1、GeometryService计算面积和长度（Calculate_Feature_Length_Or_Area）功能实现

本文主要介绍利用GeometryService实现面积和长度的计算。首先看一下实现本功能的主要过程：

接下来就一步步介绍实现本功能的具体步骤和方法：

（本文的代码来自https://developers.arcgis.com/javascript/jssamples/util_measurepoly.html，本文只是提炼部分核心代码，讲解实现的思路，如需获取所有代码，请参考上述链接）

1、创建Draw对象

 var tb = new Draw(map);
 tb.on("draw-end", lang.hitch(map, getAreaAndLength));
 tb.activate(Draw.FREEHAND_POLYGON);

2、创建GeomeService对象

var geometryService = new GeometryService("http://sampleserver6.arcgisonline.com/arcgis/rest/services/Utilities/Geometry/GeometryServer");
geometryService.on("areas-and-lengths-complete", outputAreaAndLength);

3、getAreaAndLength方法的实现，注意本方法只有在draw-end事件触发时才会执行

   　　　　function getAreaAndLength(evtObj) {
              var map = this,
                  geometry = evtObj.geometry;
              map.graphics.clear();

              var graphic = map.graphics.add(new Graphic(geometry, new SimpleFillSymbol()));

              //setup the parameters for the areas and lengths operation
              var areasAndLengthParams = new AreasAndLengthsParameters();
              areasAndLengthParams.lengthUnit = GeometryService.UNIT_FOOT;
              areasAndLengthParams.areaUnit = GeometryService.UNIT_ACRES;
              areasAndLengthParams.calculationType = "geodesic";
              geometryService.simplify([geometry], function (simplifiedGeometries) {
                  areasAndLengthParams.polygons = simplifiedGeometries;
                  geometryService.areasAndLengths(areasAndLengthParams);
              });
          }

4、outputAreaAndLength方法的实现，注意，本方法只有GeomeService对象计算完面积和周长才会触发

        function outputAreaAndLength(evtObj) {
              var result = evtObj.result;
              console.log(json.stringify(result));
              dom.byId("area").innerHTML = result.areas[0].toFixed(3) + " acres";
              dom.byId("length").innerHTML = result.lengths[0].toFixed(3) + " feet";
          }    

下面看一下效果图：