arcgis for javascript api做缓冲区分析

需求内容: 对某个arcgis server发布的服务进行缓冲区分析，并且下载缓冲区的结果要素图层

思路: 

1. 通过arcgis 服务的 find操作拿到所有的要素
2. 对要素进行缓冲区分析
3. 下载缓冲区分析的结果

项目使用的ags for javascript是3.14的。

1.获取arcgis 服务的所有的要素，可以使用 下面的两个类进行查找，接口返回的结果是要素的数组

"esri/tasks/FindTask",
"esri/tasks/FindParameters",

function findAllGeometry(layer){
    if (layer.url){
        var find = new FindTask(layer.url);
        var parameter = new FindTaskParameter();
        parameter.layerIds = [0];
        parameter.searchText = 0;
        parameter.returnGeometry = true
        try{
            find.execute(parameter,processResult)
        }catch(e){
            layer.msg("查找要素服务失败")
        }
    }
}
2. 进行缓冲区分析可以使用下面api的两个类进行分析（因为arcgis server自带的utilities/Geometry下面有buffer的服务工具）

"esri/tasks/GeometryService",
"esri/tasks/BufferParameters",

var _buffer = function (geometry, distance, callback, option) {
    if (_geometryService == undefined) {
        _geometryService = new GeometryService(_url);
    }
    if (distance == undefined)
        distance = DEFAULT_BUFFER_DISTANCE;
    on.once(_geometryService, 'buffer-complete', lang.hitch(this, callback));
    on.once(_geometryService, 'error', one rrorHandler);

    lang.mixin(_bufferOption, option);
    var params = new BufferParameters();
    params.geometries = lang.isArray(geometry) ? geometry : [geometry];
    params.distances = lang.isArray(distance) ? distance : [distance];
    params.unit = _bufferOption.unit;
    params.unionResults = option.unionResults?option.unionResults:false;
    params.bufferSpatialReference = _bufferOption.bufferSpatialReference;
    params.outSpatialReference = _bufferOption.outSpatialReference;
    _geometryService.buffer(params);
};
参数的unionResults如果为ture返回的结果只有一个要素，如果为false会返回和geometry数组一样的数组,geometry的类型是arcgis for javascript的类型。
3.进行下载需要将返回的geometry转化为geojson进行下载，调用接口进行下载

_expToShp: function (geo) {
    var features = [];
    var sr, wkt;
    for (var i = 0; i < geo.length; i++) {
        var geometry = geo[i].geometry;
        var attr = geo[i].attributes;
        var coordinates = [];
        if (geometry) {
            sr = geometry.spatialReference;
            wkt = sr.wkid;
            var rings = geometry.rings;
            for (var j = 0; j < rings.length; j++) {
                var ring = rings[j];
                coordinates.push(ring);
            }
        }
        var feature = {
            "type": "Feature",
            "geometry": {"type": geometry.type, "coordinates": coordinates},
            "properties": attr
        };
        features.push(feature);
    }
    if (sr) {
        wkt = sr.wkid;
    }
    var urlVar = {};
    urlVar.geometry = JSON.stringify({"type": "FeatureCollection", "features": features});
    urlVar.sr = wkt;
    exportFile(urlVar, root + "/geometryService/export/shp");
}