//declare custom layer to talk to mergeAndOutput.*
//request JSON
// {
//   url: <String>,
//   format: <String, jpg/png>,
//   opacity: <Number, 0 - 1>,
//   layers: <String, show:[layerIds], optional>,
//   layerDefs: <String, id1:def1;id2:def2;...>
// }
//response JSON is same as http://resources.esri.com/help/9.3/ArcGISServer/apis/REST/export.html
//scale property not returned
dojo.declare("my.PrintableLayer", esri.layers.DynamicMapServiceLayer, {
    //constructor for layer with custom required option: layers, extent
    constructor: function(url, options) {
        this.layers = options.layers;
        this.initialExtent = this.fullExtent = options.extent;
        this.spatialReference = this.initialExtent.spatialReference;

        this.loaded = true;
        this.onLoad(this);
    },

    //call mergeAndOutput.*, pass request JSON.
    //process response JSON and return image url
    getImageUrl: function(extent, width, height, callback) {
        try {
            var layersJson = [], layerJson;

            dojo.forEach(this.layers, function(layer) {
                if (layer.visible) {
                    layerJson = { url: layer.url, format: (layersJson.length == 0 ? "jpg" : "png"), opacity: layer.opacity };

                    if (layer.layers) {
                        layerJson.layers = "show:" + layer.layers.join(",");
                    }
                    if (layer.layerDefs) {
                        var defs = [];
                        dojo.forEach(layer.layerDefs, function(def, index) {
                            if (def) {
                                defs.push(index + ":" + def);
                            }
                        });
                        layerJson.layerDefs = defs.join(";");
                    }

                    layersJson.push(dojo.toJson(layerJson));
                }
            });

            //request JSON
            var json = {
                width: map.width,
                height: map.height,
                extent: dojo.toJson(extent.toJson()),
                layers: layersJson
            };

            //send request to mergeAndOutput.php, force proxy use
            esri.request({
                url: this.url,
                postData: dojo.toJson(json),
                handleAs: "json",
                load: function(response, io) {
                    //on load, call callback and pass image url
                    callback(response.href);
                },
                error: this.onError
            }, true);
        }
        catch (err) {
            this.onError(err);
        }
    }
});