只有GUI属性返回到ViewModel
我想写一个ViewModel，它总是从视图中知道一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，它从FlowDocument一次显示一个页面。 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始终知道这两个视图属性的值。
我想我可以用OneWayToSource数据绑定来做到这一点：
<FlowDocumentPageViewer
CanGoToNextPage="{Binding NextPageAvailable, Mode=OneWayToSource}" ...>
如果这是允许的话，那将是完美的：每当FlowDocumentPageViewer的CanGoToNextPage属性发生变化时，新值将被推送到ViewModel的NextPageAvailable属性中，这正是我想要的。
不幸的是，这不能编译：我得到一个错误，说'CanGoToPreviousPage'属性是只读的，不能从标记中设置。 显然，只读属性不支持任何类型的数据绑定，甚至不支持对该属性只读的数据绑定。
我可以让ViewModel的属性成为DependencyProperties，并以另一种方式创建一个OneWay绑定，但是我并不满足于分离关注违规（ViewModel需要引用View，而MVVM数据绑定应该避免）。
FlowDocumentPageViewer不公开一个CanGoToNextPageChanged事件，我不知道从DependencyProperty获取更改通知的任何好方法，但创建另一个DependencyProperty来绑定它，这似乎在这里过度杀伤。
如何让我的ViewModel知道对视图的只读属性所做的更改？
是的，我在过去使用ActualWidth和ActualHeight属性完成了这两个属性，它们都是只读的。 我创建了具有ObservedWidth和ObservedHeight附加属性的附加行为。 它还具有用于执行初始连接的Observe属性。 用法如下所示：
<UserControl ...
SizeObserver.Observe="True"
SizeObserver.ObservedWidth="{Binding Width, Mode=OneWayToSource}"
SizeObserver.ObservedHeight="{Binding Height, Mode=OneWayToSource}"
因此，视图模型的Width和Height属性始终与ObservedWidth和ObservedHeight附加属性保持同步。 Observe属性只是附加到FrameworkElement的SizeChanged事件。 在句柄中，它更新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DP应该支持OneWayToSource绑定），但它工作并支持MVVM模式。 显然， ObservedWidth和ObservedHeight DP不是只读的。
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);
}
}
我使用的通用解决方案不仅适用于ActualWidth和ActualHeight，而且适用于至少可以在阅读模式下绑定的任何数据。
标记看起来像这样，只要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();
}
}
如果其他人有兴趣，我在这里编写了一个近似的肯特解决方案：
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
}
随意在您的应用程序中使用它。 它运作良好。 （谢谢肯特！）
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