wpf - 依赖属性 - 更新源

Question

我对自定义依赖属性感到疯狂。我已经在这里检查了很多线程，但还没有找到任何解决方案。如果源提供特定值(给定示例为 null)，我想要做的是替换属性的值。无论我尝试什么，源中的属性值都保持为空并且永远不会更新。

这是我的自定义控件:

public class TextBoxEx : TextBox
{
    public TextBoxEx()
    {
        TrueValue = 0;
        this.TextChanged += (s, e) =>
        {
            TrueValue = Text.Length;
            SetCurrentValue(MyPropertyProperty, TrueValue);
            var x = BindingOperations.GetBindingExpression(this, MyPropertyProperty);
            if (x != null)
            {
                x.UpdateSource();
            }
        };
    }

    public int? TrueValue { get; set; }

    public int? MyProperty
    {
        get { return (int?)GetValue(MyPropertyProperty); }
        set { SetValue(MyPropertyProperty, value); }
    }

    public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.Register("MyProperty", typeof(int?), typeof(TextBoxEx), new PropertyMetadata(null, PropertyChangedCallback));

    private static void PropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue == null)
        {
            d.SetCurrentValue(MyPropertyProperty, (d as TextBoxEx).TrueValue);
        }
    }
}

这是我正在绑定(bind)的 DataContext:

public class VM : INotifyPropertyChanged
{
    private int? _Bar = null;

    public int? Bar
    {
        get { return _Bar; }
        set
        {
            _Bar = value;
            OnPropertyChanged("Bar");
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

我的绑定(bind)看起来像这样:

<local:TextBoxEx MyProperty="{Binding Bar, UpdateSourceTrigger=PropertyChanged}"/>

记住:我需要一个 TwoWay 绑定(bind)，所以 OneWayToSource 对我不起作用。

知道我在这里没有得到什么吗？

Answer 1

您只需将绑定(bind)设置为双向即可。但这应该是默认设置，您可以使用以下元数据相应地注册属性:

... new FrameworkPropertyMetadata(null,
                                  FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
                                  PropertyChangedCallback)

获取 TextChanged 中的表达式处理程序和手动更新源不是必需的，所以我会删除该代码。

如果你没有在绑定(bind)上显式设置模式，将使用默认模式，来自 the documentation :

Default: Uses the default Mode value of the binding target. The default value varies for each dependency property. In general, user-editable control properties, such as those of text boxes and check boxes, default to two-way bindings, whereas most other properties default to one-way bindings. A programmatic way to determine whether a dependency property binds one-way or two-way by default is to get the property metadata of the property using GetMetadata and then check the Boolean value of the BindsTwoWayByDefault property.