Show labels on layers

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Display custom labels on a feature layer.

Image of show labels on layers

Use case

Labeling features is useful to visually display a key piece of information or attribute of a feature on a map. For example, you may want to label rivers or street with their names.

How to use the sample

Pan and zoom around the United States. Labels for congressional districts will be shown in red for Republican districts and blue for Democrat districts. Notice how labels pop into view as you zoom in.

How it works

  1. Create a ServiceFeatureTable using a feature service URL.
  2. Create a FeatureLayer from the service feature table.
  3. Create a TextSymbol to use for displaying the label text.
  4. Create a JSON string for the label definition.

    • Set the "LabelExpressionInfo.expression" key to express what the text the label should display. You can use fields of the feature by using $feature.field_name in the expression.
    • To use the text symbol, set the "symbol" key to the symbol's JSON representation using textSymbol.ToJson().
  5. Create a label definition from the JSON using LabelDefinition.FromJson(json).
  6. Add the definition to the feature layer with featureLayer.LabelDefinitions.Add(labelDefinition) .
  7. Lastly, enable labels on the layer using featureLayer.LabelsEnabled.

Relevant API

  • FeatureLayer
  • LabelDefinition
  • TextSymbol

About the data

This sample uses the USA 116th Congressional Districts feature layer hosted on ArcGIS Online.

Additional information

Help regarding the JSON syntax for defining the LabelDefinition.FromJson syntax can be found in labeling info in the Web map specification.

Tags

attribute, deconfliction, label, labeling, string, symbol, text, visualization

Sample Code

ShowLabelsOnLayer.cs
                                                                                                                                                                    
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// Copyright 2018 Esri.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License.
// You may obtain a copy of the License at: http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific
// language governing permissions and limitations under the License.

using System;
using Android.App;
using Android.OS;
using Android.Widget;
using Esri.ArcGISRuntime.Data;
using Esri.ArcGISRuntime.Geometry;
using Esri.ArcGISRuntime.Mapping;
using Esri.ArcGISRuntime.UI.Controls;

namespace ArcGISRuntime.Samples.ShowLabelsOnLayer
{
    [Activity (ConfigurationChanges=Android.Content.PM.ConfigChanges.Orientation | Android.Content.PM.ConfigChanges.ScreenSize)]
    [ArcGISRuntime.Samples.Shared.Attributes.Sample(
        name: "Show labels on layers",
        category: "Layers",
        description: "Display custom labels on a feature layer.",
        instructions: "Pan and zoom around the United States. Labels for congressional districts will be shown in red for Republican districts and blue for Democrat districts. Notice how labels pop into view as you zoom in.",
        tags: new[] { "attribute", "deconfliction", "label", "labeling", "string", "symbol", "text", "visualization" })]
    public class ShowLabelsOnLayer : Activity
    {
        // Create and hold reference to the used MapView.
        private MapView _myMapView;

            // Help regarding the Json syntax for defining the LabelDefinition.FromJson syntax can be found here:
            // https://developers.arcgis.com/web-map-specification/objects/labelingInfo/
            private const string RedLabelJson =
             @"{
                    ""labelExpressionInfo"":{""expression"":""$feature.NAME + ' (' + left($feature.PARTY,1) + ')\\nDistrict' + $feature.CDFIPS""},
                    ""labelPlacement"":""esriServerPolygonPlacementAlwaysHorizontal"",
                    ""where"":""PARTY = 'Republican'"",
                    ""symbol"":
                        {
                            ""angle"":0,
                            ""backgroundColor"":[0,0,0,0],
                            ""borderLineColor"":[0,0,0,0],
                            ""borderLineSize"":0,
                            ""color"":[255,0,0,255],
                            ""font"":
                                {
                                    ""decoration"":""none"",
                                    ""size"":10,
                                    ""style"":""normal"",
                                    ""weight"":""normal""
                                },
                            ""haloColor"":[255,255,255,255],
                            ""haloSize"":2,
                            ""horizontalAlignment"":""center"",
                            ""kerning"":false,
                            ""type"":""esriTS"",
                            ""verticalAlignment"":""middle"",
                            ""xoffset"":0,
                            ""yoffset"":0
                        }
               }";

            private const string BlueLabelJson =
                @"{
                    ""labelExpressionInfo"":{""expression"":""$feature.NAME + ' (' + left($feature.PARTY,1) + ')\\nDistrict' + $feature.CDFIPS""},
                    ""labelPlacement"":""esriServerPolygonPlacementAlwaysHorizontal"",
                    ""where"":""PARTY = 'Democrat'"",
                    ""symbol"":
                        {
                            ""angle"":0,
                            ""backgroundColor"":[0,0,0,0],
                            ""borderLineColor"":[0,0,0,0],
                            ""borderLineSize"":0,
                            ""color"":[0,0,255,255],
                            ""font"":
                                {
                                    ""decoration"":""none"",
                                    ""size"":10,
                                    ""style"":""normal"",
                                    ""weight"":""normal""
                                },
                            ""haloColor"":[255,255,255,255],
                            ""haloSize"":2,
                            ""horizontalAlignment"":""center"",
                            ""kerning"":false,
                            ""type"":""esriTS"",
                            ""verticalAlignment"":""middle"",
                            ""xoffset"":0,
                            ""yoffset"":0
                        }
               }";

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            Title = "Show labels on layer";

            // Create the UI, setup the control references and execute initialization.
            CreateLayout();
            Initialize();
        }

        private async void Initialize()
        {
            // Create a map with a light gray canvas basemap.
            Map sampleMap = new Map(BasemapStyle.ArcGISLightGray);

            // Assign the map to the MapView.
            _myMapView.Map = sampleMap;

            // Define the URL string for the feature layer.
            string layerUrl = "https://services.arcgis.com/P3ePLMYs2RVChkJx/arcgis/rest/services/USA_115th_Congressional_Districts/FeatureServer/0";

            // Create a service feature table from the URL.
            ServiceFeatureTable featureTable = new ServiceFeatureTable(new System.Uri(layerUrl));

            // Create a feature layer from the service feature table.
            FeatureLayer districtFeatureLabel = new FeatureLayer(featureTable);

            // Add the feature layer to the operations layers collection of the map.
            sampleMap.OperationalLayers.Add(districtFeatureLabel);

            try
            {
                // Load the feature layer - this way we can obtain it's extent.
                await districtFeatureLabel.LoadAsync();

                // Zoom the map view to the extent of the feature layer.
                await _myMapView.SetViewpointCenterAsync(new MapPoint(-10846309.950860, 4683272.219411, SpatialReferences.WebMercator), 20000000);

                // Create a label definition from the JSON string.
                LabelDefinition redLabelDefinition = LabelDefinition.FromJson(RedLabelJson);
                LabelDefinition blueLabelDefinition = LabelDefinition.FromJson(BlueLabelJson);

                // Add the label definition to the feature layer's label definition collection.
                districtFeatureLabel.LabelDefinitions.Add(redLabelDefinition);
                districtFeatureLabel.LabelDefinitions.Add(blueLabelDefinition);

                // Enable the visibility of labels to be seen.
                districtFeatureLabel.LabelsEnabled = true;
            }
            catch (Exception e)
            {
                new AlertDialog.Builder(this).SetMessage(e.ToString()).SetTitle("Error").Show();
            }
        }

        private void CreateLayout()
        {
            // Create a new vertical layout for the app.
            LinearLayout layout = new LinearLayout(this) { Orientation = Orientation.Vertical };

            // Add the map view to the layout.
            _myMapView = new MapView(this);
            layout.AddView(_myMapView);

            // Show the layout in the app.
            SetContentView(layout);
        }
    }
}

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