ArcGIS API for Silverlight实现地图测距功能

问题：如何实现地图测距功能？

地图工具栏

 <Grid x:Name="gToolMenu"  Height="100" VerticalAlignment="Top" Opacity="0.8" HorizontalAlignment="Right" Width="467">
            <Rectangle Fill="#22919191" RadiusX="10" RadiusY="10" Margin="0,0,0,6" >
                <Rectangle.Effect>
                    <DropShadowEffect/>
                </Rectangle.Effect>
            </Rectangle>
            <Rectangle Fill="#CCFFFFFF" Stroke="DarkGray" RadiusX="5" RadiusY="5" Margin="5,10,5,10" />
            <StackPanel Orientation="Vertical">
                <esri:Toolbar x:Name="MyToolbar" MaxItemHeight="80" MaxItemWidth="80"
                    VerticalAlignment="Top" HorizontalAlignment="Center"
                    Loaded="MyToolbar_Loaded"
                    ToolbarItemClicked="MyToolbar_ToolbarItemClicked" 
                    Width="458" Height="80">
                    <esri:Toolbar.Items>
                        <esri:ToolbarItemCollection>
                            <esri:ToolbarItem Text="Zoom In">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_zoomin.png" Stretch="Uniform" Margin="5" />
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Zoom Out">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_zoomout.png" Stretch="Uniform" Margin="5" />
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Pan">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_pan.png" Stretch="Uniform" Margin="5" />
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Measure Length">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_measure.png" Margin="5">
                                        <i:Interaction.Triggers>
                                            <i:EventTrigger EventName="MouseLeftButtonDown">
                                                <esri:MeasureAction TargetName="myMap" AreaUnit="SquareMeters"  MeasureMode="Polyline" DistanceUnit="Kilometers"   FillSymbol="{StaticResource DefaultFillSymbol}"  
DisplayTotals="True" MapUnits="Kilometers"/>
                                            </i:EventTrigger>
                                        </i:Interaction.Triggers>
                                    </Image>
                                </esri:ToolbarItem.Content>

                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Previous Extent" >
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_previous.png" IsHitTestVisible="False" Opacity="0.3" Stretch="Uniform" Margin="5"/>
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Next Extent">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_next.png" IsHitTestVisible="False" Opacity="0.3" Stretch="Uniform" Margin="5"/>
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Full Extent">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_globe.png" Stretch="Uniform" Margin="5" />
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>
                            <esri:ToolbarItem Text="Full Screen">
                                <esri:ToolbarItem.Content>
                                    <Image Source="Images/i_widget.png" Stretch="Uniform" Margin="5" />
                                </esri:ToolbarItem.Content>
                            </esri:ToolbarItem>

                        </esri:ToolbarItemCollection>
                    </esri:Toolbar.Items>
                </esri:Toolbar>
            </StackPanel>
        </Grid>

上面是工具栏代码，测距的xaml代码从中抽取如下：

 <esri:ToolbarItem Text="Measure Length">
     <esri:ToolbarItem.Content>
               <Image Source="Images/i_measure.png" Margin="5">
                       <i:Interaction.Triggers>
                               <i:EventTrigger EventName="MouseLeftButtonDown">
                                      <esri:MeasureAction TargetName="myMap" AreaUnit="SquareMeters"  MeasureMode="Polyline" DistanceUnit="Kilometers"   FillSymbol="{StaticResource DefaultFillSymbol}"  
DisplayTotals="True" MapUnits="Kilometers"/>
                              </i:EventTrigger>
                        </i:Interaction.Triggers>
               </Image>
     </esri:ToolbarItem.Content>
 </esri:ToolbarItem>

代码分析：

为图片Image添加Triggers,监听Image的鼠标左键按下事件，然后执行MeasureAction，目标是myMap地图，测量方式是Polyline，单位是千米（Kilometers），符号使用的是一个静态资源。

 <esri:ToolbarItem Text="Measure Length">
              <esri:ToolbarItem.Content>
                             <Image Source="Images/i_measure.png" Margin="5">
                                        <i:Interaction.Triggers>
                                            <i:EventTrigger EventName="MouseLeftButtonDown">
                                                <esri:MeasureAction TargetName="myMap" AreaUnit="SquareMeters"  MeasureMode="Polyline" DistanceUnit="Kilometers"   FillSymbol="{StaticResource DefaultFillSymbol}"  
DisplayTotals="True" MapUnits="Kilometers"/>
                                            </i:EventTrigger>
                                        </i:Interaction.Triggers>
                           </Image>
        </esri:ToolbarItem.Content>
</esri:ToolbarItem>

当按下测距按钮时，可以在地图上画线测距了，效果如下图：

注：其它要注意引用的地方

 xmlns:esri="http://schemas.esri.com/arcgis/client/2009"
 xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity" 

<Grid.Resources>

        <esri:SimpleFillSymbol x:Key="DefaultFillSymbol" BorderBrush="Red" BorderThickness="1"/>
 </Grid.Resources>

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