R Leaflet addAwesomeMarkers如何按字符型列值条件设置标记颜色

使用leaflet包的addAwesomeMarkers功能时，需要根据数据集指定列的取值动态调整地图标记的显示颜色：

• 基于内置quakes数据集测试，若通过mutate生成的判断列为数值型，可正常实现颜色区分，测试代码如下：

data(quakes)
quakes <-
quakes %>% mutate(
  type = ifelse(stations > 60, 1,2)
)

getColor <- function(quakes) {
  sapply(quakes$type, function(type) {
  if(type == 1) {
    "green"
  } else {
    "red"
  } })
}

icons <- awesomeIcons(
  icon = 'ios-close',
  iconColor = 'black',
  library = 'ion',
  markerColor = getColor(quakes[1:20, ])
)

leaflet(quakes[1:20, ]) %>% addTiles() %>%
  addAwesomeMarkers(~long, ~lat, icon=icons, label=~as.character(mag))

上述代码运行后可正常实现符合条件的绿、红标记颜色区分。

• 当mutate生成的type列为字符型取值（stations>60时赋值"foo"，否则赋值"bar"）时，所有标记都被统一设置为else分支的红色，无法正常区分颜色，测试代码如下：

quakes <-
quakes %>% mutate(
  type = ifelse(stations > 60, "foo","bar")
)

getColor <- function(quakes) {
  sapply(quakes$type, function(type) {
  if(type == "foo") {
    "green"
  } else {
    "red"
  } })
}

icons <- awesomeIcons(
  icon = 'ios-close',
  iconColor = 'black',
  library = 'ion',
  markerColor = getColor(quakes[1:20, ])
)

运行时控制台同时输出警告：

Input to asJSON(keep_vec_names=TRUE) is a named vector. In a future version of jsonlite, this option will not be supported, and named vectors will be translated into arrays instead of objects. If you want JSON object output, please use a named list instead. See ?toJSON.

问题出在sapply的返回值上：对字符型列做sapply遍历判断时，返回的结果是带原列值作为names属性的字符向量，awesomeIcons在接收markerColor参数时，带名向量会被jsonlite序列化为对象而非数组，导致颜色映射规则解析失败，所有标记回退到默认分支的红色。
数值型场景下未触发问题属于偶然情况，本质是数值型names的序列化结果没有完全打断参数解析，并非写法本身没有隐患。

两种方案均可实现基于字符型列值的动态颜色设置，第二种更符合leaflet的设计逻辑，出错概率更低：

• 保留原有写法，仅需在getColor返回结果时去掉向量的names属性即可

getColor <- function(quakes) {
  res <- sapply(quakes$type, function(type) {
    if(type == "foo") {
      "green"
    } else {
      "red"
    }
  })
  unname(res) # 核心：移除向量的names属性，保证序列化结果为颜色值数组
}

• （推荐）不提前生成静态icons对象，直接在addAwesomeMarkers中用公式接口动态映射颜色，自动适配数据行数，也不会出现带名向量的序列化问题

library(leaflet)
library(dplyr)

data(quakes)
quakes <- quakes %>% 
  mutate(type = ifelse(stations > 60, "foo", "bar"))

leaflet(quakes[1:20, ]) %>% 
  addTiles() %>%
  addAwesomeMarkers(
    ~long, ~lat,
    icon = ~awesomeIcons(
      icon = 'ios-close',
      iconColor = 'black',
      library = 'ion',
      markerColor = ifelse(type == "foo", "green", "red")
    ),
    label = ~as.character(mag)
  )

内容的提问来源于stack exchange，提问作者r0bt