Is there a function to find the input handlers for a package, generally? There are several special input handlers for leaflet, e.g. input$mymap_shape_mouseover that are not anywhere listed in the R documentation. Really, I just want to be able to grab the coordinates of a flat png heatmap that I'm using with leaflet and reformat them to grab the coordinates in the matrix I've previously plotted.

library(shiny)
library(leaflet)
library(leaflet.extras)
library(mapview)
library(foreach)
server <- function(input, output, session) {
  points <- eventReactive(input$recalc, {
    cbind(rnorm(40) * 2 + 13, rnorm(40) + 48)
  }, ignoreNULL = FALSE)
  output$mymap <- renderLeaflet({
    bounds <- c(0, 0, 14400, 14400)
    leaflet(options = leafletOptions(
      crs = leafletCRS(crsClass = "L.CRS.Simple"),
      minZoom = -5,
      maxZoom = 5)) %>%
      fitBounds(bounds[1], bounds[2], bounds[3], bounds[4]) %>%
      htmlwidgets::onRender("
                            function(el, t) {
                            var myMap = this;
                            var bounds = myMap.getBounds();
                            var image = new L.ImageOverlay(
                            'https://github.com/theaidenlab/juicebox/wiki/images/domains_peaks.png',
                            bounds);
                            image.addTo(myMap);
                            }") %>%
    addMeasure()  %>%
      addMiniMap( toggleDisplay = TRUE,
                  position = "bottomleft") %>% addDrawToolbar() %>% addFullscreenControl() %>% 
     addMouseCoordinates(style="basic")
  })

Discovering Leaflet Input Events

For those interested in leaflet input events - such as input$MAPID_center - please thank @blondclover. They recommended a nifty trick to print out all input events:

Create the output$outputID in the UI using verbatimTextOutput; and
Store the results of renderPrint({reactiveValuesToList(input)}) in the output$outputID object in server.

As the complexity of your leaflet map grows, knowing which leaflet input events are available to use will help you customize your map.

# load necessary packages
library( shiny )
library( leaflet )
library( mapview )

ui <- fluidPage(
  leafletOutput( outputId = "map"),
  downloadButton( outputId = "dl"),
  h2("List of Input Events"),
  verbatimTextOutput( outputId = "text")
)

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

  # print list of input events
  output$text <-
    renderPrint({reactiveValuesToList(input)})

  # Create foundational leaflet map
  # and store it as a reactive expression
  foundational.map <- reactive({

    leaflet() %>% # create a leaflet map widget

      addTiles( urlTemplate = "https://{s}.tile.openstreetmap.se/hydda/base/{z}/{x}/{y}.png" ) # specify provider tile and type

  }) # end of foundational.map()

  # render foundational leaflet map
  output$map <- leaflet::renderLeaflet({

    # call reactive map
    foundational.map()

  }) # end of render leaflet

  # store the current user-created version
  # of the Leaflet map for download in 
  # a reactive expression
  user.created.map <- reactive({

    # call the foundational Leaflet map
    foundational.map() %>%

      # store the view based on UI
      setView( lng = input$map_center$lng
               ,  lat = input$map_center$lat
               , zoom = input$map_zoom
      )

  }) # end of creating user.created.map()



  # create the output file name
  # and specify how the download button will take
  # a screenshot - using the mapview::mapshot() function
  # and save as a PDF
  output$dl <- downloadHandler(
    filename = paste0( Sys.Date()
                       , "_customLeafletmap"
                       , ".pdf"
    )

    , content = function(file) {
      mapshot( x = user.created.map()
               , file = file
               , cliprect = "viewport" # the clipping rectangle matches the height & width from the viewing port
               , selfcontained = FALSE # when this was not specified, the function for produced a PDF of two pages: one of the leaflet map, the other a blank page.
      )
    } # end of content() function
  ) # end of downloadHandler() function

} # end of server

# run the Shiny app
shinyApp(ui = ui, server = server)

# end of script #