Demo - using Leaflet JS plugins in R and Shiny

app.R

# Demo - using Leaflet JS plugins in R and Shiny
# directions from https://gist.github.com/jcheng5/c084a59717f18e947a17955007dc5f92
# implementing an Opacity plugin that works for tile-layers only:
# 		https://github.com/dayjournal/Leaflet.Control.Opacity
# download js/css files first, see note (2) below

library(shiny)
library(leaflet)
library(htmlwidgets)

# workaround for the src/href problem - dnld/save locally first
urlf <- 'https://cdn.statically.io/gh/dayjournal/Leaflet.Control.Opacity/master/dist/%s'
download.file(sprintf(urlf,'L.Control.Opacity.js'), 'C:/Temp/L.Control.Opacity.js', mode="wb")
download.file(sprintf(urlf,'L.Control.Opacity.css'), 'C:/Temp/L.Control.Opacity.css', mode="wb")

ctrlOpacity <- htmltools::htmlDependency(
	name = 'ctrlOpacity',
	version = "1.0.0",
	# (1) works in R but not in Shiny due to async loading, see https://github.com/rstudio/shiny/issues/1389
	# src = c(href = 'https://cdn.statically.io/gh/dayjournal/Leaflet.Control.Opacity/master/dist/'),
	# (2) works in R and Shiny - download js/css files, then use this:
	src = c(file = normalizePath('C:/Temp')),
	script = "L.Control.Opacity.js",
	stylesheet = "L.Control.Opacity.css"
)
registerPlugin <- function(map, plugin) {
	map$dependencies <- c(map$dependencies, list(plugin))
	map
}

Location <- c("Atlanta ","Los Angeles","Chicago","New York","Dallas","Baltimore","Phoenix","Charlotte","Houston","San Antonio", "Seattle" )
Lat <- c(33.74401,33.82377,41.78798,40.767309,32.88153,39.148492,33.45444,35.2406,29.935842,29.44838,47.714965 )
Lon <- c(-84.56032,-118.2668,-87.7738,-73.978308,-96.64601,-76.796211,-112.32401,-81.04028,-95.398436,-98.39908,-122.127166 )
mdf <- data.frame(Location, Lat, Lon)

ui <- fluidPage( leafletOutput("mymap") )

server <- function(input, output, session) {
	output$mymap <- renderLeaflet({
		
map <- leaflet() %>%
	addTiles(layerId='osm', urlTemplate="https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png") %>%
	addWMSTiles(layerId='radar', baseUrl="http://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0r.cgi",
		layers = "nexrad-n0r-900913",
		options = WMSTileOptions(format = "image/png", transparent = TRUE),
		attribution = "Weather data © 2012 IEM Nexrad"
	) %>%
	addMarkers(data=mdf, lng=~Lon, lat=~Lat, label=~Location) %>%
	addControl(tags$div(HTML('<b>Note</b>: clicking sliders works better')), position='bottomright')

map %>%  
	registerPlugin(ctrlOpacity) %>%
	onRender("function(el, x) {
		var map = this;
		// not documented weird layer naming = research-time-eating Monster!
		var tileLayer1 = map.layerManager._byLayerId['tile\\nosm'];	
		var tileLayer2 = map.layerManager._byLayerId['tile\\nradar'];
		L.control.opacity({ 'osm': tileLayer1, 'radar data' : tileLayer2 }, 
				{ label: 'Tile-layer opacity' }).addTo(map);
	}")

	})
}

shinyApp(ui, server)