If you use ArcGISDynamicMapServiceLayer, REST and browser request caching, VisibleLayers can cause n! (factorial) requests to MapServer.Order of layers in REST Map Server is defined in MDX file, therefore asking for {1,2} or {2,1} it's always translated to the order defined in MDX.ExampleIf we have for instance two layers that we want to show by ArcGISDynamicMapServiceLayer by setting VisibleLayers. Let�??s assume that our layers ID are as follow: 1 and 2.
- If we will set VisibleLayers in code to {1,2}, request to REST is generated and Image is returned �?? that�??s fine.
- Now if we will set VisibleLayers to empty {} so no Images will be show, REST request is not generated and base map is showed without any images on it �?? that�??s fine.
- If we will then set VisibleLayers in code to {1,2}, REST request is not generated and Image is returned from cache.
- If we will then set VisibleLayers in code to {2,1}, request to REST is generated and Image is returned �?? that�??s fine, I understand why this generates the request I could think of few places where we can take advantage of that if map servers allows it;
- Now if we will set VisibleLayers to empty {} so no Images will be show, REST request is not generated and base map is showed without any images on it �?? that�??s fine.
- If we will then set VisibleLayers in code to {2,1}, REST request is not generated and Image is returned from cache.
This would be a proper behaviour if sort order of VisibleLayers would implicated that image returned by {1,2} is different than image returned by {2,1}. But it�??s not, is the same image. Map Server is sorting layers ID based on MDX file on the server, so our request {2,1} is transformed into {1,2} (or vice versa), but still request is generated, and we will download the Image to client.SolutionIMO if this is a known limitation, this should be provided in SDK documentation, and/or GetImageUrl method in MapService should do sorting for us.
internal void GetImageUrl(Envelope extent, int width, int height, int[] layers, string imageFormat, bool disableClientCaching, bool useMime, ObservableCollection<LayerDefinition> layerDefinitions, TimeExtent timeInterval, UrlCompleteHandler onComplete);
Declaring Type: ESRI.ArcGIS.Client.Services.MapService
Assembly: ESRI.ArcGIS.Client, Version=2.0.0.314
it can be achieved by just replacing this code:
builder.Append("&layers=show:");
for (int i = 0; i < layers.Length; i++)
{
if (i > 0)
{
builder.Append(",");
}
builder.AppendFormat("{0}", new object[] { layers });
}
with this (second line):
builder.Append("&layers=show:");
Array.Sort(layers);
for (int i = 0; i < layers.Length; i++)
{
if (i > 0)
{
builder.Append(",");
}
builder.AppendFormat("{0}", new object[] { layers });
}
WorkaroundIf we don�??t want to have this additional and unnecessary request to REST we just need to sort Array before assigning it to VisibleLayers. But this little tweak, instead of n! (if we are working on the set, n is number of elements in set, ! - factorial) requests to map server for images we will always have one. Even if we are not working on the set, so each time we select a layer we add it to VisibleLayers, it�??s still better to sort it.
