如何单独从ViewModel以编程方式突出显示DataGrid行？

Question

The SelectedItem property binding is not causing its DataGrid row to be highlighted on initial load.

I have a DataGrid with a binding on SelectedItem that is not getting highlighted until I click it again. I think I have an order of operations problem--coming from the fact that all the ViewModel code runs before the Views get rendered. It works fine once I click a row (even the same one that's already in the SelectedAccount prop), but I need to able to highlight a row from the VM.

I can easily verify the SelectedAccount property is not null because there are other ViewModels that display it's values via PubSubEvents.

I've tried several solutions methods, and the only way I've been able to get it to work so far just feels dirty:

using ApplicationName.UI.ViewModels;
public partial class AccountsView : UserControl
{
    public AccountsView()
    {
        InitializeComponent();
        Loaded += AccountsView_Loaded;
    }

    private void AccountsView_Loaded(object sender, RoutedEventArgs e)
    {
        AccountsViewModel viewModel = (AccountsViewModel)DataContext;
        AccountsDataGrid.SelectedItem = viewModel.SelectedAccount;
        AccountsDataGrid.Focus();
        UpdateLayout();
    }   
}

I don't like this because it causes the OnPropertyChanged event to fire twice, once before the views load, and and again after the above hack. This triggers a SQL call, so I want to avoid that. I also thought the point of MVVM was to decouple the view from the viewmodel, but maybe I'm not understanding that as well as I thought.

Here's the XAML for the DataGrid:

<ScrollViewer Grid.Row="1"
              Grid.Column="0"
              Grid.ColumnSpan="3"
              Margin="5,0">
    <DataGrid Name="AccountsDataGrid"
              ItemsSource="{Binding Accounts}"
              SelectedItem="{Binding SelectedAccount}"
              AutoGenerateColumns="False"
              CanUserResizeColumns="True"
              SelectionMode="Single"
              SelectionUnit="FullRow">
        <DataGrid.Columns>
            <DataGridTextColumn Header="ClinicId" 
                                TextBlock.TextAlignment="Center"
                                Width="75"
                                Binding="{Binding ClinicId}" />
            <DataGridTextColumn Header="Account#"
                                Width="75"
                                Binding="{Binding AccountNumber}"/>
            <DataGridTextColumn Header="LastName"
                                Width="1*"
                                Binding="{Binding LastName}" />
            <DataGridTextColumn Header="FirstName"
                                Width="1*"
                                Binding="{Binding FirstName}" />
            <DataGridTextColumn Header="Balance"
                                Width="Auto"
                                Binding="{Binding Balance}" />
            <DataGridTextColumn Header="Follow Up"
                                Width="100"
                                Binding="{Binding FollowUpDate}"/>
        </DataGrid.Columns>
    </DataGrid>
</ScrollViewer>

And the intial Load method in the ViewModel, which is where I want to set the highlighted row.

public void Load()
{
    RefreshGrid();
    SelectedAccount = Accounts.First();
    _accountId = SelectedAccount.Id;
}

EDIT

The issue was subtle, but makes perfect sense now.

private Account _selectedAccount;

public Account SelectedAccount
{
    get => _selectedAccount;
    set => SetSelectedAccount(value);
}

private void SetSelectedAccount(Account value)
{
    _selectedAccount = value;
    OnPropertyChanged("_selectedAccount");  // <= whoops
    if (_selectedAccount != null)
        OnAccountSelected(
           _selectedAccount.PrimaryKeyFields);
}

Raising this event for a private property doesn't make sense, as the view cannot see it, and is bound to SelectedAccount. Changing it to OnPropertyChanged("SelectedAccount") did the trick.

Answer 1

Implementing INotifyPropertyChanged should be enough, this code works on my end, I'm using a Command to call the Load() method but it's probably not needed in your code.

ViewModel and C# code :

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        DataContext = new ViewModel();
    }
}

public class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    public ViewModel()
    {
        Accounts = new List<Account>();

        Accounts.AddRange(
            Enumerable.Range(0, 10)
                .Select(r => new Account
                {
                    AccountNumber = r,
                    FirstName = $"First{r}",
                    LastName = $"Last{r}"
                }));

        LoadedCommand = new WpfCommand((param) => Load());
    }

    private void Load()
    {
        SelectedAccount = Accounts.FirstOrDefault(a => a.AccountNumber == 2);
    }

    public WpfCommand LoadedCommand { get; set; }

    public List<Account> Accounts { get; set; }

    private Account _selectedAccount = null;
    public Account SelectedAccount
    {
        get { return _selectedAccount; }
        set
        {
            _selectedAccount = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(SelectedAccount)));
        }
    }
}

public class Account
{
    public int AccountNumber { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class WpfCommand : ICommand
{
    private Action<object> _execute;
    private Func<object, bool> _canExecute;

    public event EventHandler CanExecuteChanged;

    public WpfCommand(Action<object> execute, Func<object, bool> canExecute = null)
    {
        _execute = execute;
        _canExecute = canExecute;
    }

    public void Execute(object parameter)
    {
        _execute?.Invoke(parameter);
    }

    public bool CanExecute(object parameter)
    {
        return _canExecute?.Invoke(parameter) ?? true;
    }
}

XAML :

   <!--System.Windows.Interactivity.WPF nuget package-->
   xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"

    <DataGrid 
        ItemsSource="{Binding Accounts}"
        SelectedItem="{Binding SelectedAccount}">

        <i:Interaction.Triggers>
            <i:EventTrigger EventName="Loaded">
                <i:InvokeCommandAction Command="{Binding LoadedCommand}" />
            </i:EventTrigger>
        </i:Interaction.Triggers>

        <DataGrid.Columns>
            <DataGridTextColumn Binding="{Binding AccountNumber}"/>
            <DataGridTextColumn Binding="{Binding LastName}" />
            <DataGridTextColumn Binding="{Binding FirstName}" />
        </DataGrid.Columns>
    </DataGrid>

Answer 2

In your view model, use your framework's RaisePropertyChanged(); function (or whatever the equivalent is). In the code-behind of the DataGrid element, try this code:

private void DataGrid_SelectionChanged(object sender, SelectionChangedEventArgs e)
{    
    for (int i = 0; i < DataGrid.Items.Count; i++)
    {
        DataGridRow row = (DataGridRow)DataGrid.ItemContainerGenerator.ContainerFromIndex(i);
        TextBlock cellContent = DataGrid.Columns[0].GetCellContent(row) as TextBlock;
        if (cellContent != null && cellContent.Text.Equals(DataGrid.SelectedItem))
        {
            object item = DataGrid.Items[i];
            DataGrid.SelectedItem = item;
            DataGrid.ScrollIntoView(item);
            row.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
            break;
        }
    }
}

In my example, I used a generic string list of names, so you may need to alter the line TextBlock cellContent = DataGrid.Columns[0].GetCellContent(row) as TextBlock; and cellContent.Text.Equals(DataGrid.SelectedItem)) to meet your line selection criteria.

The other alternative if you don't want to use the code-behind is an attached behavior that more-or-less does the same thing.