WPF：用户控件的依赖项属性

Question

OK, I don't get it and I know, this question has been asked and answered at least 10000 times.... but maybe I have some kind of special case here or I just don't get it.

I have a usercontrol that is is called Statisticspopup and it has a DependencyProperty as shown here:

public static readonly DependencyProperty XValueProperty = DependencyProperty.Register(
    "XValue", typeof(double), typeof(Statisticspopup),
    new FrameworkPropertyMetadata(XValueChanged));

public double XValue
{
    get
    {
        var x = GetValue(XProperty);
        return (double)x;
    }
    set
    {
        SetValue(XProperty, value);
    }
}

private static void XValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var control = (Statisticspopup)d;
    control.XValue = double.Parse(e.NewValue.ToString());
    System.Diagnostics.Debug.WriteLine("XValueChanged");
}

I use it in my xaml code like this:

<controls:Statisticspopup XValue="42" />

This works and everything is fine... Now I want to use a binding for the property, something like this:

<controls:Statisticspopup XValue="{Binding DataPoint.X,PresentationTraceSources.TraceLevel=High}" />

The DataPoint.X value is from another control (OxyPlot object), so the whole code looks like this:

<oxy:Plot x:Name="PlotThing" Title="{Binding Title}"  Style="{DynamicResource PlotStyle}" >
    <oxy:Plot.TrackerDefinitions>
        <oxy:TrackerDefinition TrackerKey="someKey" >
            <oxy:TrackerDefinition.TrackerTemplate>
                <ControlTemplate>
                    <oxy:TrackerControl Name="TrackerControl" DataContext="{Binding }" Position="{Binding Position}" LineExtents="{Binding PlotModel.PlotArea}">
                        <oxy:TrackerControl.Content>     

<controls:Statisticspopup XValue="{Binding DataPoint.X,PresentationTraceSources.TraceLevel=High}" />
<TextBlock Foreground="Aquamarine" Text="{Binding DataPoint.X, PresentationTraceSources.TraceLevel=High}"></TextBlock>
....

As you can see, I also added a TextBlock to the TrackerControl.Content tag. Unfortunately the TextBlock shows the correct value, but I don't receive the binding in my usercontrol.

I get this output error:

BindingExpression path error: 'DataPoint' property not found on 'object' ''StatisticspopupViewModel' (HashCode=3740464)'.
BindingExpression:Path=DataPoint.X; DataItem='StatisticspopupViewModel' (HashCode=3740464); target element is 'Statisticspopup' (Name=''); target property is 'XValue' (type 'Double')

If I have a look to the TextBox, everything is ok.

I think it is somehow related to the Binding.Path property, as it tries to access the StatisticspopupViewModel which is definitely wrong. The output from the TextBox:

• At level 0 - for TrackerHitResult.DataPoint found accessor ReflectPropertyDescriptor(DataPoint)
• Replace item at level 0 with TrackerHitResult, using accessor ReflectPropertyDescriptor(DataPoint)
• GetValue at level 0 from TrackerHitResult using ReflectPropertyDescriptor(DataPoint): DataPoint
• At level 1 - for DataPoint.X found accessor ReflectPropertyDescriptor(X)
• Replace item at level 1 with DataPoint , using accessor ReflectPropertyDescriptor(X)
• GetValue at level 1 from DataPoint using ReflectPropertyDescriptor(X): '9' TransferValue - got raw value '9'
• TransferValue - implicit converter produced '9'
• TransferValue - using final value '9'

An finally the value is displayed...

Any idea for this issue?

Answer 1

Your PropertyChangedCallback is broken. It must not contain the line

control.XValue = double.Parse(e.NewValue.ToString());

which btw. should be looking like control.XValue = (double)e.NewValue;

The method is a "changed" callback for the XValue property, and therefore called when the property value has already change. It should not (and must not) set the value again, because this effectivly removes the Binding from the property.

private static void XValueChanged(
    DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var control = (Statisticspopup)d;

    // here control.XValue is already identical to (double)e.NewValue;

    Debug.WriteLine("XValueChanged: {0}, {1}", e.NewValue, control.XValue);
}

Answer 2

Ok, I got it working. If I remove the ViewModel binding, the code is able to set the dependency properties and to use them in the correct way. This post here explains how to do it.