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Supported callback functions:


This callback is called before an HTTP request is performed, with the request object as an argument. If the callback returns a value other than NULL, the HTTP request is not performed at all, and the return value of the callback is returned. This mechanism can be used to replay previously recorded HTTP responses.


This callback is called after an HTTP request is performed. The callback is called with two arguments: the request object and the response object of the HTTP request. If this callback returns a value other than NULL, then this value is returned by httr.



set_callback(name, new_callback = NULL)



Character scalar, name of the callback to query or set.


The callback function to install, a function object; or NULL to remove the currently installed callback (if any).


get_callback returns the currently installed callback, or NULL if none is installed.

set_callback returns the previously installed callback, or NULL if none was installed.


Note that it is not possible to install multiple callbacks of the same type. The installed callback overwrites the previously intalled one. To uninstall a callback function, set it to NULL with set_callback().

See the httrmock package for a proper example that uses callbacks.


if (FALSE) {
## Log all HTTP requests to the screeen
req_logger <- function(req) {
  cat("HTTP request to", sQuote(req$url), "\n")

old <- set_callback("request", req_logger)
g1 <- GET("")
g2 <- GET("")
set_callback("request", old)

## Log all HTTP requests and response status codes as well
req_logger2 <- function(req) {
  cat("HTTP request to", sQuote(req$url), "... ")
res_logger <- function(req, res) {
  cat(res$status_code, "\n")

old_req <- set_callback("request", req_logger2)
old_res <- set_callback("response", res_logger)
g3 <- GET("")
g4 <- GET("")
set_callback("request", old_req)
set_callback("response", old_res)

## Return a recorded response, without performing the HTTP request
replay <- function(req) {
  if (req$url == "") g3
old_req <- set_callback("request", replay)
grec <- GET("")
grec$date == g3$date
set_callback("request", old_req)