R Shiny: Updating Multiple Dependent Dropdown Menus with Leaflet Map Click

Question

I have an interactive map rendered with leaflet in a shiny dashboard in R. The dashboard also contains two dropdown menus created with selectizeInput, where the options available in the second dropdown depend on the selection in the first dropdown. In the toy example below, the second dropdown presents a list of cities, which depends on the country selected in the first dropdown.

I would like to specify the selections in these two dropdown menus by clicking on the city in the map. In the code below, this works once you have chosen a country. For example, if I select "Australia" from the first dropdown and then click on Australian cities in the map, the selected city in the second dropdown updates correctly. However, if I then click on a city in New Zealand, "Auckland" (the first city in the New Zealand list) is selected in the city dropdown regardless of which New Zealand city I actually clicked on. Subsequent clicks on cities in New Zealand then work correctly.

How can I get the city dropdown to update correctly the first time I click on a city in a different country to that currently selected in the country dropdown?

Note: This is only intended as a simple, reproducible example of the functionality that I require.

library(shiny)
library(leaflet)

cities <- data.frame(country = c(rep('Australia',5),rep('New Zealand',3)),
                     city = c("Adelaide", "Brisbane", "Melbourne", "Perth", "Sydney", "Auckland", "Christchurch", "Wellington"),
                     lat = c(-34.9329, -27.469, -37.8142, -31.9527, -33.868, -36.85, -43.53, -41.2889),
                     long = c(138.5998, 153.0235, 144.9632, 115.8605, 151.21, 174.7833, 172.6203, 174.7772))


ui <- fluidPage(
  fluidRow(column(width=12, leafletOutput("map"))),
  fluidRow(
    column(width=4, 
           selectizeInput(inputId = "countrySelected",label = "Country", choices = cities$country),
           uiOutput("citySelectedUI"))
    )
  )


server <- function(input, output, session){
  
  # THE MAP:
  output$map <- renderLeaflet({
    leaflet(cities) %>% 
      addTiles() %>% 
      setView(lng=152, lat=-36, zoom=4) %>% 
      addCircleMarkers(lng = ~long, lat = ~lat, radius=4, label=~city) 
  })
  
  # THE CITIES DROPDOWN (CONDITIONAL ON SELECTED COUNTRY)
  output$citySelectedUI <- renderUI({
    selectizeInput("citySelected", "City", choices=cities$city[cities$country==input$countrySelected])
  })
  
  # NOT WORKING CORRECTLY: Clicking city on map should update country and city selected 
  observe({
    if(!is.null(input$map_marker_click)){
      
      updateSelectizeInput(
        session, "countrySelected", 
        selected = cities$country[(cities$lat==input$map_marker_click$lat)&(cities$long==input$map_marker_click$lng)])
      
      updateSelectizeInput(
        session, "citySelected", 
        selected = cities$city[(cities$lat==input$map_marker_click$lat)&(cities$long==input$map_marker_click$lng)])
    }
    
  })
  
}


shinyApp(ui = ui, server = server)

Answer 1

Try this

  # WORKING CORRECTLY: Clicking city on map should update country and city selected 
  observeEvent(input$map_marker_click, {
    if(!is.null(input$map_marker_click)){
      
      updateSelectizeInput(
        session, "countrySelected", 
        selected = cities$country[(cities$lat==input$map_marker_click$lat)&(cities$long==input$map_marker_click$lng)])
    }
    }, ignoreInit = TRUE)
  observe({
    if(!is.null(input$map_marker_click)){
      req(input$countrySelected)
      updateSelectizeInput(
        session, "citySelected", 
        selected = cities$city[(cities$lat==input$map_marker_click$lat)&(cities$long==input$map_marker_click$lng)])
    }
    
  })