Question

如果UpdateSourceTrigger设置为PropertyChanged，则会运行以下代码，但在UpdateSourceTrigger设置为LostFocus时初始化时会抛出异常。

此实施有什么问题？如何纠正？

异常

"'ComboBoxSample.ComboBoxBehavior' type must derive from FrameworkElement or FrameworkContentElement."

查看

<Window.DataContext>
    <local:ViewModel/>
</Window.DataContext>

<Grid>
    <ComboBox ItemsSource="{Binding Path=Apples}"
              DisplayMemberPath="Cultivar">
        <i:Interaction.Behaviors>
            <local:ComboBoxBehavior 
                SelectedValue="{Binding Path=SelectedId, 
                                        Mode=TwoWay,
                                        UpdateSourceTrigger=LostFocus}"/>
        </i:Interaction.Behaviors>
    </ComboBox>
</Grid>

行为

public class ComboBoxBehavior : Behavior<ComboBox>
{
    public static readonly DependencyProperty SelectedValueProperty
        = DependencyProperty.Register("SelectedValue",
                                      typeof(object),
                                      typeof(ComboBoxBehavior),
                                      new FrameworkPropertyMetadata(null, (target, args) => { }));

    public object SelectedValue
    {
        get { return GetValue(SelectedValueProperty); }
        set { SetValue(SelectedValueProperty, value); }
    }

    protected override void OnAttached() { base.OnAttached(); }

    protected override void OnDetaching() { base.OnDetaching(); }
}

视图模型

public class ViewModel
{
    public ObservableCollection<Apple> Apples { get; set; }

    public int SelectedId { get; set; }

    public ViewModel()
    {
        Apples = new ObservableCollection<Apple>
        {
            new Apple()
            {
                Id = 0,
                Cultivar = "Alice",
                Weight = 0.250
            },
            new Apple()
            {
                Id = 1,
                Cultivar = "Golden",
                Weight = 0.3
            },
            new Apple()
            {
                Id = 2,
                Cultivar = "Granny Smith",
                Weight = 0.275
            }
        };
    }
}

public class Apple
{
    public int Id { get; set; }

    public string Cultivar { get; set; }

    public double Weight { get; set; }
}

Answer 1

您的问题是您尝试使用<local:ComboBoxBehavior />，就好像它是一个块（框架元素）。在xaml中，您只能编写从Framework Element继承的块（如段落，TextBlock等）。由于我不确定你想要达到的目标，这几乎是我能给出的最佳答案。

Answer 2

我自己也很陌生，所以不确定它会有多大帮助。首先，您是否尝试一起更新SelectedItem和SelectedValue？如果是这样，可能不需要行为（警告，未经测试的代码！）：

<Window.DataContext>
    <local:ViewModel/>
</Window.DataContext>

<Grid>
    <ComboBox ItemsSource="{Binding Path=Apples}"
          DisplayMemberPath="Cultivar" SelectedValue="{Binding Path=SelectedId, 
                                    Mode=TwoWay,
                                    UpdateSourceTrigger=LostFocus}" >
    </ComboBox>
</Grid>

或类似的东西将组合框的SelectedValue直接绑定到视图模型中的SelectedId。然后，如果在更改SelectedValue时需要在行为中执行某些操作，请将其创建为与SelectedId更新分开的自己的行为。这有帮助吗？

与UpdateSourceTrigger绑定设置为LostFocus的异常

2 个答案: