How to set Leaflet layerId using mapview

Question

In my shiny app, I would like to click on a polygon of my mapview map and be able to extract the layerId attribute to a variable using input$map_shape_click. In the following code, when you click a polygon, it prints out the id, but it is set to null by default in mapview.

library(shiny)
library(tmap)
library(leaflet)
library(mapview)

ui <- bootstrapPage(
  title = "Standardized Crop Production Index",
  tags$style(type = "text/css", "html, body {width:100%;height:100%}"),
  mapview::mapviewOutput("map", width = "100%", height = "100%")
)

data("World") #from the tmap library

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

  out_plot <- observeEvent(input$map_shape_click, {
    p <- input$map_shape_click
    print(p)
  })

  output$map <- renderLeaflet({

    test <- mapview(World)
    test@map
  })
}

shinyApp(ui = ui, server = server)

Is there a way to set the layerId of a mapview object? I know that I could set it using addPolygons() if I were to just use Leaflet instead of mapview. However, I would eventually like to use the data gathered from clicking the polygon as input into mapview's popupGraph().

Even better would be if there is a way to retrieve the attributes from the table that pops up when you click a polygon. For example, if I click on Antarctica, the following attribute table pops up: Antarctica Attribute Table. Is there any way to retrieve the "name" attribute and store it as a variable when a polygon clicked?

Thanks!

Carlos Xavier Bonilla Carlos Xavier Bonilla · Accepted Answer · 2018-10-05T19:45:39

Not entirely sure about your first question, as I'm not aware of any way that can be assigned using mapview(). However, here is a reproducible solution using addPolygons():

library(dplyr)
library(shiny)
library(leaflet)
library(leaflet.extras)
library(rgdal)
library(sp)
library(tigris)
library(htmltools)

setwd(dirname(rstudioapi::getActiveDocumentContext()$path))  # set your working directory

philly <- tracts(state = 'PA', county = c('Philadelphia'))

ui <- fluidPage(
  title = "Test Map",
  leafletOutput("mymap", width = 600)
)


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

  RV <- reactiveValues(Clicks=list()) # used for storing leaflet variables

  tract_labels <- sprintf( # labels for mouseover tooltip
    "<strong>%s</strong>, <strong>%s</strong>
    <br/><b>Land Area:</b> %s",
    philly$COUNTYFP,
    philly$STATEFP,
    philly$ALAND
  ) %>% lapply(htmltools::HTML)


  output$mymap <- renderLeaflet({  # leaflet map
    leaflet(data = philly) %>%
      setView(-75.16, 39.9523, zoom = 10) %>%
      addTiles(urlTemplate = "https://{s}.tile.openstreetmap.se/hydda/full/{z}/{x}/{y}.png",
               attribution = NULL) %>%
      addPolygons(data = philly,
                  layerId = philly@data$ALAND,
                  group = "regions",
                  fillColor = "#bdd7e7",
                  weight = 1,
                  opacity = 1.0, 
                  fillOpacity = 0.5,
                  smoothFactor = 0.5,
                  label = tract_labels,
                  labelOptions = labelOptions(
                    style = list("font-weight" = "normal", padding = "3px 8px"),
                    textsize = "12px",
                    direction = "auto"),
                  highlightOptions = highlightOptions(color = "white",
                                                      weight = 2,
                                                      bringToFront = TRUE))
  })


  observeEvent({input$mymap_shape_click}, {

    #create object for clicked polygon
    click <- input$mymap_shape_click
    RV$Clicks <- c(RV$Clicks,click$id)

    #define leaflet proxy for second regional level map
    proxy <- leafletProxy("mymap")

    #subset regions shapefile by the clicked on polygons
    selectedReg <- philly[philly@data$ALAND == click$id,]

    #map clicked on polygons
    proxy %>% addPolygons(data = selectedReg,
                          fillColor = "red",
                          fillOpacity = 1,
                          weight = 1,
                          color = "black",
                          stroke = T,
                          group = "selected",
                          layerId = selectedReg@data$ALAND)

    # remove polygon group that are clicked twice
    if(click$group == "selected"){
      proxy %>%
        clearGroup(group = "selected")

      RV$Clicks <- 0  # resets values if polygons are clicked twice
    }

    mean.land <- mean(as.numeric(RV$Clicks))  # stores the values of polygons that are clicked
    print(mean.land)

  })


}

shinyApp(ui, server)

Essentially the map has two layers: the base tracts layer, and another tracts polygon that highlights what you click. You can click on each polygon to 'retrieve' a value (in this case it's land area, or variable ALAND) from each polygon and make calculations on it. Here I have selected three polygons, and I have used mean.land variable to display the average land area of all three.

The reactiveValues RV object is used to store a numeric value of the layerId variable on any polygon that you click. This allows you to store and 'retrieve' it for other calculations you may want to make.

[1] 717210  # first click, first value
[1] 571940  # second click, averaged value
[1] 488678.3  # third click, averaged value

You can change extract the layerId attribute by changing any references to variable ALAND in the code.

How to set Leaflet layerId using mapview

2 Answers