lnkpaulo / leaflet-realtime

Put realtime data on a Leaflet map

Leaflet Realtime

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

Example

Checkout the Leaflet Realtime Demo, or the slightly more advanced trains in Sweden (made possible with Tågkartan's nice data. 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.

Technically, L.Realtime extends L.GeoJSON, with the additional functionality that it automatically updates the data with the result from a periodic HTTP request. You can basically do anything you can do with L.GeoJSON with L.Realtime - styling, onEachFeature, gettings bounds, etc.

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.

For ease of use and flexibility, Leaflet Realtime uses reqwest for getting data over HTTP(S), and it is bundled in the distribution. If you need to use the bundled reqwest library directly, it is exported as L.Realtime.reqwest.

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, <ILayer> newLayer)	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

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 either be an options object that is passed to reqwest for fetching the data, or 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