text: define([
          '../Core/AssociativeArray',
          '../Core/Cartesian2',
          '../Core/Cartesian3',
          '../Core/Color',
          '../Core/defaultValue',
          '../Core/defined',
          '../Core/destroyObject',
          '../Core/DeveloperError',
          '../Core/DistanceDisplayCondition',
          '../Core/NearFarScalar',
          '../Scene/HeightReference',
          '../Scene/HorizontalOrigin',
          '../Scene/LabelStyle',
          '../Scene/VerticalOrigin',
          './BoundingSphereState',
          './Property'
      ], function(
          AssociativeArray,
          Cartesian2,
          Cartesian3,
          Color,
          defaultValue,
          defined,
          destroyObject,
          DeveloperError,
          DistanceDisplayCondition,
          NearFarScalar,
          HeightReference,
          HorizontalOrigin,
          LabelStyle,
          VerticalOrigin,
          BoundingSphereState,
          Property) {
      'use strict';
  
      var defaultScale = 1.0;
      var defaultFont = '30px sans-serif';
      var defaultStyle = LabelStyle.FILL;
      var defaultFillColor = Color.WHITE;
      var defaultOutlineColor = Color.BLACK;
      var defaultOutlineWidth = 1.0;
      var defaultShowBackground = false;
      var defaultBackgroundColor = new Color(0.165, 0.165, 0.165, 0.8);
      var defaultBackgroundPadding = new Cartesian2(7, 5);
      var defaultPixelOffset = Cartesian2.ZERO;
      var defaultEyeOffset = Cartesian3.ZERO;
      var defaultHeightReference = HeightReference.NONE;
      var defaultHorizontalOrigin = HorizontalOrigin.CENTER;
      var defaultVerticalOrigin = VerticalOrigin.CENTER;
  
      var positionScratch = new Cartesian3();
      var fillColorScratch = new Color();
      var outlineColorScratch = new Color();
      var backgroundColorScratch = new Color();
      var backgroundPaddingScratch = new Cartesian2();
      var eyeOffsetScratch = new Cartesian3();
      var pixelOffsetScratch = new Cartesian2();
      var translucencyByDistanceScratch = new NearFarScalar();
      var pixelOffsetScaleByDistanceScratch = new NearFarScalar();
      var scaleByDistanceScratch = new NearFarScalar();
      var distanceDisplayConditionScratch = new DistanceDisplayCondition();
  
      function EntityData(entity) {
          this.entity = entity;
          this.label = undefined;
          this.index = undefined;
      }
  
      /**
       * A {@link Visualizer} which maps the {@link LabelGraphics} instance
       * in {@link Entity#label} to a {@link Label}.
       * @alias LabelVisualizer
       * @constructor
       *
       * @param {EntityCluster} entityCluster The entity cluster to manage the collection of billboards and optionally cluster with other entities.
       * @param {EntityCollection} entityCollection The entityCollection to visualize.
       */
      function LabelVisualizer(entityCluster, entityCollection) {
          //>>includeStart('debug', pragmas.debug);
          if (!defined(entityCluster)) {
              throw new DeveloperError('entityCluster is required.');
          }
          if (!defined(entityCollection)) {
              throw new DeveloperError('entityCollection is required.');
          }
          //>>includeEnd('debug');
  
          entityCollection.collectionChanged.addEventListener(LabelVisualizer.prototype._onCollectionChanged, this);
  
          this._cluster = entityCluster;
          this._entityCollection = entityCollection;
          this._items = new AssociativeArray();
  
          this._onCollectionChanged(entityCollection, entityCollection.values, [], []);
      }
  
      /**
       * Updates the primitives created by this visualizer to match their
       * Entity counterpart at the given time.
       *
       * @param {JulianDate} time The time to update to.
       * @returns {Boolean} This function always returns true.
       */
      LabelVisualizer.prototype.update = function(time) {
          //>>includeStart('debug', pragmas.debug);
          if (!defined(time)) {
              throw new DeveloperError('time is required.');
          }
          //>>includeEnd('debug');
  
          var items = this._items.values;
          var cluster = this._cluster;
  
          for (var i = 0, len = items.length; i < len; i++) {
              var item = items[i];
              var entity = item.entity;
              var labelGraphics = entity._label;
              var text;
              var label = item.label;
              var show = entity.isShowing && entity.isAvailable(time) && Property.getValueOrDefault(labelGraphics._show, time, true);
              var allowPicking = entity.allowPicking;
              var position;
              if (show) {
                  position = Property.getValueOrUndefined(entity._position, time, positionScratch);
                  text = Property.getValueOrUndefined(labelGraphics._text, time);
                  show = defined(position) && defined(text);
              }
  
              if (!show) {
                  //don't bother creating or updating anything else
                  returnPrimitive(item, entity, cluster);
                  continue;
              }
  
              if (!Property.isConstant(entity._position)) {
                  cluster._clusterDirty = true;
              }
  
              var updateClamping = false;
              var heightReference = Property.getValueOrDefault(labelGraphics._heightReference, time, defaultHeightReference);
  
              if (!defined(label)) {
                  label = cluster.getLabel(entity);
                  label.id = entity;
                  item.label = label;
  
                  // If this new label happens to have a position and height reference that match our new values,
                  // label._updateClamping will not be called automatically. That's a problem because the clamped
                  // height may be based on different terrain than is now loaded. So we'll manually call
                  // _updateClamping below.
                  updateClamping = Cartesian3.equals(label.position, position) && label.heightReference === heightReference;
              }
  
              label.show = true;
              label.position = position;
              label.text = text;
              label.scale = Property.getValueOrDefault(labelGraphics._scale, time, defaultScale);
              label.font = Property.getValueOrDefault(labelGraphics._font, time, defaultFont);
              label.style = Property.getValueOrDefault(labelGraphics._style, time, defaultStyle);
              label.fillColor = Property.getValueOrDefault(labelGraphics._fillColor, time, defaultFillColor, fillColorScratch);
              label.outlineColor = Property.getValueOrDefault(labelGraphics._outlineColor, time, defaultOutlineColor, outlineColorScratch);
              label.outlineWidth = Property.getValueOrDefault(labelGraphics._outlineWidth, time, defaultOutlineWidth);
              label.showBackground = Property.getValueOrDefault(labelGraphics._showBackground, time, defaultShowBackground);
              label.backgroundColor = Property.getValueOrDefault(labelGraphics._backgroundColor, time, defaultBackgroundColor, backgroundColorScratch);
              label.backgroundPadding = Property.getValueOrDefault(labelGraphics._backgroundPadding, time, defaultBackgroundPadding, backgroundPaddingScratch);
              label.pixelOffset = Property.getValueOrDefault(labelGraphics._pixelOffset, time, defaultPixelOffset, pixelOffsetScratch);
              label.eyeOffset = Property.getValueOrDefault(labelGraphics._eyeOffset, time, defaultEyeOffset, eyeOffsetScratch);
              label.heightReference = heightReference;
              label.horizontalOrigin = Property.getValueOrDefault(labelGraphics._horizontalOrigin, time, defaultHorizontalOrigin);
              label.verticalOrigin = Property.getValueOrDefault(labelGraphics._verticalOrigin, time, defaultVerticalOrigin);
              label.allowPicking = allowPicking;
              label.translucencyByDistance = Property.getValueOrUndefined(labelGraphics._translucencyByDistance, time, translucencyByDistanceScratch);
              label.pixelOffsetScaleByDistance = Property.getValueOrUndefined(labelGraphics._pixelOffsetScaleByDistance, time, pixelOffsetScaleByDistanceScratch);
              label.scaleByDistance = Property.getValueOrUndefined(labelGraphics._scaleByDistance, time, scaleByDistanceScratch);
              label.distanceDisplayCondition = Property.getValueOrUndefined(labelGraphics._distanceDisplayCondition, time, distanceDisplayConditionScratch);
              label.disableDepthTestDistance = Property.getValueOrUndefined(labelGraphics._disableDepthTestDistance, time);
  
              if (updateClamping) {
                  label._updateClamping();
              }
          }
          return true;
      };
  
      /**
       * Computes a bounding sphere which encloses the visualization produced for the specified entity.
       * The bounding sphere is in the fixed frame of the scene's globe.
       *
       * @param {Entity} entity The entity whose bounding sphere to compute.
       * @param {BoundingSphere} result The bounding sphere onto which to store the result.
       * @returns {BoundingSphereState} BoundingSphereState.DONE if the result contains the bounding sphere,
       *                       BoundingSphereState.PENDING if the result is still being computed, or
       *                       BoundingSphereState.FAILED if the entity has no visualization in the current scene.
       * @private
       */
      LabelVisualizer.prototype.getBoundingSphere = function(entity, result) {
          //>>includeStart('debug', pragmas.debug);
          if (!defined(entity)) {
              throw new DeveloperError('entity is required.');
          }
          if (!defined(result)) {
              throw new DeveloperError('result is required.');
          }
          //>>includeEnd('debug');
  
          var item = this._items.get(entity.id);
          if (!defined(item) || !defined(item.label)) {
              return BoundingSphereState.FAILED;
          }
  
          var label = item.label;
          result.center = Cartesian3.clone(defaultValue(label._clampedPosition, label.position), result.center);
          result.radius = 0;
          return BoundingSphereState.DONE;
      };
  
      /**
       * Returns true if this object was destroyed; otherwise, false.
       *
       * @returns {Boolean} True if this object was destroyed; otherwise, false.
       */
      LabelVisualizer.prototype.isDestroyed = function() {
          return false;
      };
  
      /**
       * Removes and destroys all primitives created by this instance.
       */
      LabelVisualizer.prototype.destroy = function() {
          this._entityCollection.collectionChanged.removeEventListener(LabelVisualizer.prototype._onCollectionChanged, this);
          var entities = this._entityCollection.values;
          for (var i = 0; i < entities.length; i++) {
              this._cluster.removeLabel(entities[i]);
          }
          return destroyObject(this);
      };
  
      LabelVisualizer.prototype._onCollectionChanged = function(entityCollection, added, removed, changed) {
          var i;
          var entity;
          var items = this._items;
          var cluster = this._cluster;
  
          for (i = added.length - 1; i > -1; i--) {
              entity = added[i];
              if (defined(entity._label) && defined(entity._position)) {
                  items.set(entity.id, new EntityData(entity));
              }
          }
  
          for (i = changed.length - 1; i > -1; i--) {
              entity = changed[i];
              if (defined(entity._label) && defined(entity._position)) {
                  if (!items.contains(entity.id)) {
                      items.set(entity.id, new EntityData(entity));
                  }
              } else {
                  returnPrimitive(items.get(entity.id), entity, cluster);
                  items.remove(entity.id);
              }
          }
  
          for (i = removed.length - 1; i > -1; i--) {
              entity = removed[i];
              returnPrimitive(items.get(entity.id), entity, cluster);
              items.remove(entity.id);
          }
      };
  
      function returnPrimitive(item, entity, cluster) {
          if (defined(item)) {
              item.label = undefined;
              cluster.removeLabel(entity);
          }
      }
  
      return LabelVisualizer;
  });
  