Put realtime data on a Leaflet map
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README.md

Leaflet Realtime

Build status NPM version Leaflet 1.0 compatible! CDNJS Greenkeeper badge

Put realtime data on a Leaflet map: live tracking GPS units, sensor data or just about anything.

Note: version 2 and up of this plugin is _only compatible with Leaflet 1.0 and later. Use earlier versions of Leaflet Realtime if you need Leaflet 0.7 compatibility.

Example

Checkout the Leaflet Realtime Demo. Basic example:

var map = L.map('map'),
    realtime = L.realtime({
        url: 'https://wanderdrone.appspot.com/',
        crossOrigin: true,
        type: 'json'
    }, {
        interval: 3 * 1000
    }).addTo(map);

realtime.on('update', function() {
    map.fitBounds(realtime.getBounds(), {maxZoom: 3});
});

Usage

Overview

By default, Leaflet Realtime reads and displays GeoJSON from a provided source. A "source" is usually a URL, and data can be fetched using AJAX (XHR), JSONP. This means Leaflet Realtime will poll for data, pulling it from the source. Alternatively, you can write your own source, to provide data in just about any way you want. Leaflet Realtime can also be made work with push data, for example data pushed from the server using socket.io or similar.

To be able to figure out when new features are added, when old features are removed, and which features are just updated, Leaflet Realtime needs to identify each feature uniquely. This is done using a feature id. Usually, this can be done using one of the feature's properties. By default, Leaflet Realtime will try to look for a called property id and use that.

By default, L.Realtime uses a L.GeoJSON layer to display the results. You can basically do anything you can do with L.GeoJSON with L.Realtime - styling, onEachFeature, gettings bounds, etc. as if you were working directly with a normal GeoJSON layer.

L.Realtime can also use other layer types to display the results, for example it can use a MarkerClusterGroup from Leaflet MarkerCluster: pass a LayerGroup (or any class that implements addLayer and removeLayer) to L.Realtime's container option. (This feature was added in version 2.1.0.)

Typical usage involves instantiating L.Realtime with options for style and/or onEachFeature, to customize styling and interaction, as well as adding a listener for the update event, to for example list the features currently visible in the map.

Since version 2.0, Leaflet Realtime uses the Fetch API to request data (AJAX). If you are in the unfortunate situation that you need to support a browser without Fetch, you either need to use a polyfill, or write your own source function to make the AJAX requests.

Push data

If you prefer getting data pushed from the server, in contrast to Leaflet Realtime pulling it with a standard HTTP request, you can feed added and updated GeoJSON data to Leaflet Realtime using the update method. In this scenario, you will also need to remove features by explicit calls to remove.

Since automatic updates do not make sense in a push scenario, you want to create the layer with the option start set to to false.

API

L.Realtime

This is a realtime updated layer that can be added to the map. It extends L.GeoJSON.

Creation
Factory Description
L.Realtime(<Source> source, <RealtimeOptions> options?) Instantiates a new realtime layer with the provided source and options
Options

Provides these options, in addition to the options of L.GeoJSON.

Option Type Default Description
start Boolean true Should automatic updates be enabled when class is instantiated
interval Number 60000 Automatic update interval, in milliseconds
getFeatureId(<GeoJSON> featureData) Function Returns featureData.properties.id Function used to get an identifier uniquely identify a feature over time
updateFeature(<GeoJSON> featureData, <ILayer> oldLayer) Function Special Used to update an existing feature's layer; by default, points (markers) are updated, other layers are discarded and replaced with a new, updated layer. Allows to create more complex transitions, for example, when a feature is updated
container LayerGroup L.geoJson() Specifies the layer instance to display the results in
Events
Event Data Description
update UpdateEvent Fires when the layer's data is updated
Methods
Method Returns Description
start() this Starts automatic updates
stop() this Stops automatic updates
isRunning() Boolean Tells if automatic updates are running
update(<GeoJSON> featureData?) this Updates the layer's data. If featureData is provided, it is used to add or update data in the layer, otherwise the layer's source is queried for new data asynchronously
remove(<GeoJSON> featureData) this Removes the provided feature or features from the layer
getLayer(<FeatureId> featureId) ILayer Retrieves the layer used for a certain feature
getFeature(<FeatureId> featureId) GeoJSON Retrieves the feature data for the given featureId

Source

The source can be one of:

  • a string with the URL to get data from
  • an options object that is passed to fetch for fetching the data
  • a function in case you need more freedom.

In case you use a function, the function should take two callbacks as arguments: fn(success, error), with the callbacks:

  • a success callback that takes GeoJSON as argument: success(<GeoJSON> features)
  • an error callback that should take an error object and an error message (string) as argument: error(<Object> error, <String> message)

UpdateEvent

An update event is fired when the layer's data is updated from its source. The data included loosely resembles D3's join semantics, to make it easy to handle new features (the enter set), updated features (the update set) and removed features (the exit set).

property type description
features Object Complete hash of current features, with feature id as key
enter Object Features added by this update, with feature id as key
update Object Existing features updated by this update, with feature id as key
exit Object Existing features that were removed by this update, with feature id as key