Question

gsvc.project(params, function (projectedPoints) {
    pt = projectedPoints[0];
});

geometry.addPath(pt);

有谁可以告诉我为什么projectedPoints只会在抛出错误后返回？代码不会进入回调并尝试在未定义数组时分配pt。这会导致代码出错，即回调最终启动时。

我一直在尝试引用here中的代码，但似乎无法弄清楚如何让回调像xample一样工作。

` var map，gsvc，pt;

  require([
    "esri/map", "esri/graphic", "esri/symbols/SimpleMarkerSymbol",
    "esri/tasks/GeometryService", "esri/tasks/ProjectParameters",
    "esri/SpatialReference", "esri/InfoTemplate", "dojo/dom", "dojo/on",
    "dojo/domReady!"
  ], function(
    Map, Graphic, SimpleMarkerSymbol,
    GeometryService, ProjectParameters,
    SpatialReference, InfoTemplate, dom, on
  ) {
    map = new Map("map", {
      basemap: "streets",
      center: [-98.445, 46.147],
      zoom: 3
    });

    gsvc = new GeometryService("http://tasks.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer");
    map.on("click", projectToWebMercator);

    function projectToWebMercator(evt) {
      map.graphics.clear();

      var point = evt.mapPoint;
      var symbol = new SimpleMarkerSymbol().setStyle("diamond");
      var graphic = new Graphic(point, symbol);
      var outSR = new SpatialReference(102100);

      map.graphics.add(graphic);

      gsvc.project([ point ], outSR, function(projectedPoints) {
        pt = projectedPoints[0];
        graphic.setInfoTemplate(new InfoTemplate("Coordinates",
          "<span>X:</span>" + pt.x.toFixed() + "<br>" + 
          "<span>Y:</span>" + pt.y.toFixed() + "<br>" + 
          "<input type='button' value='Convert back to LatLong' id='convert'>" +
          "<div id='latlong'></div>"));
        map.infoWindow.setTitle(graphic.getTitle());
        map.infoWindow.setContent(graphic.getContent());
        map.infoWindow.show(evt.screenPoint, map.getInfoWindowAnchor(evt.screenPoint));
        on.once(dom.byId("convert"), "click", projectToLatLong);
      });
    }

    function projectToLatLong() {
      var outSR = new SpatialReference(4326);
      var params = new ProjectParameters();
      params.geometries = [pt.normalize()];
      params.outSR = outSR;

      gsvc.project(params, function(projectedPoints) {
        pt = projectedPoints[0];
        dom.byId("latlong").innerHTML = "<span>Latitude: </span> " + 
          pt.y.toFixed(3) + "<br><span>Longitude:</span>" + pt.x.toFixed(3);
      });
    }
  });
</script>`

Answer 1

您有可变范围问题。需要在回调之外定义pt。

var pt;

gsvc.project(params, function(projectedPoints) {
    pt = projectedPoints[0];
});

geometry.addPath(pt);

修改

如果.project()方法是异步的，那么Arun的回答是正确的方法。

Answer 2

可能是因为项目是异步方法（如果是这样）

//looks like project is a async method
gsvc.project(params, function (projectedPoints) {
    var pt = projectedPoints[0];
    //do all operations which depends on pt in the callback
    geometry.addPath(pt);
});

//so this part will get executed before the callback is executed so pt won't be defined

How do I return the response from an asynchronous call? - 它遵循与ajax请求相同的原则

为什么这个回调不会在JavaScript中返回值？

2 个答案: