get circle id from leaflet in shiny for later use - r

Question

I am quite new to shiny, and I am facing a difficulty. I want to have a map with interactive circles. When clicked, these circles will allow me to make a query to a SQL database to get the corresponding data and so make plots.
I don't manage to get the circles info into a variable, although I am able to print it to the shiny ui.
Here is the example code:
library(shiny)
library(leaflet)
ui <- fluidPage(
leafletOutput("mymap"),
verbatimTextOutput("marker")
)
server <- function(input, output, session) {
output$mymap <- renderLeaflet({
leaflet(data = mapStates, options = leafletOptions(minZoom = 3, maxZoom = 18)) %>%
addTiles() %>%
addProviderTiles(providers$Stamen.TonerLite,
options = providerTileOptions(noWrap = TRUE))%>%
addCircleMarkers(data = data.frame(lat = 51, lng = 13,STANAME = "somewhere",STAID = "1" ), lng = ~lng, lat = ~lat,radius = 1, color = "red", fill = "red", popup = ~STANAME,layerId = ~STAID)
})
# here the circle info
output$marker <- renderPrint(input$mymap_marker_click)
}
shinyApp(ui, server)
but I don't manage to get the id of the marker into a variable in the server function. I tried:
input$mymap_marker_click$id
But it tells me that I need a reactive context. If I do:
renderPrint(input$mymap_marker_click)$id
Error : object of type 'closure' is not subsettable
I can't use the output in the server side, but I need this variable in the server side to do the query and the plots.
I should I proceed ?
Thank you for your help.

Answer 1

In Shiny you need to create an observer to "listen" for the click event (or any event/change to input) and perform a certain response.
Removing the map tiles, bc I don't know where mapStates comes from but the idea is identical.
library(shiny)
library(leaflet)
ui <- fluidPage(
leafletOutput("mymap"),
verbatimTextOutput("marker")
)
server <- function(input, output, session) {
output$mymap <- renderLeaflet({
leaflet(options = leafletOptions(minZoom = 3, maxZoom = 18)) %>%
addCircleMarkers(data = data.frame(lat = 51, lng = 13,STANAME = "somewhere",STAID = "1" ), lng = ~lng, lat = ~lat,radius = 1, color = "red", fill = "red", popup = ~STANAME,layerId = ~STAID)
})
# needs to be in a observer to "listen" for events
observeEvent(input$mymap_marker_click, {
output$marker <- renderPrint(input$mymap_marker_click$id)
})
}
shinyApp(ui, server)
Live demo