var collectionView = CollectionViewSource.GetDefaultView(MyItems);
        collectionView.MoveCurrentTo(_mySelectedItem);
    }

    private bool IsItemValid(object value)
    {
        return value is MyItem;
    }

    private void RaiseOnMySelectedItemChanged()
    {
        var handler = PropertyChanged;
        if (handler != null)
        {
            var args = new PropertyChangedEventArgs("MySelectedItem");

            handler(this, args);
        }
    }
  }
}

Answer 1

1.One way to approach this is to extend the DataGrid . Then in the DataGridExtended determine whether certain Row or Cell needs to be skipped from selection (by maybe moving the focus to the next cell or row). The processing part is easy. What's harder is to get the cell or the row. Here's a post how to do that: How to get a cell from DataGrid?

In your ViewModel or model create properties that mark an item as not-selectable, and respond to it in the DataGridExtended.

2.There is also an interesting post on the subject that might work for you, or be an additional helper. It basically creates a style to make the row unfocusable. Making a row non-focusable in a WPF datagrid

3.you can directly respond and cancel selection events in code behind. Together with (part 2.) this might do the trick. Here's another link how to get started: http://wpf.codeplex.com/wikipage?title=Single-Click%20Editing&referringTitle=Tips%20%26%20Tricks