如何将嵌套的视图模型绑定到控件的属性

Question

I used Microsoft's Chart Control of the WPF toolkit to write my own chart control. I blogged about it here . My Chart control stacks the yaxes in the chart on top of each other. As you can read in the article this all works quite well. Now I want to create a viewmodel that controls the data and axes in the chart. So far I'm able to add axes to the chart and show them in the chart. But I have a problem when I try to add the lineseries because it has one DependentAxis and one InDependentAxis property. I don't know how to assign the proper xAxis and yAxis controls to it.
Below you see part of the LineSeriesViewModel. It has a nested XAxisViewModel and YAxisViewModel property.

public class LineSeriesViewModel : ViewModelBase, IChartComponent
{

    XAxisViewModel _xAxis;
    public XAxisViewModel XAxis
    {
        get { return _xAxis; }
        set
        {
            _xAxis = value;
            RaisePropertyChanged(() => XAxis);
        }
    }

    //The YAxis Property look the same
}

The viewmodels all have their own datatemplate. The xaml code looks like this:

<UserControl.Resources>
    <DataTemplate x:Key="xAxisTemplate" DataType="{x:Type l:YAxisViewModel}">
        <chart:LinearAxis  x:Name="yAxis"  Orientation="Y" Location="Left" Minimum="0"  Maximum="10" IsHitTestVisible="False" Width="50" />
    </DataTemplate>
    <DataTemplate x:Key="yAxisTemplate" DataType="{x:Type l:XAxisViewModel}">
        <chart:LinearAxis x:Name="xAxis"  Orientation="X" Location="Bottom" Minimum="0"  Maximum="100" IsHitTestVisible="False" Height="50" />
    </DataTemplate>

    <DataTemplate DataType="{x:Type l:LineSeriesViewModel}">
        <!--Binding doesn't work on the Dependent and IndependentAxis! -->
        <!--YAxis XAxis and Series are properties of the LineSeriesViewModel -->
        <l:FastLineSeries DependentAxis="{Binding Path=YAxis}" 
                          IndependentAxis="{Binding Path=XAxis}"
                          ItemsSource="{Binding Path=Series}"/>
    </DataTemplate>
    <Style TargetType="ItemsControl">
        <Setter Property="ItemsPanel">
            <Setter.Value>
                <ItemsPanelTemplate>
                    <!--My stacked chart control -->
                    <l:StackedPanel x:Name="stackedPanel" Width="Auto" Height="Auto" Background="LightBlue">
                    </l:StackedPanel>
                </ItemsPanelTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</UserControl.Resources>
<Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch" ClipToBounds="True">
    <!-- View is an ObservableCollection of all axes and series-->
    <ItemsControl x:Name="chartItems" ItemsSource="{Binding Path=View}" Focusable="False">
    </ItemsControl>
</Grid>

This code works quite well. When I add axes they get drawn. But the DependentAxis and InDependentAxis of the lineseries control stay null, so the series doesn't get drawn. How can I bind the nested viewmodels to the properties of a control?

Answer 1

It should work. A few things you can check:

• Does the Series Binding work? If so, try to figure out what's the difference.
• Are you sure that the XAxis and YAxis properties actually have values? Try putting a breakpoint in the getter. If it's reached, the Binding works. You can also put a converter (IValueConverter) on the Binding (that simply returns the value it receives) and place a breakpoint there.
• Use PresentationTraceSources.TraceLevel=High on the Binding to get more verbose tracing (that will appear in the VS Output window).
• Are DependentAxis/IndependentAxis defined as dependency properties on FastLineSeries?

Hope that helps,

Aelij.

Answer 2

You've probably already checked this but I find that when I'm debugging bindings the first and easiest place to start is running a debug session from VS as the debug output tells which objects and properties are failing to bind. I usually end up discovering I need to explicitly set a DataContext or something else like a typo. The output to look for start like this:

System.Windows.Data Error: 39 : BindingExpression path error:

this is followed by the property name you tried to bind to and usually most importantly the class against which its actually trying to bind. If this doesn't help there's a great article here on the debugging bindings: http://www.beacosta.com/blog/?p=52 which discusses the use of PresentationTraceSources.TraceLevel=High which Aelij mentioned, as well as a few other techniques. Hope this gets onto the right track.

Regards,

Mike