How to render leaflet-maps in loops in RMDs with knitr

Question

I am currently fighting trying to get knitr to render my leaflet maps, taken from a collection to appear correctly in a rendered RMD html-output. I'm already aware about some potential problems when looping throug collections and generating graphical output with RMD/knitr, but still I can't figure out, how to make my example work for leaflet-maps.

Reproducible working example (Test_1.Rmd):

---
title: "test1"
author: "phabee"
date: "22 Mai 2018"
output: html_document
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

## Title 1

```{r, fig.show='asis', echo=FALSE, results='asis'}
for (i in 1:4) {
    cat("### Plot Number ", i, "\n")
    plot(1,1)
    # use plot.new() here to force rendering of potential plot-duplicates
    plot.new()
    cat("\n\n")
}
```

The above example renders as expected (at least after adding plot.new(), which I've learned here from Freedomtowin). But when I try to do the same with leaflet-maps, it doesn't work at all. No single map is being rendered:

Reproducible failing example (Test_2.Rmd)

---
title: "test2"
author: "phabee"
date: "22 Mai 2018"
output: html_document
---

```{r setup, include=FALSE}
library(leaflet)
knitr::opts_chunk$set(echo = TRUE)
```

## Title 1

```{r, fig.show='asis', echo=FALSE, results='asis'}
for (i in 1:4) {
  cat("### Map Number ", i, "\n")
  leaflet() %>%
  addTiles() %>%  # Add default OpenStreetMap map tiles
  addMarkers(lng=174.768, lat=-36.852, popup="The birthplace of R")  
  cat("\n")
}
```

I would expect the second Rmd to render 4 times the same map, showing different titles ("Plot Number 1-4"). But the output doesn't render any map at all. The output looks as follows:

After inspecting the generated html-output the output, it can be seen that there is nothing rendered at all and it's not just a visibility-issue:

However, when I evaluate the leaflet-section within the 2nd Rmd directly by 'highlighting' the code and hitting ctrl-Enter, the map renders as expected:

I already tried to

• convert the leaflet-statement to an assignment statement
• introduce cat() or print() commands to force map-output
• play around with additional newline-characters '\n' before and/or after the map output section
• fiddle around with the 'asis' directives from fig.show or results

without any effect. Does anyone have a clue here?

Answer 1

You need to put things in a tagList, and have that list print from the chunk. This just uses default settings for fig.show and results; it also uses the htmltools::h3() function to turn the title into an HTML title directly, not using the Markdown ### marker. (You might want h2 or h4 instead.)

---
title: "test3"
output: html_document
---

```{r setup, include=FALSE}
library(leaflet)
library(htmltools)
knitr::opts_chunk$set(echo = TRUE)
```

## Title 1

```{r echo=FALSE}
html <- list()
for (i in 1:4) {
  html <- c(html, 
            list(h3(paste0("Map Number ", i)),
                 leaflet() %>%
                 addTiles() %>%  # Add default OpenStreetMap map tiles
                 addMarkers(lng=174.768, lat=-36.852, popup="The birthplace of R")  
                 )
            )
}
tagList(html)
```