#' get names and weights of the holdings of SPDR ETFs
#' @param Symbols character vector of SPDR ETF symbols
#' @param env where to store holdings
#' @param auto.assign should the results be assigned in \code{env}?
#' @return if \code{auto.assign} is TRUE, holdings will be assigned as 
#'   the ETF symbols appended with \dQuote{.h}, and the names of those objects
#'   will be returned. Otherwise, if \code{Symbols} is only one symbol, its'
#'   holdings will be returned.  If \code{Symbols} is of length greater than
#'   one, a list will be returned where each element is the holdings of a
#'   different ETF.
#' @author Garrett See
#' @seealso \code{FinancialInstrument:::getHoldings}, 
#'   \code{gsee:::getHoldings.iShares.AsOf}
#' @references \url{https://www.spdrs.com/}
#' @examples
#' \dontrun{
#' getHoldings.SPDR("SPY")
#' SPY.h
#' }
#' @export
getHoldings.SPDR <- function(Symbols, env=.GlobalEnv, auto.assign=TRUE) {
  hlist <- lapply(Symbols, function(symbol) {
    lnk <- paste0("https://www.spdrs.com/site-content/csv/", symbol, 
                  "_All_Holdings.csv?fund=", symbol, "&docname=All+Holdings&",
                  "onyx_code1=1286&onyx_code2=1521")
    lnk <- paste0("https://www.spdrs.com/site-content/csv/", symbol, 
                  "_All_Holdings.csv?fund=", symbol, "&docname=All+Holdings")
    tmp <- tempfile()
    download.file(lnk, destfile=tmp, method='curl')
    fr <- read.csv(tmp, skip=3)
    unlink(tmp)
    out <- fr[, c(2, 1, 4)]
    colnames(out)[1] <- paste(symbol, "Weight", sep=".")
    rownames(out) <- fr[, 3]
    out
  })
  names(hlist) <- Symbols
  if (isTRUE(auto.assign)) {
    lapply(Symbols, function(x) {
      assign(paste(x, "h", sep="."), hlist[[x]], pos=env)
    })
    return(paste(Symbols, "h", sep="."))
  }
  if (length(hlist) > 1) {
    return(hlist)
  } else return(hlist[[1L]])
}