Best way to draw on leaflet map in R - r

Question

I want to draw several things on a leaflet map (through Shiny/R)
I initialize the map like this
map = leaflet() %>% addProviderTiles("Stamen.TonerLite") %>% setView(-1.5, 53.4, 9)
output$myMap = renderLeaflet(map)
Then, depending on what is clicked in the App I ether want to draw Markers or a Polygon
sp <- reactiveValues()
ep <- reactiveValues()
area <- reactiveValues()
area$mp <- matrix(...) # empty matrix with 2 cols named lat/lng
observeEvent(input$map_click, {
coords <- input$map_click
if ( (!is.null(as.integer(input$button)) && (!is.null(coords))) ) {
if (as.integer(input$button) == 1) {
sp[["lat"]] <- coords$lat
sp[["lng"]] <- coords$lng
} else if (as.integer(input$button) == 2) {
ep[["lat"]] <- coords$lat
ep[["lng"]] <- coords$lng
} else if (as.integer(input$button) == 3) {
cm <- matrix(data = c(coords$lat, coords$lng), nrow = 1, ncol = 2)
area$mp <- rbind(area$mp, cm)
} else {
print("Kawum!")
}
})
What I cannot get into my head is how to draw something now on the leaflet map.
What is group ID, what is layer ID. Where comes leafletProxy into play?
How would I, depending on which if else statement kicks in, send the data to leaflet and add a marker or a polygon?
Any help or pointing into the right direction is highly appreciated!

Answer 1

Maybe this can clarify things:
library(shiny)
library(leaflet)
ui <- shinyUI(fluidPage(
actionButton("button", "Change style!"),
leafletOutput("myMap")
))
server <- function(input, output){
map = leaflet() %>% addProviderTiles("Stamen.TonerLite") %>% setView(-1.5, 53.4, 9)
output$myMap = renderLeaflet(map)
sp <- reactiveValues()
ep <- reactiveValues()
area <- reactiveValues()
observeEvent(sp$lat, {
leafletProxy("myMap") %>% addMarkers(lat = sp$lat, lng = sp$lng)
})
observeEvent(ep$lat, {
leafletProxy("myMap") %>% addCircles(lat = ep$lat, lng = ep$lng)
})
observeEvent(area$mp, {
leafletProxy("myMap") %>% addPolygons(lat = area$mp[ , 1], lng = area$mp[ , 2])
})
observeEvent(input$myMap_click, {
coords <- input$myMap_click
if ( (!is.null(input$button) && (!is.null(coords))) ) {
if (input$button %% 4 == 1) {
sp[["lat"]] <- coords$lat
sp[["lng"]] <- coords$lng
} else if (input$button %% 4 == 2) {
ep[["lat"]] <- coords$lat
ep[["lng"]] <- coords$lng
} else if (input$button %% 4 == 3) {
cm <- matrix(data = c(coords$lat, coords$lng), nrow = 1, ncol = 2)
area$mp <- if(!is.null(area$mp)){rbind(area$mp, cm)}else{cm}
} else {
print("Kawum!")
}
}
})
}
shinyApp(ui, server)
First thing, the click event needs to be named after the output element. So input$myMap_click gives you the coords. Second, the leaflet proxy is designed to draw points, things etc. into existing maps. Imagine you'd always re-render the map to do leaflet() %>% addMarkers(...). leafletProxy just needs the output element's name and draws the markers on top of it.
The code above shows some things you can do with that. E.g. using the polygons.
Try using it and comment, if there is something unclear.