r - Shiny leaflet easyPrint plugin

Question

I'm trying to incorporate the easyPrint plugin into my shiny leaflet app. What I want is something that looks like the demo, but in shiny.

I have tried to mimic the examples, but have been unsuccessful.

Here's my code for my R code so far:

    library(shiny)
    library(shinydashboard)
    library(shinyjs)
    library(htmlwidgets)
    library(htmltools)
    library(leaflet)
    library(leaflet.extras)
    library(sp)

    shinyApp(
  ui = fluidPage(
    leafletOutput("map", height = 750)
  ),
  server = function(input, output) {

    registerPlugin <- function(map, plugin) {
      map$dependencies <- c(map$dependencies, list(plugin))
      map
    }

    easyPrintPlugin <- htmlDependency("leaflet-easyprint", "2.1.8",
                                      src = c(href = "https://github.com/rowanwins/leaflet-easyPrint/blob/gh-pages/dist/"),
                                      script = "index.js")

    # Map
    output$map <- renderLeaflet({
      leaflet() %>%
        addProviderTiles(providers$CartoDB.Positron) %>%
        registerPlugin(easyPrintPlugin) %>%
        onRender("function(el, x) {
                 L.easyPrint({
                 position: 'topleft',
                 sizeModes: ['A4Portrait', 'A4Landscape']
                 }).addTo(map);}")
    })

  }
)

However, nothing is happening. It's literally a white screen. If I remove the onRender part, the leaflet acts normal.

Unfortunately, I'm relatively new to Shiny, leaflet, .js, and github, so I'm struggling to identify which aspect is causing the problem.

Answer 1

  library(leaflet)
  library(shiny)
  library(htmlwidgets)

  jsfile <- "https://rawgit.com/rowanwins/leaflet-easyPrint/gh-pages/dist/bundle.js" 
  ui <- fluidPage(
    tags$head(tags$script(src = jsfile)),
    leafletOutput("map")
  )
  
  server <- function(input, output, session) {
    
    output$map <- renderLeaflet({
      leaflet() %>% 
        addProviderTiles("OpenStreetMap.Mapnik") %>%
        setView(-122.23, 37.75, zoom = 10) %>%
        onRender(
          "function(el, x) {
            L.easyPrint({
              sizeModes: ['Current', 'A4Landscape', 'A4Portrait'],
              filename: 'mymap',
              exportOnly: true,
              hideControlContainer: true
            }).addTo(this);
            }"
        )
      })
    
    }
  
  shinyApp(ui, server)

Note: leaflet-easyPrint depends on dom-to-image. Per the dom-to-image Readme, Safari and Internet Explorer are not supported. However, the print button will work in Chrome and Firefox.

Troubleshooting Process

If we run the app and inspect element, we see the following errors:

Let's start with the second and third errors.

Failed to load resource

This error is pretty self-explanatory: the URL https://github.com/rowanwins/leaflet-easyPrint/blob/gh-pages/dist//index.js doesn’t exist. The path is wrong: index.js doesn’t exist in the dist directory.

We want to use bundle.js with this path: https://github.com/rowanwins/leaflet-easyPrint/blob/gh-pages/dist/bundle.js.

Did not load script

GitHub uses strict MIME type checking, so the browser isn’t using the file as intended. We need to use a rawgit.com path instead. Read more here. To write a rawgit.com path, follow these steps (from the linked answer):

1. Find your link on GitHub, and click to the "Raw" version of the file.
2. Copy the URL, and link to it.
3. Change raw.githubusercontent.com to rawgit.com (non-production) or cdn.rawgit.com (production)

We should use this path: https://rawgit.com/rowanwins/leaflet-easyPrint/gh-pages/dist/bundle.js

TypeError: L.easyPrint is not a function

The error occurred before the errors from loading leaflet-easyPrint. This tells us that onRender is getting called before leaflet-easyPrint is loaded and attached to the widget. Per Joe Cheng in this thread, htmldependency injection at runtime can be asynchronous. He recommends against using htmlDependency(src = c(href = "http://...")) for any dependency that's intended to be used with Shiny.

Instead, we can just include the remote JS file in the header of the app. Then leaflet-easyPrint will be loaded before onRender is called.