Question

我想编写一个ViewModel，它始终知道View中某些只读依赖项属性的当前状态。

具体来说，我的GUI包含一个FlowDocumentPageViewer，它一次从FlowDocument显示一个页面。 FlowDocumentPageViewer公开了两个名为CanGoToPreviousPage和CanGoToNextPage的只读依赖项属性。我希望我的ViewModel始终知道这两个View属性的值。

我想我可以用OneWayToSource数据绑定来完成这个：

<FlowDocumentPageViewer
    CanGoToNextPage="{Binding NextPageAvailable, Mode=OneWayToSource}" ...>

如果允许，那将是完美的：每当FlowDocumentPageViewer的CanGoToNextPage属性发生更改时，新值将被推送到ViewModel的NextPageAvailable属性，这正是我想要的。

不幸的是，这不能编译：我收到错误，说'CanGoToPreviousPage'属性是只读的，无法通过标记设置。显然只读属性不支持任何类型的数据绑定，甚至不是与该属性相关的只读数据绑定。

我可以让我的ViewModel的属性为DependencyProperties，并使OneWay绑定以另一种方式运行，但我并不为关注点分离违反而疯狂（ViewModel需要对View的引用，MVVM数据绑定是应该避免）。

FlowDocumentPageViewer没有暴露CanGoToNextPageChanged事件，我不知道从DependencyProperty获取更改通知的任何好方法，没有创建另一个DependencyProperty来绑定它，这在这里看起来有点过分。

如何让我的ViewModel了解视图的只读属性的更改？

Answer 1

是的，我过去使用ActualWidth和ActualHeight属性执行此操作，这两个属性都是只读的。我创建了一个附加了ObservedWidth和ObservedHeight附加属性的附加行为。它还有一个Observe属性，用于执行初始连接。用法如下：

<UserControl ...
    SizeObserver.Observe="True"
    SizeObserver.ObservedWidth="{Binding Width, Mode=OneWayToSource}"
    SizeObserver.ObservedHeight="{Binding Height, Mode=OneWayToSource}"

因此，视图模型具有Width和Height属性，这些属性始终与ObservedWidth和ObservedHeight附加属性同步。 Observe属性只是附加到SizeChanged的{{1}}事件。在句柄中，它会更新其FrameworkElement和ObservedWidth属性。因此，视图模型的ObservedHeight和Width始终与Height的{{1}}和ActualWidth同步。

也许不是完美的解决方案（我同意 - 只读DP 应该支持ActualHeight绑定），但它可以工作并且它支持MVVM模式。显然，UserControl和OneWayToSource DP 不是只读。

更新：这是实现上述功能的代码：

ObservedWidth

Answer 2

我使用的通用解决方案不仅适用于ActualWidth和ActualHeight，还适用于至少在阅读模式下可以绑定的任何数据。

如果ViewportWidth和ViewportHeight是视图模型的属性，则标记如下所示

<Canvas>
    <u:DataPiping.DataPipes>
         <u:DataPipeCollection>
             <u:DataPipe Source="{Binding RelativeSource={RelativeSource AncestorType={x:Type Canvas}}, Path=ActualWidth}"
                         Target="{Binding Path=ViewportWidth, Mode=OneWayToSource}"/>
             <u:DataPipe Source="{Binding RelativeSource={RelativeSource AncestorType={x:Type Canvas}}, Path=ActualHeight}"
                         Target="{Binding Path=ViewportHeight, Mode=OneWayToSource}"/>
          </u:DataPipeCollection>
     </u:DataPiping.DataPipes>
<Canvas>

以下是自定义元素的源代码

public class DataPiping
{
    #region DataPipes (Attached DependencyProperty)

    public static readonly DependencyProperty DataPipesProperty =
        DependencyProperty.RegisterAttached("DataPipes",
        typeof(DataPipeCollection),
        typeof(DataPiping),
        new UIPropertyMetadata(null));

    public static void SetDataPipes(DependencyObject o, DataPipeCollection value)
    {
        o.SetValue(DataPipesProperty, value);
    }

    public static DataPipeCollection GetDataPipes(DependencyObject o)
    {
        return (DataPipeCollection)o.GetValue(DataPipesProperty);
    }

    #endregion
}

public class DataPipeCollection : FreezableCollection<DataPipe>
{

}

public class DataPipe : Freezable
{
    #region Source (DependencyProperty)

    public object Source
    {
        get { return (object)GetValue(SourceProperty); }
        set { SetValue(SourceProperty, value); }
    }
    public static readonly DependencyProperty SourceProperty =
        DependencyProperty.Register("Source", typeof(object), typeof(DataPipe),
        new FrameworkPropertyMetadata(null, new PropertyChangedCallback(OnSourceChanged)));

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((DataPipe)d).OnSourceChanged(e);
    }

    protected virtual void OnSourceChanged(DependencyPropertyChangedEventArgs e)
    {
        Target = e.NewValue;
    }

    #endregion

    #region Target (DependencyProperty)

    public object Target
    {
        get { return (object)GetValue(TargetProperty); }
        set { SetValue(TargetProperty, value); }
    }
    public static readonly DependencyProperty TargetProperty =
        DependencyProperty.Register("Target", typeof(object), typeof(DataPipe),
        new FrameworkPropertyMetadata(null));

    #endregion

    protected override Freezable CreateInstanceCore()
    {
        return new DataPipe();
    }
}

Answer 3

如果有其他人感兴趣，我在这里编写了肯特解决方案的近似值：

class SizeObserver
{
    #region " Observe "

    public static bool GetObserve(FrameworkElement elem)
    {
        return (bool)elem.GetValue(ObserveProperty);
    }

    public static void SetObserve(
      FrameworkElement elem, bool value)
    {
        elem.SetValue(ObserveProperty, value);
    }

    public static readonly DependencyProperty ObserveProperty =
        DependencyProperty.RegisterAttached("Observe", typeof(bool), typeof(SizeObserver),
        new UIPropertyMetadata(false, OnObserveChanged));

    static void OnObserveChanged(
      DependencyObject depObj, DependencyPropertyChangedEventArgs e)
    {
        FrameworkElement elem = depObj as FrameworkElement;
        if (elem == null)
            return;

        if (e.NewValue is bool == false)
            return;

        if ((bool)e.NewValue)
            elem.SizeChanged += OnSizeChanged;
        else
            elem.SizeChanged -= OnSizeChanged;
    }

    static void OnSizeChanged(object sender, RoutedEventArgs e)
    {
        if (!Object.ReferenceEquals(sender, e.OriginalSource))
            return;

        FrameworkElement elem = e.OriginalSource as FrameworkElement;
        if (elem != null)
        {
            SetObservedWidth(elem, elem.ActualWidth);
            SetObservedHeight(elem, elem.ActualHeight);
        }
    }

    #endregion

    #region " ObservedWidth "

    public static double GetObservedWidth(DependencyObject obj)
    {
        return (double)obj.GetValue(ObservedWidthProperty);
    }

    public static void SetObservedWidth(DependencyObject obj, double value)
    {
        obj.SetValue(ObservedWidthProperty, value);
    }

    // Using a DependencyProperty as the backing store for ObservedWidth.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ObservedWidthProperty =
        DependencyProperty.RegisterAttached("ObservedWidth", typeof(double), typeof(SizeObserver), new UIPropertyMetadata(0.0));

    #endregion

    #region " ObservedHeight "

    public static double GetObservedHeight(DependencyObject obj)
    {
        return (double)obj.GetValue(ObservedHeightProperty);
    }

    public static void SetObservedHeight(DependencyObject obj, double value)
    {
        obj.SetValue(ObservedHeightProperty, value);
    }

    // Using a DependencyProperty as the backing store for ObservedHeight.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty ObservedHeightProperty =
        DependencyProperty.RegisterAttached("ObservedHeight", typeof(double), typeof(SizeObserver), new UIPropertyMetadata(0.0));

    #endregion
}

随意在您的应用中使用它。它运作良好。（谢谢肯特！）

Answer 4

这是我在这里写博客的这个“bug”的另一个解决方案：
OneWayToSource Binding for ReadOnly Dependency Property

它使用两个依赖属性，监听器和镜像。 Listener将OneWay绑定到TargetProperty，并在PropertyChangedCallback中更新Mirror属性，该属性将OneWayToSource绑定到Binding中指定的任何内容。我将其称为PushBinding，它可以在任何只读的依赖属性上设置，如此

<TextBlock Name="myTextBlock"
           Background="LightBlue">
    <pb:PushBindingManager.PushBindings>
        <pb:PushBinding TargetProperty="ActualHeight" Path="Height"/>
        <pb:PushBinding TargetProperty="ActualWidth" Path="Width"/>
    </pb:PushBindingManager.PushBindings>
</TextBlock>

Download Demo Project Here。
它包含源代码和简短的样本用法，如果您对实现细节感兴趣，请访问my WPF blog。

最后一点，自.NET 4.0起，我们甚至远离内置支持，因为OneWayToSource Binding reads the value back from the Source after it has updated it

Answer 5

我喜欢Dmitry Tashkinov的解决方案！然而它在设计模式下撞毁了我的VS.这就是为什么我在OnSourceChanged方法中添加了一行：

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (!((bool)DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue))
            ((DataPipe)d).OnSourceChanged(e);
    }

Answer 6

我认为可以做得更简单一些：

XAML：

behavior:ReadOnlyPropertyToModelBindingBehavior.ReadOnlyDependencyProperty="{Binding ActualWidth, RelativeSource={RelativeSource Self}}"
behavior:ReadOnlyPropertyToModelBindingBehavior.ModelProperty="{Binding MyViewModelProperty}"

CS：

public class ReadOnlyPropertyToModelBindingBehavior
{
  public static readonly DependencyProperty ReadOnlyDependencyPropertyProperty = DependencyProperty.RegisterAttached(
     "ReadOnlyDependencyProperty", 
     typeof(object), 
     typeof(ReadOnlyPropertyToModelBindingBehavior),
     new PropertyMetadata(OnReadOnlyDependencyPropertyPropertyChanged));

  public static void SetReadOnlyDependencyProperty(DependencyObject element, object value)
  {
     element.SetValue(ReadOnlyDependencyPropertyProperty, value);
  }

  public static object GetReadOnlyDependencyProperty(DependencyObject element)
  {
     return element.GetValue(ReadOnlyDependencyPropertyProperty);
  }

  private static void OnReadOnlyDependencyPropertyPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
  {
     SetModelProperty(obj, e.NewValue);
  }


  public static readonly DependencyProperty ModelPropertyProperty = DependencyProperty.RegisterAttached(
     "ModelProperty", 
     typeof(object), 
     typeof(ReadOnlyPropertyToModelBindingBehavior), 
     new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

  public static void SetModelProperty(DependencyObject element, object value)
  {
     element.SetValue(ModelPropertyProperty, value);
  }

  public static object GetModelProperty(DependencyObject element)
  {
     return element.GetValue(ModelPropertyProperty);
  }
}

将只读GUI属性推送回ViewModel

6 个答案: