README.md

This is an example Shiny app to use a dynamic Leaflet map like a selectInput. To run the app, install the necessary packages then enter the command shiny::runGist('4988b5164258917687cb').

app.R

library(shiny)
# requires the dev version of leaflet; devtools::install_github('rstudio/leaflet')
library(leaflet)
library(tigris)
library(sp)

# Using counties in Washington here from the tigris package; any spatial data frame in WGS84
# will do!  (Also NAD83 works fine, that's what the Census shapefiles use)
wa <- counties('WA', cb = TRUE, resolution = '20m')

ui <- fluidPage(
  
  titlePanel("Leaflet map as selectInput"),
  
  sidebarLayout(
    sidebarPanel(
      selectInput('county', 
                  'Select a county:', 
                  # Using the county name as the common ID here; be sure
                  # to keep this consistent across your app
                  choices = sort(wa$NAME), 
                  selected = 'King'), 
      leafletOutput('map')
    ), 
    
    
    mainPanel(
      verbatimTextOutput('expression')
    )
  )
)

server <- function(input, output, session) {
  
  # Here, grab a single county based on user input
  selected_county <- reactive({
    
    s <- wa[wa$NAME == input$county, ]
    
    return(s)
    
  })
  
  output$map <- renderLeaflet({
    
    wamap <- leaflet() %>%
      addProviderTiles('CartoDB.Positron') %>%
      # First add all of the counties...
      addPolygons(data = wa, weight = 1, smoothFactor = 0.2, color = '#00008B', 
                  fillColor = '#00008B', label = ~NAME, 
                  layerId = ~NAME) %>%
      # ... then superimpose the outline of the selected county in a different color - I'm using yellow.  
      addPolygons(data = selected_county(), fill = FALSE, color = '#FFFF00', 
                  opacity = 1, layerId = 'sel_cty') %>%
      # Zoom to the bounds of the selected county
      fitBounds(lng1 = bbox(selected_county())[1], 
                lat1 = bbox(selected_county())[2], 
                lng2 = bbox(selected_county())[3], 
                lat2 = bbox(selected_county())[4])
    
    wamap
    
    
  })
  
  # Get the ID of the clicked county.  The returned ID will be
  # the layerId specified in the first addPolygons call - in this
  # case the value of the NAME field for the clicked county.  
  county_click <- eventReactive(input$map_shape_click, {
    
    x <- input$map_shape_click
    
    y <- x$id
    
    return(y)
    
  })
  
  # Here, we update our drop-down menu based on the county clicked.
  observe({
    
    updateSelectInput(session, 'county', selected = county_click())
    
  })
  
  # Here, we generate a new map based on the county click.  We remove the old
  # highlighted county, then add the new one using the same styling we did before.  
  observe({
    
    wamap <- leafletProxy('map', session) %>%
      removeShape('sel_cty') %>%
      addPolygons(data = selected_county(), fill = FALSE, color = '#FFFF00', 
                  opacity = 1, layerId = 'sel_cty') %>%
      fitBounds(lng1 = bbox(selected_county())[1], 
                lat1 = bbox(selected_county())[2], 
                lng2 = bbox(selected_county())[3], 
                lat2 = bbox(selected_county())[4])
    
    
    
  })
  
  # This is just an example of the output that can be generated.  
  # As you've changed the value for NAME - in this case the 
  # name of the county - it could be passed to a reactive expression to subset another
  # dataset by county and then create a custom county plot.  
  output$expression <- renderPrint({
    
    paste0('You clicked on ', input$county, '!  Pass me to a reactive expression!')
    
  })
  
}

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