将只读 GUI 属性推回 ViewModel

Question

我想写一个 ViewModel，它总是知道视图中一些只读依赖属性的当前状态。

具体来说，我的 GUI 包含一个 FlowDocumentPageViewer，它一次显示一个来自 FlowDocument 的页面。 FlowDocumentPageViewer 公开了两个只读依赖属性，称为 CanGoToPreviousPage 和 CanGoToNextPage。我希望我的 ViewModel 始终知道这两个 View 属性的值。

我想我可以使用 OneWayToSource 数据绑定来做到这一点：

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

如果允许，那就太完美了：每当 FlowDocumentPageViewer 的 CanGoToNextPage 属性发生变化时，新值就会被下推到 ViewModel 的 NextPageAvailable 属性中，这正是我想要的。

很遗憾，这无法编译：我收到一条错误消息，提示 'CanGoToPreviousPage' 属性是只读的，无法从标记中设置。 显然只读属性不支持 任何类型的数据绑定，甚至不包括对该属性的只读数据绑定。

我可以让我的 ViewModel 的属性为 DependencyProperties，并以另一种方式进行 OneWay 绑定，但我并不担心违反关注点分离（ViewModel 需要对 View 的引用，MVVM 数据绑定是应该避免）。

FlowDocumentPageViewer 没有公开 CanGoToNextPageChanged 事件，而且我不知道有什么好的方法可以从 DependencyProperty 获取更改通知，除了创建另一个 DependencyProperty 来绑定它，这在这里看起来有点过头了。

如何让我的 ViewModel 了解视图只读属性的更改？

Answer 1

是的，我过去曾使用 ActualWidth 和 ActualHeight 属性完成此操作，这两个属性都是只读的。我创建了一个具有ObservedWidth 和ObservedHeight 附加属性的附加行为。它还有一个Observe 属性，用于进行初始连接。用法如下所示：

<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，它们始终与ObservedWidth 和ObservedHeight 附加属性同步。 Observe 属性只是附加到FrameworkElement 的SizeChanged 事件。在句柄中，它更新其ObservedWidth 和ObservedHeight 属性。因此，视图模型的Width 和Height 始终与UserControl 的ActualWidth 和ActualHeight 同步。

也许不是完美的解决方案（我同意 - 只读 DPs应该 支持 OneWayToSource 绑定），但它有效并且支持 MVVM 模式。显然，ObservedWidth 和 ObservedHeight DP 是非只读的。

更新：这里是实现上述功能的代码：

public static class SizeObserver
{
    public static readonly DependencyProperty ObserveProperty = DependencyProperty.RegisterAttached(
        "Observe",
        typeof(bool),
        typeof(SizeObserver),
        new FrameworkPropertyMetadata(OnObserveChanged));

    public static readonly DependencyProperty ObservedWidthProperty = DependencyProperty.RegisterAttached(
        "ObservedWidth",
        typeof(double),
        typeof(SizeObserver));

    public static readonly DependencyProperty ObservedHeightProperty = DependencyProperty.RegisterAttached(
        "ObservedHeight",
        typeof(double),
        typeof(SizeObserver));

    public static bool GetObserve(FrameworkElement frameworkElement)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        return (bool)frameworkElement.GetValue(ObserveProperty);
    }

    public static void SetObserve(FrameworkElement frameworkElement, bool observe)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        frameworkElement.SetValue(ObserveProperty, observe);
    }

    public static double GetObservedWidth(FrameworkElement frameworkElement)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        return (double)frameworkElement.GetValue(ObservedWidthProperty);
    }

    public static void SetObservedWidth(FrameworkElement frameworkElement, double observedWidth)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        frameworkElement.SetValue(ObservedWidthProperty, observedWidth);
    }

    public static double GetObservedHeight(FrameworkElement frameworkElement)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        return (double)frameworkElement.GetValue(ObservedHeightProperty);
    }

    public static void SetObservedHeight(FrameworkElement frameworkElement, double observedHeight)
    {
        frameworkElement.AssertNotNull("frameworkElement");
        frameworkElement.SetValue(ObservedHeightProperty, observedHeight);
    }

    private static void OnObserveChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var frameworkElement = (FrameworkElement)dependencyObject;

        if ((bool)e.NewValue)
        {
            frameworkElement.SizeChanged += OnFrameworkElementSizeChanged;
            UpdateObservedSizesForFrameworkElement(frameworkElement);
        }
        else
        {
            frameworkElement.SizeChanged -= OnFrameworkElementSizeChanged;
        }
    }

    private static void OnFrameworkElementSizeChanged(object sender, SizeChangedEventArgs e)
    {
        UpdateObservedSizesForFrameworkElement((FrameworkElement)sender);
    }

    private static void UpdateObservedSizesForFrameworkElement(FrameworkElement frameworkElement)
    {
        // WPF 4.0 onwards
        frameworkElement.SetCurrentValue(ObservedWidthProperty, frameworkElement.ActualWidth);
        frameworkElement.SetCurrentValue(ObservedHeightProperty, frameworkElement.ActualHeight);

        // WPF 3.5 and prior
        ////SetObservedWidth(frameworkElement, frameworkElement.ActualWidth);
        ////SetObservedHeight(frameworkElement, frameworkElement.ActualHeight);
    }
}

Answer 2

我使用了一种通用解决方案，它不仅适用于 ActualWidth 和 ActualHeight，而且适用于至少在阅读模式下可以绑定的任何数据。

标记看起来像这样，前提是 ViewportWidth 和 ViewportHeight 是视图模型的属性

<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>

这里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

如果其他人有兴趣，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

这是我在博客中提到的这个“错误”的另一个解决方案：
OneWayToSource Binding for ReadOnly Dependency Property

它通过使用两个依赖属性，监听器和镜像来工作。侦听器绑定到 TargetProperty 的 OneWay，并且在 PropertyChangedCallback 中它更新绑定 OneWayToSource 到绑定中指定的任何内容的 Mirror 属性。我称之为PushBinding，它可以像这样设置在任何只读依赖属性上

<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示例用法。

最后一点，从 .NET 4.0 开始，我们离内置支持更远了，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！ 但是它在设计模式下使我的 VS 崩溃了。这就是为什么我在 OnSourceChanged 方法中添加了一行：

私有静态无效 OnSourceChanged（DependencyObject d，DependencyPropertyChangedEventArgs e） { if (!((bool)DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue)) ((DataPipe)d).OnSourceChanged(e); }

Answer 6

我觉得可以简单一点：

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);
  }
}