library(shiny)
library(dplyr)
library(leaflet)

ui <- fluidPage(
  titlePanel("Hello Shiny!"),
  sidebarLayout(
    sidebarPanel(
      actionButton(inputId = "my_button1",
                   label = "Use leafletProxy()"),
      actionButton(inputId = "my_button2",
                   label = "Use renderLeaflet()")
    ),
    mainPanel(
      leafletOutput(
        outputId = "map",
        width = "100%",
        height = "300px"
      )
    )
  )
)

server <- function(input, output, session) {

  some_data <- data.frame(
    "lon"=c(4.905167,4.906357,4.905831),
    "lat"=c(52.37712,52.37783,52.37755),
    "number_var"=c(5,9,7),
    "name"=c("Jane","Harold","Mike"),
    stringsAsFactors = F
  )

  output$map <- renderLeaflet({
    return(
      leaflet(data = some_data[0,]) %>%
         addProviderTiles(providers$CartoDB.Positron) %>%
        fitBounds(
          min(some_data$lon),
          min(some_data$lat),
          max(some_data$lon),
          max(some_data$lat)
        ) %>%
        addMarkers(
          lng = ~lon,
          lat = ~lat,
          clusterOptions = markerClusterOptions(
            iconCreateFunction = JS(paste0("function(cluster) {",
                                           "return new L.DivIcon({",
                                           "html: '<div style=\"background-color:rgba(77,77,77,0.5)\"><span>' + cluster.getChildCount() + '</div><span>',",
                                           "className: 'marker-cluster'",
                                           "});",
                                           "}"))



          )
        )
    )
  })

  observeEvent(input$my_button1,{
      leafletProxy(mapId = "map",
                   session = session,
                   data = some_data) %>%
        addProviderTiles(providers$CartoDB.Positron) %>%
        clearMarkerClusters() %>%
        clearMarkers() %>%
        fitBounds(
          min(some_data$lon),
          min(some_data$lat),
          max(some_data$lon),
          max(some_data$lat)
        ) %>%
        addMarkers(
          lng = ~lon,
          lat = ~lat,
          clusterOptions = markerClusterOptions(
            iconCreateFunction = JS(paste0("function(cluster) {",
                                           "console.log('Here comes cluster',cluster); ",
                                           "return new L.DivIcon({",
                                           "html: '<div style=\"background-color:rgba(77,77,77,0.5)\"><span>' + cluster.getChildCount() + '</div><span>',",
                                           "className: 'marker-cluster'",
                                           "});",
                                           "}"))
          )
        )
  })

  observeEvent(input$my_button2,{
    output$map <- renderLeaflet({

      leaflet(data = some_data) %>%
        addProviderTiles(providers$CartoDB.Positron) %>%
        fitBounds(
          min(some_data$lon),
          min(some_data$lat),
          max(some_data$lon),
          max(some_data$lat)
        ) %>%
        addMarkers(
          lng = ~lon,
          lat = ~lat,
          clusterOptions = markerClusterOptions(
            iconCreateFunction = JS(paste0("function(cluster) {",
                                           "console.log('Here comes cluster',cluster); ",
                                           "return new L.DivIcon({",
                                           "html: '<div style=\"background-color:rgba(77,77,77,0.5)\"><span>' + cluster.getChildCount() + '</div><span>',",
                                           "className: 'marker-cluster'",
                                           "});",
                                           "}"))
          )
        )
    })
  })
}

shinyApp(ui = ui, server = server)