Leaflet map that toggles selection on click

app.R

# Draw a map which responds to clicks by toggling the selection state
# of the clicked polygon.
library(shiny)
library(leaflet)
library(sf)
library(tidyverse)

nc = read_sf(system.file("gpkg/nc.gpkg", package="sf")) %>% 
  st_transform(4326)

# Initial selection
nc$selected = sample(c(0, 1), nrow(nc), replace=TRUE)

ui <- fluidPage(
   titlePanel("Leaflet selection demo"),
   leafletOutput('map')
)

server <- function(input, output) {
   output$map = renderLeaflet({
     leaflet() %>% addTiles() %>% 
       addPolygons(data=nc, fillColor=~c('red', 'blue')[selected+1],
                   layerId=~FIPS)
   })
   
   observeEvent(input$map_shape_click, ignoreInit=TRUE, {
     id = input$map_shape_click$id
     map = leafletProxy('map') %>% 
       removeShape(id)
     
     nc_local = nc
     ix = which(nc_local$FIPS==id)
     nc_local$selected[ix] = 1 - nc_local$selected[ix]
     nc <<- nc_local
     
     map %>% addPolygons(data=nc_local[ix,], fillColor=~c('red', 'blue')[selected+1],
                   layerId=id)
   })
}

shinyApp(ui = ui, server = server)