Show labels on layers

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Display custom labels on a feature layer.

screenshot

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

To show custom labels on a feature layer:

  1. Create a ServiceFeatureTable using a feature service URL.
  2. Create a FeatureLayer from the service feature table.
  3. Create an ArcadeLabelExpression for the label definition.
    • You can use fields of the feature by using $feature.field_name in the expression.
  4. Create a TextSymbol to control how the label text is styled.
  5. Create a LabelDefinition by passing in the ArcadeLabelExpression and TextSymbol.
  6. Add the definition to the feature layer with featureLayer.labelDefinitions().append(labelDefinition) .
  7. Lastly, enable labels on the layer using featureLayer.setLabelsEnabled().

Relevant API

  • ArcadeLabelExpression
  • 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 Arcade label expression script for defining a label definition can be found on the ArcGIS Developers site.

Tags

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

Sample Code

ShowLabelsOnLayers.cppShowLabelsOnLayers.cppShowLabelsOnLayers.hShowLabelsOnLayers.qml
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// [WriteFile Name=ShowLabelsOnLayers, Category=DisplayInformation]
// [Legal]
// 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.
// [Legal]

#ifdef PCH_BUILD
#include "pch.hpp"
#endif // PCH_BUILD

#include "ShowLabelsOnLayers.h"

#include "ArcadeLabelExpression.h"
#include "FeatureLayer.h"
#include "LabelDefinition.h"
#include "Map.h"
#include "MapQuickView.h"
#include "ServiceFeatureTable.h"
#include "TextSymbol.h"

using namespace Esri::ArcGISRuntime;

ShowLabelsOnLayers::ShowLabelsOnLayers(QQuickItem* parent /* = nullptr */):
  QQuickItem(parent)
{
}

void ShowLabelsOnLayers::init()
{
  // Register the map view for QML
  qmlRegisterType<MapQuickView>("Esri.Samples", 1, 0, "MapView");
  qmlRegisterType<ShowLabelsOnLayers>("Esri.Samples", 1, 0, "ShowLabelsOnLayersSample");
}

void ShowLabelsOnLayers::componentComplete()
{
  QQuickItem::componentComplete();

  // find QML MapView component
  m_mapView = findChild<MapQuickView*>("mapView");

  // Create a map using the light gray basemap
  m_map = new Map(BasemapStyle::ArcGISLightGray, this);

  // Create a feature layer
  ServiceFeatureTable* featureTable = new ServiceFeatureTable(QUrl("https://services.arcgis.com/P3ePLMYs2RVChkJx/arcgis/rest/services/USA_115th_Congressional_Districts/FeatureServer/0"), this);
  FeatureLayer* featureLayer = new FeatureLayer(featureTable, this);
  connect(featureLayer, &FeatureLayer::doneLoading, this, [this, featureLayer](Error e)
  {
    if (!e.isEmpty())
      return;

    m_mapView->setViewpoint(Viewpoint(featureLayer->fullExtent().center(), 56759600));
  });
  m_map->operationalLayers()->append(featureLayer);

  // Apply labels to the feature layer
  LabelDefinition* republicanLabelDef = createRepublicanLabelDefinition();
  LabelDefinition* democratLabelDef = createDemocratLabelDefinition();
  featureLayer->labelDefinitions()->append(republicanLabelDef);
  featureLayer->labelDefinitions()->append(democratLabelDef);
  featureLayer->setLabelsEnabled(true);

  // Set map to map view
  m_mapView->setViewpointCenter(Point(-10846309.950860, 4683272.219411, SpatialReference::webMercator()), 20000000);
  m_mapView->setMap(m_map);
}

LabelDefinition* ShowLabelsOnLayers::createRepublicanLabelDefinition()
{
  // This particular LabelDefinition will have the following characteristics:
  // (1) The 'ArcadeLabelExpression' defines that the label text displayed comes from the fields 'NAME',
  //     the first letter of PARTY' (R or D), and 'CDFIPS' in the feature service in the format:
  //         Firstname Lastname (R)
  //         District #
  // (2) The 'TextSymbol' for the labeled text will be red with a white halo centered in the target polygon.
  // (3) The 'where' clause of the 'LabelDefinition' restricts the labels to data from Republican districts.

  ArcadeLabelExpression* republicanArcadeLabelExpression = new ArcadeLabelExpression("$feature.NAME + ' (' + left($feature.PARTY,1) + ')\\nDistrict ' + $feature.CDFIPS", this);

  TextSymbol* republicanTextSymbol = new TextSymbol(this);
  republicanTextSymbol->setSize(11);
  republicanTextSymbol->setColor(Qt::red);
  republicanTextSymbol->setHaloColor(Qt::white);
  republicanTextSymbol->setHaloWidth(2);
  republicanTextSymbol->setHorizontalAlignment(HorizontalAlignment::Center);
  republicanTextSymbol->setVerticalAlignment(VerticalAlignment::Middle);

  LabelDefinition* republicanLabelDefinition = new LabelDefinition(republicanArcadeLabelExpression, republicanTextSymbol, this);
  republicanLabelDefinition->setWhereClause("PARTY = 'Republican'");

  return republicanLabelDefinition;
}

LabelDefinition* ShowLabelsOnLayers::createDemocratLabelDefinition()
{
  // This particular LabelDefinition will have the following characteristics:
  // (1) The 'ArcadeLabelExpression' defines that the label text displayed comes from the fields 'NAME',
  //     the first letter of PARTY' (R or D), and 'CDFIPS' in the feature service in the format:
  //         Firstname Lastname (D)
  //         District #
  // (2) The 'TextSymbol' for the labeled text will be blue with a white halo centered in the target polygon.
  // (3) The 'where' clause of the 'LabelDefinition' restricts the labels to data from Democrat districts.

  ArcadeLabelExpression* democratArcadeLabelExpression = new ArcadeLabelExpression("$feature.NAME + ' (' + left($feature.PARTY,1) + ')\\nDistrict ' + $feature.CDFIPS", this);

  TextSymbol* democratTextSymbol = new TextSymbol(this);
  democratTextSymbol->setSize(11);
  democratTextSymbol->setColor(Qt::blue);
  democratTextSymbol->setHaloColor(Qt::white);
  democratTextSymbol->setHaloWidth(2);
  democratTextSymbol->setHorizontalAlignment(HorizontalAlignment::Center);
  democratTextSymbol->setVerticalAlignment(VerticalAlignment::Middle);

  LabelDefinition* democratLabelDefinition = new LabelDefinition(democratArcadeLabelExpression, democratTextSymbol, this);
  democratLabelDefinition->setWhereClause("PARTY = 'Democrat'");

  return democratLabelDefinition;
}

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