Retrieve positions of all markers in a leaflet map of Shiny APP

Question

I have a leaflet map which has the draggable markers and shapes, user drawn features (with leaflet.draw plugin). Now I want to programmably retrieve all features in a layer/group from the leaflet map.

How should I achieve this function? Thanks for any suggestions.

Here I provided a simple example, a random set of markers is added into leaflet map with group markers (codes directly copied from https://rstudio.github.io/leaflet/shiny.html and made some changes). My target here is to retrieve the position of all new markers.

My real question is for polygons and may be complicated than this, but method should be similar.

library(shiny)
library(leaflet)

r_colors <- rgb(t(col2rgb(colors()) / 255))
names(r_colors) <- colors()

ui <- fluidPage(
  leafletOutput("mymap"),
  p(),
  actionButton("recalc", "New points"),
  verbatimTextOutput('summary')
)

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

  points <- eventReactive(input$recalc, {
    cbind(rnorm(40) * 2 + 13, rnorm(40) + 48)
  }, ignoreNULL = FALSE)

  output$mymap <- renderLeaflet({
    leaflet() %>%
      addProviderTiles("Stamen.TonerLite",
                       options = providerTileOptions(noWrap = TRUE)
      ) %>%
      addMarkers(data = points(), group = 'markers')
  })

  output$summary <- renderPrint({
    # Add some codes here
    # .....

  })
}

shinyApp(ui, server)

Answer 1

To get the points from the map object, you can still use reactiveValues() to store the map object, and update this by observe()ing the button press. Then you can access all its properties.

library(shiny)
library(leaflet)

r_colors <- rgb(t(col2rgb(colors()) / 255))
names(r_colors) <- colors()

ui <- fluidPage(
    leafletOutput("mymap"),
    p(),
    actionButton("recalc", "New points"),
    verbatimTextOutput('summary')
)

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

    rv <- reactiveValues()
    rv$m <- NULL
    rv$p <- NULL

    points <- eventReactive(input$recalc, {
        cbind(rnorm(40) * 2 + 13, rnorm(40) + 48)
    }, ignoreNULL = FALSE)

    output$mymap <- renderLeaflet({
        m <- leaflet() %>%
            addProviderTiles("Stamen.TonerLite",
                    options = providerTileOptions(noWrap = TRUE)) %>%
            addMarkers(data = points(), group = 'markers')
        rv$m <- m
        return(m)
    })

    observe({
        input$recalc
        ## I'm 90% confident these are the arguments you want...
        rv$p <- data.frame(x = rv$m$x$calls[[2]]$args[[1]],
                           y = rv$m$x$calls[[2]]$args[[2]])
    })

    output$summary <- renderPrint({
        # print points
        rv$p
    })
}

shinyApp(ui, server) 

---

Old solution

I'm leaving this here as it may still be useful to others

You can make use of reactiveValues() to store your point data in, and then output that data in your print

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

  rv <- reactiveValues()
  rv$points <- cbind(rnorm(40) * 2 + 13, rnorm(40) + 48)

  observe({

    if(input$recalc){
      p <-  cbind(rnorm(40) * 2 + 13, rnorm(40) + 48)
      rv$points <- p
    }
  })

  output$mymap <- renderLeaflet({
    leaflet() %>%
      addProviderTiles("Stamen.TonerLite",
                       options = providerTileOptions(noWrap = TRUE)
      ) %>%
      addMarkers(data = rv$points, group = 'markers')
  })

  output$summary <- renderPrint({
    # Add some codes here
    # .....
    rv$points
  })
}
shinyApp(ui, server)