Switch between R Plumber Serializers

Question

#* @get /json
#* @serializer unboxedJSON
function() {
  return(iris)
}

#* @get /csv
#* @serializer csv list(type="text/plain; charset=UTF-8")
function() {
  return(iris)
}

#* @param type csv or json
#* @get /data
function(type = 'csv') {
  if (type == 'cvs') {
    #* @serializer csv list(type="text/plain; charset=UTF-8")
    return(iris)
  } else {
    #* @serializer unboxedJSON
    return(iris)
  }
}

The first 2 endpoints above each work fine, but the 3rd endpoint does not work as the #* @serializer unboxedJSON cannot be inside the function it seems. However, it would be great if I could somehow have a single endpoint which handles serializing before returning. plumber has plumber::serializer_csv and plumber::serializer_unboxed_json() and all of their serializers as functions, however I'm not sure how to use them inside the endpoint functions (or if this is even possible)

Thanks!

Answer 1

You can create a custom serializer that changes the serialization format depending on the the response object. Here is an example. In this case, I attach an attribute to my response that provides the format that I want:

library(plumber)

#* @apiTitle Example Plumber API with Serializer to switch formats using object
#* attributes

serializer_switch <- function() {
  function(val, req, res, errorHandler) {
    tryCatch({
      format <- attr(val, "serialize_format")
      if (is.null(format) || format  == "json") {
        type <- "application/json"
        sfn <- jsonlite::toJSON
      } else if (format == "csv") {
        type <- "text/csv; charset=UTF-8"
        sfn <- readr::format_csv
      } else if (format == "rds") {
        type <- "application/rds"
        sfn <- function(x) base::serialize(x, NULL)
      }
      val <- sfn(val)
      res$setHeader("Content-Type", type)
      res$body <- val
      res$toResponse()
    }, error = function(err) {
      errorHandler(req, res, err)
    })
  }
}

register_serializer("switch", serializer_switch)

#* Return a data frame of random values
#* @param n size of data frame
#* @param format one of "json", "csv", or "rds"
#* @serializer switch
#* @get /random_df
function(n = 10, format = "json") {
  out <- data.frame(value = rnorm(n))
  attr(out, "serialize_format") <- format
  out
}