wpf XAML 中只读属性的 OneWayToSource 绑定

OneWayToSource binding from readonly property in XAML

提问by Inferis

I'm trying to bind to a Readonlyproperty with OneWayToSourceas mode, but it seems this cannot be done in XAML:

我正在尝试Readonly使用OneWayToSourceas 模式绑定到一个属性，但似乎这无法在 XAML 中完成：

<controls:FlagThingy IsModified="{Binding FlagIsModified, 
                                          ElementName=container, 
                                          Mode=OneWayToSource}" />

I get:

我得到：

The property 'FlagThingy.IsModified' cannot be set because it does not have an accessible set accessor.

无法设置属性“FlagThingy.IsModified”，因为它没有可访问的 set 访问器。

IsModifiedis a readonly DependencyPropertyon FlagThingy. I want to bind that value to the FlagIsModifiedproperty on the container.

IsModified是只读DependencyProperty的FlagThingy。我想将该值绑定到FlagIsModified容器上的属性。

To be clear:

要清楚：

FlagThingy.IsModified --> container.FlagIsModified
------ READONLY -----     ----- READWRITE --------

Is this possible using just XAML?

这可以仅使用 XAML 吗？

---

---

Update:Well, I fixed this case by setting the binding on the container and not on the FlagThingy. But I'd still like to know if this is possible.

更新：嗯，我通过在容器上而不是在FlagThingy. 但我仍然想知道这是否可能。

采纳答案by alex2k8

Some research results for OneWayToSource...

OneWayToSource 的一些研究结果...

Option # 1.

选项1。

// Control definition
public partial class FlagThingy : UserControl
{
    public static readonly DependencyProperty IsModifiedProperty = 
            DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());
}

<controls:FlagThingy x:Name="_flagThingy" />

// Binding Code
Binding binding = new Binding();
binding.Path = new PropertyPath("FlagIsModified");
binding.ElementName = "container";
binding.Mode = BindingMode.OneWayToSource;
_flagThingy.SetBinding(FlagThingy.IsModifiedProperty, binding);

Option # 2

选项#2

// Control definition
public partial class FlagThingy : UserControl
{
    public static readonly DependencyProperty IsModifiedProperty = 
            DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());

    public bool IsModified
    {
        get { return (bool)GetValue(IsModifiedProperty); }
        set { throw new Exception("An attempt ot modify Read-Only property"); }
    }
}

<controls:FlagThingy IsModified="{Binding Path=FlagIsModified, 
    ElementName=container, Mode=OneWayToSource}" />

Option # 3 (True read-only dependency property)

选项 #3（真正的只读依赖属性）

System.ArgumentException: 'IsModified' property cannot be data-bound.

System.ArgumentException: 'IsModified' 属性不能是数据绑定的。

// Control definition
public partial class FlagThingy : UserControl
{
    private static readonly DependencyPropertyKey IsModifiedKey =
        DependencyProperty.RegisterReadOnly("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());

    public static readonly DependencyProperty IsModifiedProperty = 
        IsModifiedKey.DependencyProperty;
}

<controls:FlagThingy x:Name="_flagThingy" />

// Binding Code
Same binding code...

Reflector gives the answer:

反射器给出了答案：

internal static BindingExpression CreateBindingExpression(DependencyObject d, DependencyProperty dp, Binding binding, BindingExpressionBase parent)
{
    FrameworkPropertyMetadata fwMetaData = dp.GetMetadata(d.DependencyObjectType) as FrameworkPropertyMetadata;
    if (((fwMetaData != null) && !fwMetaData.IsDataBindingAllowed) || dp.ReadOnly)
    {
        throw new ArgumentException(System.Windows.SR.Get(System.Windows.SRID.PropertyNotBindable, new object[] { dp.Name }), "dp");
    }
 ....

回答by Fredrik Hedblad

This is a limitation of WPF and it is by design. It is reported on Connect here:
OneWayToSource binding from a readonly dependency property

这是 WPF 的一个限制，它是设计使然。在 Connect 上报告：
OneWayToSource binding from a readonly dependency property

I made a solution to dynamically be able to push read-only dependency properties to the source called PushBindingwhich I blogged about here. The example below does OneWayToSourceBindings from the read-only DP's ActualWidthand ActualHeightto the Width and Height properties of the DataContext

我提出了一个解决方案，可以动态地将只读依赖项属性推送到PushBinding我在博客中提到的源代码。下面的示例OneWayToSource从只读 DPActualWidth和ActualHeightWidth 和 Height 属性进行绑定DataContext

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

PushBindingworks by using two Dependency Properties, Listener and Mirror. Listener is bound OneWayto the TargetProperty and in the PropertyChangedCallbackit updates the Mirror property which is bound OneWayToSourceto whatever was specified in the Binding.

PushBinding通过使用两个依赖属性，侦听器和镜像来工作。侦听器绑定OneWay到 TargetProperty 并在PropertyChangedCallback其中更新 Mirror 属性，该属性绑定OneWayToSource到 Binding 中指定的任何内容。

Demo Project can be Downloaded Here.
It contains source code and short sample usage, or visit my WPF blogif you're interested in the implementation details.

演示项目可以在这里下载。
它包含源代码和简短的示例用法，如果您对实现细节感兴趣，请访问我的 WPF 博客。

回答by Johan Larsson

Wrote this:

写了这个：

Usage:

用法：

<TextBox Text="{Binding Text}"
         p:OneWayToSource.Bind="{p:Paths From={x:Static Validation.HasErrorProperty},
                                         To=SomeDataContextProperty}" />

Code:

代码：

using System;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;

public static class OneWayToSource
{
    public static readonly DependencyProperty BindProperty = DependencyProperty.RegisterAttached(
        "Bind",
        typeof(ProxyBinding),
        typeof(OneWayToSource),
        new PropertyMetadata(default(Paths), OnBindChanged));

    public static void SetBind(this UIElement element, ProxyBinding value)
    {
        element.SetValue(BindProperty, value);
    }

    [AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
    [AttachedPropertyBrowsableForType(typeof(UIElement))]
    public static ProxyBinding GetBind(this UIElement element)
    {
        return (ProxyBinding)element.GetValue(BindProperty);
    }

    private static void OnBindChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((ProxyBinding)e.OldValue)?.Dispose();
    }

    public class ProxyBinding : DependencyObject, IDisposable
    {
        private static readonly DependencyProperty SourceProxyProperty = DependencyProperty.Register(
            "SourceProxy",
            typeof(object),
            typeof(ProxyBinding),
            new PropertyMetadata(default(object), OnSourceProxyChanged));

        private static readonly DependencyProperty TargetProxyProperty = DependencyProperty.Register(
            "TargetProxy",
            typeof(object),
            typeof(ProxyBinding),
            new PropertyMetadata(default(object)));

        public ProxyBinding(DependencyObject source, DependencyProperty sourceProperty, string targetProperty)
        {
            var sourceBinding = new Binding
            {
                Path = new PropertyPath(sourceProperty),
                Source = source,
                Mode = BindingMode.OneWay,
            };

            BindingOperations.SetBinding(this, SourceProxyProperty, sourceBinding);

            var targetBinding = new Binding()
            {
                Path = new PropertyPath($"{nameof(FrameworkElement.DataContext)}.{targetProperty}"),
                Mode = BindingMode.OneWayToSource,
                Source = source
            };

            BindingOperations.SetBinding(this, TargetProxyProperty, targetBinding);
        }

        public void Dispose()
        {
            BindingOperations.ClearAllBindings(this);
        }

        private static void OnSourceProxyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            d.SetCurrentValue(TargetProxyProperty, e.NewValue);
        }
    }
}

[MarkupExtensionReturnType(typeof(OneWayToSource.ProxyBinding))]
public class Paths : MarkupExtension
{
    public DependencyProperty From { get; set; }

    public string To { get; set; }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var provideValueTarget = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
        var targetObject = (UIElement)provideValueTarget.TargetObject;
        return new OneWayToSource.ProxyBinding(targetObject, this.From, this.To);
    }
}

Have not tested it in styles and templates yet, guess it needs special casing.

尚未在样式和模板中对其进行测试，猜测它需要特殊的外壳。

回答by jv_

Here's another attached property solution based on SizeObserver detailed here Pushing read-only GUI properties back into ViewModel

这是另一个基于 SizeObserver 的附加属性解决方案，这里详细介绍了将只读 GUI 属性推回 ViewModel

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

    public static readonly DependencyProperty ObservedMouseOverProperty = DependencyProperty.RegisterAttached(
        "ObservedMouseOver",
        typeof(bool),
        typeof(MouseObserver));


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

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

    public static bool GetObservedMouseOver(FrameworkElement frameworkElement)
    {
        return (bool)frameworkElement.GetValue(ObservedMouseOverProperty);
    }

    public static void SetObservedMouseOver(FrameworkElement frameworkElement, bool observedMouseOver)
    {
        frameworkElement.SetValue(ObservedMouseOverProperty, observedMouseOver);
    }

    private static void OnObserveChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
        var frameworkElement = (FrameworkElement)dependencyObject;
        if ((bool)e.NewValue)
        {
            frameworkElement.MouseEnter += OnFrameworkElementMouseOverChanged;
            frameworkElement.MouseLeave += OnFrameworkElementMouseOverChanged;
            UpdateObservedMouseOverForFrameworkElement(frameworkElement);
        }
        else
        {
            frameworkElement.MouseEnter -= OnFrameworkElementMouseOverChanged;
            frameworkElement.MouseLeave -= OnFrameworkElementMouseOverChanged;
        }
    }

    private static void OnFrameworkElementMouseOverChanged(object sender, MouseEventArgs e)
    {
        UpdateObservedMouseOverForFrameworkElement((FrameworkElement)sender);
    }

    private static void UpdateObservedMouseOverForFrameworkElement(FrameworkElement frameworkElement)
    {
        frameworkElement.SetCurrentValue(ObservedMouseOverProperty, frameworkElement.IsMouseOver);
    }
}

Declare attached property in control

声明受控制的附加财产

<ListView ItemsSource="{Binding SomeGridItems}"                             
     ut:MouseObserver.Observe="True"
     ut:MouseObserver.ObservedMouseOver="{Binding IsMouseOverGrid, Mode=OneWayToSource}">    

回答by Johan Larsson

Here is another implementation for binding to Validation.HasError

这是绑定到 Validation.HasError 的另一个实现

public static class OneWayToSource
{
    public static readonly DependencyProperty BindingsProperty = DependencyProperty.RegisterAttached(
        "Bindings",
        typeof(OneWayToSourceBindings),
        typeof(OneWayToSource),
        new PropertyMetadata(default(OneWayToSourceBindings), OnBinidngsChanged));

    public static void SetBindings(this FrameworkElement element, OneWayToSourceBindings value)
    {
        element.SetValue(BindingsProperty, value);
    }

    [AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
    [AttachedPropertyBrowsableForType(typeof(FrameworkElement))]
    public static OneWayToSourceBindings GetBindings(this FrameworkElement element)
    {
        return (OneWayToSourceBindings)element.GetValue(BindingsProperty);
    }

    private static void OnBinidngsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ((OneWayToSourceBindings)e.OldValue)?.ClearValue(OneWayToSourceBindings.ElementProperty);
        ((OneWayToSourceBindings)e.NewValue)?.SetValue(OneWayToSourceBindings.ElementProperty, d);
    }
}

public class OneWayToSourceBindings : FrameworkElement
{
    private static readonly PropertyPath DataContextPath = new PropertyPath(nameof(DataContext));
    private static readonly PropertyPath HasErrorPath = new PropertyPath($"({typeof(Validation).Name}.{Validation.HasErrorProperty.Name})");
    public static readonly DependencyProperty HasErrorProperty = DependencyProperty.Register(
        nameof(HasError),
        typeof(bool),
        typeof(OneWayToSourceBindings),
        new FrameworkPropertyMetadata(default(bool), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));

    internal static readonly DependencyProperty ElementProperty = DependencyProperty.Register(
        "Element",
        typeof(UIElement),
        typeof(OneWayToSourceBindings),
        new PropertyMetadata(default(UIElement), OnElementChanged));

    private static readonly DependencyProperty HasErrorProxyProperty = DependencyProperty.RegisterAttached(
        "HasErrorProxy",
        typeof(bool),
        typeof(OneWayToSourceBindings),
        new PropertyMetadata(default(bool), OnHasErrorProxyChanged));

    public bool HasError
    {
        get { return (bool)this.GetValue(HasErrorProperty); }
        set { this.SetValue(HasErrorProperty, value); }
    }

    private static void OnHasErrorProxyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        d.SetCurrentValue(HasErrorProperty, e.NewValue);
    }

    private static void OnElementChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue == null)
        {
            BindingOperations.ClearBinding(d, DataContextProperty);
            BindingOperations.ClearBinding(d, HasErrorProxyProperty);
        }
        else
        {
            var dataContextBinding = new Binding
                                         {
                                             Path = DataContextPath,
                                             Mode = BindingMode.OneWay,
                                             Source = e.NewValue
                                         };
            BindingOperations.SetBinding(d, DataContextProperty, dataContextBinding);

            var hasErrorBinding = new Binding
                                      {
                                          Path = HasErrorPath,
                                          Mode = BindingMode.OneWay,
                                          Source = e.NewValue
                                      };
            BindingOperations.SetBinding(d, HasErrorProxyProperty, hasErrorBinding);
        }
    }
}

Usage in xaml

xaml 中的用法

<StackPanel>
    <TextBox Text="{Binding Value, UpdateSourceTrigger=PropertyChanged}">
        <local:OneWayToSource.Bindings>
            <local:OneWayToSourceBindings HasError="{Binding HasError}" />
        </local:OneWayToSource.Bindings>
    </TextBox>
    <CheckBox IsChecked="{Binding HasError, Mode=OneWay}" />
</StackPanel>

This implementation is specific to binding Validation.HasError

此实现特定于绑定 Validation.HasError

回答by Mario

Hmmm... I'm not sure I agree with any of these solutions. How about specifying a coercion callback in your property registration that ignores external change? For instance, I needed to implement a read-only Position dependency property to get the position of a MediaElement control inside a user control. Here's how I did it:

嗯...我不确定我是否同意这些解决方案中的任何一个。如何在您的属性注册中指定一个忽略外部更改的强制回调？例如，我需要实现一个只读的 Position 依赖属性来获取用户控件中 MediaElement 控件的位置。这是我如何做到的：

    public static readonly DependencyProperty PositionProperty = DependencyProperty.Register("Position", typeof(double), typeof(MediaViewer),
        new FrameworkPropertyMetadata(0d, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Journal, OnPositionChanged, OnPositionCoerce));

    private static void OnPositionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var ctrl = d as MediaViewer;
    }

    private static object OnPositionCoerce(DependencyObject d, object value)
    {
        var ctrl = d as MediaViewer;
        var position = ctrl.MediaRenderer.Position.TotalSeconds;

        if (ctrl.MediaRenderer.NaturalDuration.HasTimeSpan == false)
            return 0d;
        else
            return Math.Min(position, ctrl.Duration);
    }

    public double Position
    {
        get { return (double)GetValue(PositionProperty); }
        set { SetValue(PositionProperty, value); }
    }

In other words, simply ignore the change and return the value backed by a different member that does not have a public modifier. -- In the above example, MediaRenderer is actually the private MediaElement control.

换句话说，只需忽略更改并返回由没有 public 修饰符的不同成员支持的值。-- 在上面的例子中，MediaRenderer 实际上是私有的 MediaElement 控件。

回答by alex2k8

WPF will not use the CLR property setter, but seems it does some odd validation based on it.

WPF 不会使用 CLR 属性设置器，但它似乎基于它进行了一些奇怪的验证。

May be in your situation this can be ok:

可能在你的情况下这可以：

    public bool IsModified
    {
        get { return (bool)GetValue(IsModifiedProperty); }
        set { throw new Exception("An attempt ot modify Read-Only property"); }
    }

回答by JaredPar

You're doing the binding in the wrong direction right now. OneWayToSource will try and update FlagIsModified on container whenever IsModified changes on the control you are creating. You want the opposite, which is to have IsModified bind to container.FlagIsModified. For that you should use the binding mode OneWay

您现在在错误的方向上进行绑定。每当您创建的控件上的 IsModified 更改时，OneWayToSource 将尝试更新容器上的 FlagIsModified。您想要相反的情况，即让 IsModified 绑定到 container.FlagIsModified。为此，您应该使用绑定模式 OneWay

<controls:FlagThingy IsModified="{Binding FlagIsModified, 
                                          ElementName=container, 
                                          Mode=OneWay}" />

Full list of enumeration members: http://msdn.microsoft.com/en-us/library/system.windows.data.bindingmode.aspx

枚举成员的完整列表：http: //msdn.microsoft.com/en-us/library/system.windows.data.bindingmode.aspx