Queries and Hit Tests

GeoBlazor is continually adding new interactive features to the SDK. Queries are a common GIS task, and one that we are actively working to support.


Querying a FeatureLayer

FeatureLayer supports the following query methods on hosted layers accessed with a Url or PortalItem.Id. For each, you define a Query, RelationshipQuery, or TopQuery in code, and then pass it to the method.

  • QueryExtent - returns an Extent of features that matches the Query
  • QueryFeatures - returns a FeatureSet of features (graphics) that match the Query. See Server Side Queries Sample Page.
  • QueryFeatureCount - returns the number of features that match the Query
  • QueryObjectIds - returns an array of object ids that match the Query
  • QueryRelatedFeatures - returns a FeatureSet of related features that match the RelationshipQuery. See Query Related Features Sample Page.
  • QueryRelatedFeatureCount - returns the number of related features that match the RelationshipQuery
  • QueryTopFeatures - returns a FeatureSet of the features that match the TopFeaturesQuery. See Query Top Features Sample Page.
  • QueryTopFeatureCount - returns the number of the features that match the TopFeaturesQuery
  • QueryTopObjectIds - returns an array of the top object ids that match the TopFeaturesQuery
  • QueryTopFeaturesExtent - returns an Extent of the top features that match the TopFeaturesQuery

Querying a LayerView

FeatureLayerView supports a lot of the same queries as FeatureLayer. The difference is that while layer queries run against the hosted layer service, layerView queries run against the rendered view. See the Hit Tests Sample Page for an example where the query results are used to call LayerView.Highlight().

Querying a Layer from the MapView

In an early prototype of GeoBlazor, we added a custom query method to MapView called QueryFeatures. Unlike the methods on FeatureLayer and FeatureLayerView, this method does not return the results, but instead automatically updates the display based on the inputs. It takes in a Symbol and PopupTemplate which define how the queried features will be displayed on the map. See the Sql Query Sample Page for an example of how to use this method.

Query layerQuery = await _layerView.CreateQuery();
layerQuery.Where = $"YEAR = {newYear} AND NAME = '{newName}'";
int[] ids = await _layerView.QueryObjectIds(layerQuery);
_highlightHandle = await _layerView!.Highlight(ids);

Hit Tests

When reacting to an event handler, one common type of query is the HitTest, which determines which graphics intersect with a particular point on the map. This is useful for things like highlighting a feature when the user clicks on it. See the Hit Tests Sample Page

HitTestOptions options = new()
    IncludeByGeoBlazorId = new[] { _hurricaneLayer!.Id }
HitTestResult result = await _mapView.HitTest(pointerEvent, options);
Graphic? graphic = result.Results.OfType<GraphicHit>().FirstOrDefault()?.Graphic;
graphic?.Attributes.TryGetValue("OBJECTID", out object? objectId);
int? graphicId = objectId as int?;

Note that while ArcGIS supports other types of Hits than Graphics, GeoBlazor currently only fully supports GraphicHit.