웹 서버의보고가 일관성이 있다고 가정하고 파일의 마지막 수정 날짜 기록을 유지해야하며 다운로드하기 전에 httr::HEAD()
으로 확인해야합니다 (즉, 저장할 때 수행 할 작업이 있습니다. 마지막으로 수정 한 값을 어딘가에, 아마도 URL이있는 데이터 프레임에 넣을 수 있습니다.
library(httr)
URL <- "http://www.pcr.uu.se/digitalAssets/124/124932_1ucdponesided2015.rdata"
#' Download a file only if it hasn't changed since \code{last_modified}
#'
#' @param URL url of file
#' @param fil path to write file
#' @param last_modified \code{POSIXct}. Ideally, the output from the first
#' successful run of \code{get_file()}
#' @param overwrite overwrite the file if it exists?
#' @param .verbose output a message if the file was unchanged?
get_file <- function(URL, fil, last_modified=NULL, overwrite=TRUE, .verbose=TRUE) {
if ((!file.exists(fil)) || is.null(last_modified)) {
res <- GET(URL, write_disk(fil, overwrite))
return(httr::parse_http_date(res$headers$`last-modified`))
} else if (inherits(last_modified, "POSIXct")) {
res <- HEAD(URL)
cur_last_mod <- httr::parse_http_date(res$headers$`last-modified`)
if (cur_last_mod != last_modified) {
res <- GET(URL, write_disk(fil, overwrite))
return(httr::parse_http_date(res$headers$`last-modified`))
}
if (.verbose) message(sprintf("'%s' unchanged since %s", URL, last_modified))
return(last_modified)
}
}
# first run == you don't know the last-modified date.
# you need to pair this with the URL in some data structure for later use.
last_mod <- get_file(URL, basename(URL))
class(last_mod)
## [1] "POSIXct" "POSIXt"
last_mod
## [1] "2015-11-16 17:34:06 GMT"
last_mod <- get_file(URL, basename(URL), last_mod)
#> 'http://www.pcr.uu.se/digitalAssets/124/124932_1ucdponesided2015.rdata' unchanged since 2015-11-16 17:34:06