example_leaflet_reactive.R

library(shiny)
library(leaflet)
library(sf)

# NC counties - a shapefile shipped with the sf package
shape <- sf::st_read(system.file("shape/nc.shp", 
                                 package ="sf"),
                     stringsAsFactors = F) 


# Define UI 
ui <- fluidPage(
  
  # Application title
  titlePanel("Go Tar Heels!"),
  
  # Top panel with county name
  verticalLayout(
    
    wellPanel(textOutput("cnty")),
    
    # the map itself
    mainPanel(
      leafletOutput("map")
    )
  )
)

# Define server logic 
server <- function(input, output) {
  
  output$map <- renderLeaflet({
    leaflet() %>% 
      addProviderTiles("Stamen.Toner") %>% 
      addPolygons(data = shape, 
                  fillColor = "aliceblue", 
                  color = "grey",
                  layerId = ~CNTY_ID)
  })
  
  # this is the fun part!!! :)
  observe({ 
    event <- input$map_shape_click
    output$cnty <- renderText(shape$NAME[shape$CNTY_ID == event$id])
    
  })
}

# Run the application 
shinyApp(ui = ui, server = server)