Shiny RStudio : Select polygon feature on a Leaflet map

app.R

# select polygon feature example
# with leaflet & shiny

library(shiny)
library(leaflet)

data(gadmCHE)

ui <- bootstrapPage(
    tags$style(type = "text/css", "html, body {width:100%;height:100%}"),
    leafletOutput("mymap", width = "100%", height = "100%"),
    absolutePanel(
        top = 10, right = 10,
        p(strong("Click on Region")),
        p("MAP_SHAPE_CLICK Event"),
        verbatimTextOutput("click_on_shape"),
        p("Selected feat. attributes"),
        verbatimTextOutput("feat_selected")
    )
)

server <- function(input, output, session) {
    
    rv <- reactiveValues()
    rv$selected <- NULL

    output$mymap <- renderLeaflet({
        hl_opts <- highlightOptions(
            color = "#CC0000", weight = 3, bringToFront = TRUE)
        leaflet() %>% addTiles() %>% 
            addPolygons(
                layerId = ~ID_1,
                group = "cantons",
                data = gadmCHE,
                label = ~NAME_1,
                fillColor = "#FCFFA4",
                weight = 1,
                color = "#666666",
                opacity = 0.4,
                fillOpacity = 0.8,
                highlightOptions = hl_opts) %>%
            setView(8.34, 46.77, 8)
    })
    
    output$click_on_shape <- renderPrint({
        input$mymap_shape_click
    })

    output$feat_selected <- renderPrint({
        feature <- req(rv$selected)
        #feature_id <- feature$id
        i <- which(gadmCHE$ID_1==feature$id) 
        as.list(gadmCHE@data[i,c("ID_1", "NAME_1", "HASC_1")])
    })
    
    observeEvent(input$mymap_click, {
        new_selected <- req(input$mymap_shape_click)
        isolate(old_selected <- rv$selected)
        if (is.null(old_selected) || new_selected$.nonce != old_selected$.nonce) {
            validate(
                need(new_selected$group!="selection", message=FALSE)
            )
            rv$selected <- new_selected
            i <- which(gadmCHE$ID_1==new_selected$id) 
            gadmCHE_filtered <- gadmCHE[i,]
            leafletProxy("mymap") %>%
                clearGroup("selection") %>%
                addPolygons(
                    layerId = ~ID_1,
                    group = "selection",
                    data = gadmCHE_filtered,
                    fillColor = "cyan",
                    weight = 1.2,
                    color = "#666666",
                    opacity = 0.4,
                    fillOpacity = 0.8)
        } else {
            rv$selected <- NULL
            leafletProxy("mymap") %>%
                clearGroup("selection")
        }
    })
        
}

shinyApp(ui, server)