Silverlight 中实现 marquee 的效果 滚动显示 火麒

<ScrollViewer HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden" BorderThickness="0" Height="36">
                <Canvas Loaded="MarqueeControl_Loaded" Width="592" HorizontalAlignment="Left">
                              <Canvas.Resources>
                        <Storyboard x:Name="sb">
                            <DoubleAnimation x:Name="da" BeginTime="00:00:02" Storyboard.TargetName="txtLorem" Storyboard.TargetProperty="(Canvas.Left)" From="0" RepeatBehavior="Forever"/>
                        </Storyboard>
                    </Canvas.Resources>
                    <TextBlock x:Name="txtLorem" Width="596" Height="33" Text="滚动文字内容..."/>
                </Canvas>
</ScrollViewer>

void MarqueeControl_Loaded(object sender, RoutedEventArgs e)
        {
            var canvas = sender as Canvas;
            if (txtLorem.ActualWidth <= canvas.Width) return;
            const double speed = 50;
            da.To = -txtLorem.ActualWidth;
            da.Duration = new Duration(TimeSpan.FromSeconds(txtLorem.ActualWidth / speed));
            sb.Begin();
        }