如何使用Knitr在RMD中的循环中渲染传单地图 [英] How to render leaflet-maps in loops in RMDs with knitr

问题描述

我目前正在努力尝试使knitr呈现我的传单地图，该传单地图来自一个集合，以正确显示在呈现的RMD html输出中.我已经知道在循环遍历集合并使用RMD/knitr生成图形输出时存在一些潜在的问题，但是我仍然不知道如何使我的示例适用于传单地图.

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.

可复制的工作示例(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")
}
```

上面的示例呈现了预期的效果(至少在添加plot.new()之后，我学会了

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:

可复制的失败示例(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")
}
```

我希望第二个Rmd渲染同一张地图的4倍，显示不同的标题(地块编号1-4").但是输出根本不会渲染任何地图.输出结果如下:

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:

检查生成的html-output后，可以看到根本没有渲染任何东西，而不仅仅是可见性问题:

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:

但是，当我直接通过突出显示"代码并按ctrl-Enter来直接评估第二个Rmd中的小叶部分时，地图将按预期方式呈现:

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:

我已经尝试过

• 将传单声明转换为赋值语句
• 引入cat()或print()命令以强制地图输出
• 在地图输出部分之前和/或之后使用其他换行符'\ n'
• 在 fig.show 或 results
中使用"asis"指令

• 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

没有任何效果.有人在这里有线索吗?

without any effect. Does anyone have a clue here?

推荐答案

您需要将内容放入tagList，并从块中打印该列表.这仅使用fig.show和results的默认设置.它还使用htmltools::h3()函数将标题直接转换为HTML标题，而不使用Markdown ###标记. (您可能需要h2或h4.)

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)
```

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