Extending Leaflet package
The Leaflet Javascript library has a plethora of plugins available that extend the functionality of the core package. We have incorporated a chosen few in the R package. It may be desirable to use plugins availble outside of what are supported by this pakage.
The way to achieve that is by extending the Leaflet package. By extending we mean writing your own code/package that incorporate your desired leaflet plugins and hook into the leaflet package.
Certain functions have been made available for you to use in your code while extending Leaflet.
derivePoints/derivePolygons
These functions can be used to extract point or shape (polygon/line/circle/rectangle) data from a data.frame or a spatial object from the sp package. It tries to auto determine the latitude/longitude colnames if not specified or use user supplied column mappings.
evalFormula
evalFormula is used to evaluate a formula on a given data and return the results. e.g. leaflet(some.data.frame) %>% addMarkers(label=~name) internally uses evalFormula to calculate the correct label values from the data using the ~name formula.
expandLimits
You can call expandLimits to make sure that your map’s view is just enough to show every point/shape in your data. This way you don’t have to determine the exact bounds for your map.
filterNULL
Often when passing a list from R to Javascript it is desirable to remove any null elements, and that’s exactly what filterNULL does.
getMapData
Accesses the data object passed when calling leaflet() funtion.
invokeMethod
The glue between the R code and Javascript code. Requires a corresponding method on the Javascript side.
Example
Here is a small example which shows how you can integrate the Bing.com basemap layer plugin
library(leaflet)
library(htmltools)
library(htmlwidgets)
bingPlugin <- htmlDependency(
"leaflet.plugins", "2.0.0",
src = normalizePath("./js"),
script = "Bing.min.js"
)
registerPlugin <- function(map, plugin) {
map$dependencies <- c(map$dependencies, list(plugin))
map
}
leaflet() %>% setView(-122.23, 37.75, zoom = 10) %>%
registerPlugin(bingPlugin) %>%
onRender("function(el, x) {
var imagerySet = 'Aerial';
var bing = new L.BingLayer('LfO3DMI9S6GnXD7d0WGs~bq2DRVkmIAzSOFdodzZLvw~Arx8dclDxmZA0Y38tHIJlJfnMbGq5GXeYmrGOUIbS2VLFzRKCK0Yv_bAl6oe-DOc',
{type: imagerySet});
this.addLayer(bing);
}")