handler_callback: Progression Handler: Progress Reported via a User-Defined...

In progressr: An Inclusive, Unifying API for Progress Updates

Progression Handler: Progress Reported via a User-Defined Hook Function

Description

A progression handler that calls a custom hook function every time this handler receives a progression update, e.g. to forward progress information to logging, GUIs, or other external systems not supported by progressr.

Usage

handler_callback(
  fcn = NULL,
  intrusiveness = getOption("progressr.intrusiveness.callback", 1),
  target = "callback",
  ...
)

Arguments

fcn	(function) A hook function that is called whenever this handler reports on an "initiate", "update", or "finish" event. It is called as fcn(type, step, max_steps, message, amount), where type (character string) is one of "initiate", "update", or "finish", step (integer) is the number of steps completed so far, max_steps (integer) is the total number of steps, message (character string) is the latest progress message, and amount (numeric) is the increment in step since the previous call. The function should accept ... to remain forward compatible with future arguments. Any value returned by fcn is ignored. If NULL (default), no hook function is called, i.e. this handler produces no output.
intrusiveness	(numeric) A non-negative scalar on how intrusive (disruptive) the reporter to the user.
target	(character vector) Specifies where progression updates are rendered.
...	Additional arguments passed to make_progression_handler().

Value

A function of class progression_handler that takes a progression condition as its first and only argument.

Examples

handlers(handler_callback(fcn = function(type, step, max_steps, message, amount, ...) {
  message(sprintf("[%s] step %d/%d (%+g): %s", type, step, max_steps, amount, message))
}))
with_progress({ y <- slow_sum_p(1:10) })
print(y)

progressr documentation built on Oct. 18, 2026, 5:08 p.m.