You can use ArcGIS Maps SDKs for Native Apps to create real-time apps that consume a stream of data from a supported data source, such as an ArcGIS stream service. ArcGIS Maps SDK for Java abstracts much of the work required to connect to a stream, consume incoming data, and display it dynamically in your app. The real-world objects described by data in a stream are called dynamic entities.
Dynamic entities allow you to do things like:
Track moving assets, such as vehicles in a delivery fleet, with updates to their current location, speed, estimated arrival, fuel level, and so on.
Monitor stationary sensors, such as weather stations, with updates to attributes such as wind speed and direction, precipitation, and temperature.
Get notified of new incidents, such as crime or accidents, and locate available responders in the area.
Dynamic entities and observations
Real-world objects that change over time are represented by the DynamicEntity class. DynamicEntity, like Graphic and ArcGISFeature, is a type of GeoElement. This means it has a collection of attributes and a geometry to describe its location. Dynamic entities might be moving (like vehicles or people), stationary (like monitoring stations or gauges), or point-in-time events (such as crime or accident incidents).
Dynamic entities are updated with data coming from a stream. These updates are referred to as observations and are represented by the DynamicEntityObservation class (also a type of GeoElement). An observation is a static snapshot of the state of a dynamic entity at a given time. This snapshot includes the associated dynamic entity's location (geometry) as well as all its attributes. For example, an observation for a moving aircraft may include updates to attributes like speed, altitude, and heading in addition to its updated location. While observations remain static, a dynamic entity changes because it's essentially a pointer to the most recent observation. Observations can be identified in a map view or scene view.
For any dynamic entity, you can get a collection of all its observations. Likewise, for any observation, you can find the dynamic entity to which it belongs. The collection of all observations for a given dynamic entity is often referred to as a track. Observations with the same track id belong to the same track and refer to the same dynamic entity. Tracks can be easily visualized with a track line connecting all observations of the same track.
Working with dynamic entities in your app involves the following broad steps:
In ArcGIS Maps SDK for Java, stream data sources are represented by the DynamicEntityDataSource base class. This data source plays a central role in your real-time app and can be used to create a DynamicEntityLayer for display in a map or scene.
The API abstracts many of the details of working with the underlying stream. For example, it manages the connection to the stream, notifies of connection status changes, and automatically attempts to reconnect when needed. It also notifies of incoming data updates and when data is purged (stale data is removed) from the DynamicEntityDataSource.
Members on DynamicEntityDataSource can be used to connect or disconnect from the data stream, apply a filter to restrict the amount of data received from the stream, purge all data from the local data cache, or define data management options for the local cache.
If you are using a dynamic entity layer, the stream service will automatically load and connect when it is added to the map. If you are using the dynamic entity data source with out a dynamic entity layer, then you will need to explicitly load and connect the dynamic entity data source to begin recieving events/updates. Once the connection is successfully established, data will begin to flow from the stream.
A DynamicEntityDataSource may have metadata that describes the service. ArcGISStreamService, for example, provides the ArcGISStreamServiceInfo class that you can use to get information about the data provided by the stream. Once the data source is loaded, you can use the metadata to find things like:
Fields: Field definitions for data in the stream
Track ID field: The field that contains a unique ID for each track in a track-aware service
Archive service URL: A service that contains the latest observations for dynamic entities provided by this stream
Geometry type: The type of geometry returned (point, polyline, or polygon, for example)
Dynamic entity filter
You can apply a filter that limits the amount of data received from the data source. For an ArcGISStreamService, use an ArcGISStreamServiceFilter to define which observations should be included based on spatial or attribute criteria (or both). For streams that deliver a lot of observations frequently, applying a filter can help keep the display uncluttered and the dynamic entity data source lean.
Data received from the DynamicEntityDataSource is stored in memory on the client and can grow quickly depending on the number of dynamic entities and frequency of updates. You can use the DynamicEntityDataSourcePurgeOptions to control app data storage for the local cache. For example, you can define a limit on the number of observations stored either for the entire dynamic entity data source (maximum observations) or for each dynamic entity (maximum observations per track). By default, the maximum total number of observations stored is 100,000 (with no limit to the number of observations per track). You can also define the limit according to the age of observations, using the maximum duration purge option.
To clear all observations from the cache, use DynamicEntityDataSource.purgeAllAsync(). This will clear all data from the local data store, but will not disconnect from the stream. Dynamic entities and observations will continue to be added to the dynamic entity data source as they come from the stream.
ArcGIS Maps SDK for Java provides a special type of layer for displaying dynamic entities, called DynamicEntityLayer. This layer is created from a DynamicEntityDataSource and manages the display of all dynamic entities and observations as they are added, purged, or updated.
The renderer you define for the layer controls the display of dynamic entities and can be one of any of the available renderer types. You can define labels for dynamic entities as you would for other layers, but labels are not displayed for previous observations. See Symbols, renderers, and styles for more information about defining a renderer for a layer and Add labels for information about defining labels for geoelements.
A dynamic entity layer's TrackDisplayProperties allow you to control the display of previous observations by setting a maximum number of observations to show, displaying a line that connects them (track line), and applying renderers to the observations and/or track line. You can apply any of the available renderers to the previous observations, but only SimpleRenderer is supported for track lines (since it contains no attributes).
The maximum observations defined for the track display properties applies strictly to the display. Don't confuse this property, used to control the display, with the maximum observations for the data source's purge options, used to limit the number of observations stored in the cache.
DynamicEntityLayer has methods for selecting or unselecting dynamic entities and observations in the display. When selecting a dynamic entity, the selection will move with the entity (as new observations come from the stream). Since observations are static, selected observations will appear like any static geoelement on the display.
As information is delivered by the stream, the dynamic entity data source is continuously updated to reflect the dynamic entities and observations being added and removed (purged). While the DynamicEntityLayer manages the display of the dynamic entities in the map or scene, you can also respond to notifications about dynamic entities being added or removed from the local data store.
The DynamicEntityDataSource exposes the following notifications as it receives data from the stream:
DynamicEntityObservationPurgedEvent—an observation is purged from the local data source; for example, when the maximum observations or maximum observation duration has been surpassed.
DynamicEntityReceivedEvent—a new dynamic entity is received from the stream; for example, when the first observation for this dynamic entity is received
DynamicEntityPurgedEvent—a dynamic entity is purged from the local data source; for example, when it no longer has any associated observations in the cache
As a simple example, you could handle these events in order to increment or decrement a numeric variable to keep a count of the current number of entities in the data store.
You can also handle updates for a specific dynamic entity by handling its DynamicEntityChangedEvent event. In the event handler, you can respond to observations as they are received or purged. You can also be notified when the entity itself is purged.
You can get notifications from connectionStatus to respond to changes with the connection status. For example, you might want to display the connection status to the user. You might also want to clean up resources in your app or unsubscribe to notification events when the connection closes.
