如何使用 MVVM 将 DataGridComboBoxColumn 绑定到 EntityFramework？

Question

我正在尝试让 DataGridComboBoxColumn 与我的 ViewModel 一起使用。一切似乎都正常工作，但是当我更改组合框的值时，实体没有改变。

窗口的数据上下文具有以下属性：

ItemsSource

Public Property AllEnergySources() As ObservableCollection(Of EnergySourceViewModel)

SelectedItemBinding

  Private _CurrentEnergySource As EnergySourceViewModel
    Public Property CurrentEnergySource() As EnergySourceViewModel
        Get
            Return _CurrentEnergySource
        End Get
        Set(ByVal value As EnergySourceViewModel)
            _CurrentEnergySource = value
            OnPropertyChanged("CurrentEnergySource")
        End Set
    End Property

我觉得问题在于我如何在 ViewModel 中填充 CurrentEnergySource，即 DataContext：

Sub New(SelectedEntity as EquipmentEnergySource)
     AllEnergySources = New ObservableCollection(Of EnergySourceViewModel)

    //Select all EnergySources from the EntityFramework
     Dim EnergyEntities = From esr in db.EnergySources Select esr

                //Loop through to convert Entity POCO to Collection of ViewModels
                For Each es In EnergyEntities
                    _AllEnergySources.Add(New EnergySourceViewModel(es))

                    //Optionally Set the newly created ViewModel to SelectedItemBinding object
                    If es.EnergySourceID = SelectedEntity.EnergySourceID Then
                        _CurrentEnergySource = _AllEnergySources.Last
                    End If
                Next
End Sub

当我为组合框创建支持集合时，如果模型是选定的模型，我将该视图模型设置为 CurrentEnergySource，但在那之后它会断开连接（这就是问题所在）

我应该在 CurrentEnergySource 中引用什么，以便在组合框更改时更新模型？

Answer 1

似乎是错误的一件事是您可能应该使用 SelectedValueBinding 而不是 SelectedItemBinding。

这是一个适合我的示例：

<Page.Resources>
    <ViewModel:DataGridComboBoxViewModel x:Key="model"/>
    <Style x:Key="ElementStyle" TargetType="ComboBox">
        <Setter 
            Property="ItemsControl.ItemsSource" 
            Value="{Binding Path=DataContext.DetailItems, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}}" 
            />
    </Style>
</Page.Resources>

<Grid DataContext="{StaticResource model}">
    <DataGrid ItemsSource="{Binding Items}" AutoGenerateColumns="False">
        <DataGrid.Columns>
            <DataGridTextColumn Header="Id" Binding="{Binding Id}"/>
            <DataGridTextColumn Header="Name" Binding="{Binding Name}"/>
            <DataGridComboBoxColumn Header="Combo" 
                                    DisplayMemberPath="Name" 
                                    SelectedValueBinding="{Binding DetailItem}" 
                                    ElementStyle="{StaticResource ElementStyle}"
                                    EditingElementStyle="{StaticResource ElementStyle}"
                                    >
            </DataGridComboBoxColumn>
        </DataGrid.Columns>
    </DataGrid>
</Grid>

public class DataItem : ViewModelBase
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Description { get; set; } 

    private DetailItem _detailItem;
    public DetailItem DetailItem
    {
        get { return _detailItem; }
        set
        {
            Debug.WriteLine(value != null
                                ? string.Format("Setting detail item to: {0}", value.Name)
                                : "Setting detail item to null.");

            Set(() => DetailItem, ref _detailItem, value);
        }
    }
}

public class DetailItem : ViewModelBase
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class DataGridComboBoxViewModel : ViewModelBase
{
    public DataGridComboBoxViewModel()
    {
        DetailItems = new List<DetailItem>
                          {
                              new DetailItem {Id = 0, Name = "Zero"},
                              new DetailItem {Id = 1, Name = "One"},
                              new DetailItem {Id = 2, Name = "Two"},
                              new DetailItem {Id = 3, Name = "Three"},
                          };

        Items = new List<DataItem>
                    {
                        new DataItem {Id = 0, Name = "Item 1", Description = "This is item 1"},
                        new DataItem {Id = 1, Name = "Item 2", Description = "This is item 2"},
                        new DataItem {Id = 2, Name = "Item 3", Description = "This is item 3"},
                        new DataItem {Id = 3, Name = "Item 4", Description = "This is item 4"},
                    };
    }

    public List<DataItem> Items { get; set; }
    public List<DetailItem> DetailItems { get; private set; }
}

Answer 2

当然，问题出在你的绑定上，DataGridComboBoxColumn 自动从 CurrentEnergySource 中取一项，而 CurrentEnergySource 上没有 AllEnergySources，你就不用这个了，

            <DataGridTemplateColumn>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <TextBlock Text="{Binding SelectedItemFromItemsSource}"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
                <DataGridTemplateColumn.CellEditingTemplate>
                    <DataTemplate>
                        <ComboBox ItemsSource="{Binding ItemsSource}" />
                    </DataTemplate>
                </DataGridTemplateColumn.CellEditingTemplate>
            </DataGridTemplateColumn>

Answer 3

您是否在绑定上尝试过RelativeSource？通常我发现当我绑定到模板等中的列时，绑定会在控件的绑定中查找，而不是在视图的数据上下文中。

试试：

"{Binding Path=DataContext.CurrentEnergySource, 
          RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},
          Mode=TwoWay}"

或

"{Binding ElementName=NameOfTheView, 
          Path=DataContext.CurrentEnergySource, 
          Mode=TwoWay}"

然后将x:Name="NameOfTheView"添加到视图的属性中（在>括号内的xmlns位置下方）

Answer 4

我的回答是你需要手动更改外键（我现在在CurrentEnergySource的setter中更改，也就是SelectedItemBinding绑定属性）

     Private _CurrentEnergySource As EnergySourceViewModel
    Public Property CurrentEnergySource() As EnergySourceViewModel
        Get
            Return _CurrentEnergySource
        End Get
        Set(ByVal value As EnergySourceViewModel)
            _CurrentEnergySource = value
            Me.Model.EnergySourceID = value.Model.EnergySourceID
            OnPropertyChanged("CurrentEnergySource")
        End Set
    End Property

在加载时，填充私有后备存储 _CurrentEnergySource 而不是属性以避免所有对象以修改状态开始