Changing pallete values based on input (Shiny & leaflet) - shiny

Question

I'm using leaflet and shiny. I would like to color my markers based on a column that can be changed via input. It's almost the same as Modifying Existing Maps with leafletProxy. In this example, the user can change the color palette. In my example, I would like to change the column that the palette is applied on.
I'm trying to use something like:
fillColor = ~pal(!!sym(input$col_to_apply)) # input$col_to_apply is a column name (string) I want to use
However, this doesn't work. I'm also not sure if I have to use reactive() in this case.

Answer 1

Sure. My suggestion would be to do it before the palette is created. Palettes are tricky enough as it is. See the below minimal example:
library(leaflet)
library(maps)
library(shiny)
ui <- fluidPage(
leafletOutput("map_1"),
selectInput(inputId = "input_species", label = "Species Selection", choices = c("Species_1", "Species_2", "Species_3"))
)
server <- function(input, output, session) {
#Load a map of the US from the 'map' package (runs once when apps starts)
shp_map = map("state", fill = TRUE, plot = FALSE)
#Make up a dataframe with some data for three species for each state (runs once when apps starts)
df_data <- data.frame(state = unique(shp_map$names), Species_1 = sample(100:199, 63), Species_2 = sample(200:299, 63), Species_3 = sample(300:399, 63))
#Create map
output$map_1 <- renderLeaflet({
df_map <- df_data
#Create a column called species selected based on which is selected in the dropdown box
df_map$Species_Selected <- df_map[, paste(input$input_species)]
#Create a palette function
palette <- colorNumeric(palette = "Blues", domain = df_map$Species_Selected)
#Use the palette function created above to add the appropriate RGB value to our dataframe
df_map$color <- palette(df_map$Species_Selected)
#Create map
map_1 <- leaflet(data = shp_map) %>%
addPolygons(fillColor = df_map$color, fillOpacity = 1, weight = 1, color = "#000000", popup = paste(sep = "", "<b>", paste(shp_map$names), " ", "</b><br>", df_map$Species_Selected))
map_1
})
}
shinyApp(ui, server)