Canvas UserControl DataContext更新问题

Question

我开发了一个usercontrol，里面有一个画布。我在画布上画了一些线条和弧线。每当datacontext发生更改时，我都需要在更新新的datacontext之前强制usercontrol的datacontext为null。除非新图纸没有更新。这是为什么？如何更新Usercontrol的内容（实际上是canvas的内容），而不是在更新新的datacontext之前将datacontext重置为null。

我使用以下代码。 FlashControl是包含canvas内部的usercontrol。

        (FlashControl.Content as FrameworkElement).DataContext = null;
        (FlashControl.Content as FrameworkElement).DataContext = this.DataContext;
        FlashControl.DataContext = this.DataContext;

更新 :::::::这是我的用户控件

<UserControl x:Class="PricingRoutine.FlashGeneratorUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:Converters="clr-namespace:PricingRoutine.Converters"
             xmlns:local ="clr-namespace:CustomFlashGenerator"
             mc:Ignorable="d" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"  HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch">

    <UserControl.Resources>
        <Converters:TextPositionConverter x:Key="TextPositionConverter" />
        <Converters:LinetPositionConverter x:Key="LinePositionConverter" />
        <Converters:PointPositionConverter x:Key="PointPositionConverter" />
    </UserControl.Resources>

    <Grid x:Name="MajorGrid" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
        <ItemsControl ItemsSource="{Binding Path = CompositeObjectList}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <!--<ViewBox Stretch="Uniform">-->
                    <Canvas x:Name="MainCanvas" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"  ClipToBounds="False">
                        <Canvas.RenderTransformOrigin>
                            <Point X="0.5" Y="0.5" />
                        </Canvas.RenderTransformOrigin>

                        <Canvas.RenderTransform>
                            <TransformGroup>
                                <TranslateTransform X="0" Y="0" />                                           <!--Handle this one through binding-->
                                <ScaleTransform ScaleX="1" ScaleY="1" CenterX=".5" CenterY=".5" />       <!--Handle this one through binding-->
                            </TransformGroup>
                        </Canvas.RenderTransform>
                    </Canvas>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.Resources>
                <DataTemplate DataType="{x:Type local:Line}">
                    <Line Stroke="Black" StrokeThickness="1">
                        <Line.X1>
                            <MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="X1">
                                <Binding ElementName="MainCanvas"  Path="ActualWidth" />
                                <Binding ElementName="MainCanvas" Path="ActualHeight" />
                                <Binding Path = "StartPoint.X" />
                            </MultiBinding>
                        </Line.X1>
                        <Line.Y1>
                            <MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="Y1">
                                <Binding ElementName="MainCanvas"  Path="ActualWidth" />
                                <Binding ElementName="MainCanvas" Path="ActualHeight" />
                                <Binding Path = "StartPoint.Y" />
                            </MultiBinding>
                        </Line.Y1>
                        <Line.X2>
                            <MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="X2">
                                <Binding ElementName="MainCanvas"  Path="ActualWidth" />
                                <Binding ElementName="MainCanvas" Path="ActualHeight" />
                                <Binding Path = "EndPoint.X" />
                            </MultiBinding>
                        </Line.X2>
                        <Line.Y2>
                            <MultiBinding Converter="{StaticResource LinePositionConverter}" ConverterParameter="Y2">
                                <Binding ElementName="MainCanvas"  Path="ActualWidth" />
                                <Binding ElementName="MainCanvas" Path="ActualHeight" />
                                <Binding Path = "EndPoint.Y" />
                            </MultiBinding>
                        </Line.Y2>
                    </Line>
                </DataTemplate>
            </ItemsControl.Resources>
        </ItemsControl>
    </Grid>
</UserControl>