wpf - 来自 XAML 中只读属性的 OneWayToSource 绑定(bind)

Question

我正在尝试以 OneWayToSource 作为模式绑定(bind)到 Readonly 属性，但似乎无法在 XAML 中完成:

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

我得到:

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

IsModified 是 FlagThingy 上的只读 DependencyProperty。我想将该值绑定(bind)到容器上的 FlagIsModified 属性。

需要明确的是:

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

仅使用 XAML 可以吗？

<小时/>

更新:嗯，我通过在容器上而不是在 FlagThingy 上设置绑定(bind)来解决此问题。但我还是想知道这是否可能。

Answer 1

OneWayToSource 的一些研究结果...

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);

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}" />

选项#3(真正的只读依赖属性)

System.ArgumentException:“IsModified”属性无法进行数据绑定(bind)。

// 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 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");
    }
 ....