development/superseded functions/query_ms_layer.R

In MatthewJWhittle/getarc: An R Wrapper for the ArcGIS Rest API

#' Query Map Server Layer
#'
#' Download feature layer from a Map Server
#'
#' @param host url for the ArcGIS Server host e.g 'https://services.arcgis.com'
#' @param instance a string defining the ArcGIS Server instance. The default is "arcgis"
#' @param folder a string defining the folder name
#' @param service_name a string defining the name of the service name
#' @param layer_id an integar defining the layer id (starting at 0)
#' @param my_token an access token acquired via \code{get_token}
#' @param query a named vector of parameters to include in the query
#' @param crs the output crs, defaulting to 4326
#'
#' @return an sf object
#' @export query_ms_layer
#'
#' @import httr
#' @import sf
#' @importFrom magrittr %>%
query_ms_layer <- function(host,
                           instance = "arcgis",
                           folder,
                           service_name,
                           layer_id = 0,
                           crs = 4326,
                           my_token = NULL,
                           query = NULL) {
  rest <- "rest"
  services <- "services"
  server_type <- "MapServer"
  request_type <- "query"

  endpoint <-
    list(
      host,
      instance,
      rest,
      services,
      folder,
      service_name,
      server_type,
      layer_id,
      request_type
    ) %>% paste0(collapse = "/")


  # Parameter spec -------------------------
  f <- "json"
  # "where=FID>=0" was causing a 400 error
  # This where clause is taken from the natural england arc gis api example
  where <- "1=1"

  # Get the token from the supplied access token
  if (!is.null(my_token)) {
    token <- parse_access_token(my_token)
  } else{
    token <- my_token
  }

  # Define a list of essential parameters
  essential_parameters <-
    list(
      f = f,
      token = token,
      # Get all features with sql query 1 = 1
      where = where,
      # Assert that the data is lat lon if writing to geojson
      outSR = 4326
    )

  # Define a list of default parameters
  default_parameters <-  list(
    returnIdsOnly = "false",
    outFields = "*",
    returnCountOnly = "false"
  )


  # Drop any parameters specified by the user in query from the list of standard parameters
  default_parameters <-
    default_parameters[!names(default_parameters) %in% names(query)]



  query_list <- c(essential_parameters, default_parameters, query)



  # Collapse the parameters into a string of length 1
  # The drop_null argument enables the function to drop NULL tokens
  # This is accounts for services that don't require an api token
  query_string <-
    collapse_query_parameters(query_list, drop_null = TRUE)

  # Create a temporary file for caching the spatial data
  temp_file <- tempfile(fileext = ".geojson")
  # Request the spatial data and write it to a temporary file as JSON
  request <-
    httr::GET(paste0(endpoint, "?", query_string),
              httr::write_disk(temp_file, overwrite = T))
  # Fail on error
  stopifnot(httr::status_code(request) == 200)
  # Read the data from the temporary file
  possible_read <- purrr::possibly(sf::st_read, otherwise = NULL)
  data <- possible_read(temp_file, stringsAsFactors = FALSE)

  if (is.null(data)) {
    stop(paste0("Error: ",
                print(httr::content(request))))
  }

  # If the specified crs is not 4326 (the current crs) then transform the data
  if (crs != 4326) {
    data <- data %>% sf::st_transform(crs = crs)
  }
  return(data)
}