爲什麼這個回調不會在JavaScript中返回一個值？

Question

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

geometry.addPath(pt); 

誰能告訴我爲什麼projectedPoints只有在返回錯誤時纔會返回？代碼不會進入回調，並嘗試將未定義的數組分配給數組。這會導致代碼出錯，這是回調最終啓動的時間。

我一直試圖引用here的代碼，但似乎無法弄清楚如何讓回調工作，就像在xample中一樣。

` 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

您有一個可變範圍問題。需要在回調之外定義pt。

var pt; 

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

geometry.addPath(pt); 

編輯：

如果.project()方法是異步的，那麼阿倫的答案是正確的做法。

Answer 2

這可能是因爲項目是一個異步方法，如果這樣

//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 

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