# MGLHeatmapStyleLayer

    @interface MGLHeatmapStyleLayer : MGLVectorStyleLayer

An `MGLHeatmapStyleLayer` is a style layer that renders a heatmap.

A heatmap visualizes the spatial distribution of a large, dense set of point data, using color to avoid cluttering the map with individual points at low zoom levels. The points are weighted by an attribute you specify. Use a heatmap style layer in conjunction with point or point collection features. These features can come from vector tiles loaded by an `MGLVectorTileSource` object, or they can be `MGLPointAnnotation`, `MGLPointFeature`, `MGLPointCollection`, or `MGLPointCollectionFeature` instances in an `MGLShapeSource` or `MGLComputedShapeSource` object.

Consider accompanying a heatmap style layer with an `MGLCircleStyleLayer` or `MGLSymbolStyleLayer` at high zoom levels. If you are unsure whether the point data in an `MGLShapeSource` is dense enough to warrant a heatmap, you can alternatively cluster the source using the `MGLShapeSourceOptionClustered` option and render the data using an `MGLCircleStyleLayer` or `MGLSymbolStyleLayer`.

You can access an existing heatmap style layer using the `-[MGLStyle layerWithIdentifier:]` method if you know its identifier; otherwise, find it using the `MGLStyle.layers` property. You can also create a new heatmap style layer and add it to the style using a method such as `-[MGLStyle addLayer:]`.

#### Related examples

See the Create a heatmap layer example to learn how to add this style layer to your map.

### Example

    let layer = MGLHeatmapStyleLayer(identifier: "earthquake-heat", source: earthquakes)
    layer.heatmapWeight = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:(magnitude, 'linear', nil, %@)",
                                       [0: 0,
                                        6: 1])
    layer.heatmapIntensity = NSExpression(format: "mgl_interpolate:withCurveType:parameters:stops:($zoomLevel, 'linear', nil, %@)",
                                          [0: 1,
                                           9: 3])
    mapView.style?.addLayer(layer)

- `-initWithIdentifier:source:`

  Returns a heatmap style layer initialized with an identifier and source.

  After initializing and configuring the style layer, add it to a map view’s style using the `-[MGLStyle addLayer:]` or `-[MGLStyle insertLayer:belowLayer:]` method.

  #### Declaration

  Objective-C

      - (nonnull instancetype)initWithIdentifier:(nonnull NSString *)identifier
                                          source:(nonnull MGLSource *)source;

  Swift

      init(identifier: String, source: MGLSource)

  #### Parameters

  `identifier` | A string that uniquely identifies the source in the style to which it is added.
  `source` | The source from which to obtain the data to style. If the source has not yet been added to the current style, the behavior is undefined.

  #### Return Value

  An initialized foreground style layer.

  View Source on GitHub

### Accessing the Paint Attributes

- `heatmapColor`

  The color of each screen point based on its density value in a heatmap. This property is normally set to an interpolation or step expression with the `$heatmapDensity` value as its input.

  The default value of this property is an expression that evaluates to a rainbow color scale from blue to red. Set this property to `nil` to reset it to the default value.

  You can set this property to an expression containing any of the following:

  - Constant `UIColor` values
  - Predefined functions, including mathematical and string operators
  - Conditional expressions
  - Variable assignments and references to assigned variables
  - Interpolation and step functions applied to the `$heatmapDensity` variable

  This property does not support applying interpolation or step functions to feature attributes.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic, null_resettable)
          NSExpression *heatmapColor;

  Swift

      var heatmapColor: NSExpression! { get set }

  View Source on GitHub
- `heatmapIntensity`

  Similar to `heatmapWeight` but controls the intensity of the heatmap globally. Primarily used for adjusting the heatmap based on zoom level.

  The default value of this property is an expression that evaluates to the float `1`. Set this property to `nil` to reset it to the default value.

  You can set this property to an expression containing any of the following:

  - Constant numeric values no less than 0
  - Predefined functions, including mathematical and string operators
  - Conditional expressions
  - Variable assignments and references to assigned variables
  - Interpolation and step functions applied to the `$zoomLevel` variable

  This property does not support applying interpolation or step functions to feature attributes.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic, null_resettable)
          NSExpression *heatmapIntensity;

  Swift

      var heatmapIntensity: NSExpression! { get set }

  View Source on GitHub
- `heatmapIntensityTransition`

  The transition affecting any changes to this layer’s `heatmapIntensity` property.

  This property corresponds to the `heatmap-intensity-transition` property in the style JSON file format.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic)
          MGLTransition heatmapIntensityTransition;

  Swift

      var heatmapIntensityTransition: MGLTransition { get set }

  View Source on GitHub
- `heatmapOpacity`

  The global opacity at which the heatmap layer will be drawn.

  The default value of this property is an expression that evaluates to the float `1`. Set this property to `nil` to reset it to the default value.

  You can set this property to an expression containing any of the following:

  - Constant numeric values between 0 and 1 inclusive
  - Predefined functions, including mathematical and string operators
  - Conditional expressions
  - Variable assignments and references to assigned variables
  - Interpolation and step functions applied to the `$zoomLevel` variable

  This property does not support applying interpolation or step functions to feature attributes.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic, null_resettable)
          NSExpression *heatmapOpacity;

  Swift

      var heatmapOpacity: NSExpression! { get set }

  View Source on GitHub
- `heatmapOpacityTransition`

  The transition affecting any changes to this layer’s `heatmapOpacity` property.

  This property corresponds to the `heatmap-opacity-transition` property in the style JSON file format.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic) MGLTransition heatmapOpacityTransition;

  Swift

      var heatmapOpacityTransition: MGLTransition { get set }

  View Source on GitHub
- `heatmapRadius`

  Radius of influence of one heatmap point in points. Increasing the value makes the heatmap smoother, but less detailed.

  This property is measured in points.

  The default value of this property is an expression that evaluates to the float `30`. Set this property to `nil` to reset it to the default value.

  You can set this property to an expression containing any of the following:

  - Constant numeric values no less than 1
  - Predefined functions, including mathematical and string operators
  - Conditional expressions
  - Variable assignments and references to assigned variables
  - Interpolation and step functions applied to the `$zoomLevel` variable and/or feature attributes

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic, null_resettable)
          NSExpression *heatmapRadius;

  Swift

      var heatmapRadius: NSExpression! { get set }

  View Source on GitHub
- `heatmapRadiusTransition`

  The transition affecting any changes to this layer’s `heatmapRadius` property.

  This property corresponds to the `heatmap-radius-transition` property in the style JSON file format.

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic) MGLTransition heatmapRadiusTransition;

  Swift

      var heatmapRadiusTransition: MGLTransition { get set }

  View Source on GitHub
- `heatmapWeight`

  A measure of how much an individual point contributes to the heatmap. A value of 10 would be equivalent to having 10 points of weight 1 in the same spot. Especially useful when combined with clustering.

  The default value of this property is an expression that evaluates to the float `1`. Set this property to `nil` to reset it to the default value.

  You can set this property to an expression containing any of the following:

  - Constant numeric values no less than 0
  - Predefined functions, including mathematical and string operators
  - Conditional expressions
  - Variable assignments and references to assigned variables
  - Interpolation and step functions applied to the `$zoomLevel` variable and/or feature attributes

  #### Declaration

  Objective-C

      @property (assign, readwrite, nonatomic, null_resettable)
          NSExpression *heatmapWeight;

  Swift

      var heatmapWeight: NSExpression! { get set }

  View Source on GitHub