R/register-s3.R

In pillar: Coloured Formatting for Columns

# This source code file is licensed under the unlicense license
# https://unlicense.org

# nocov start

#' Register a method for a suggested dependency
#'
#' Generally, the recommended way to register an S3 method is to use the
#' `S3Method()` namespace directive (often generated automatically by the
#' `@export` roxygen2 tag). However, this technique requires that the generic
#' be in an imported package, and sometimes you want to suggest a package,
#' and only provide a method when that package is loaded. `s3_register()`
#' can be called from your package's `.onLoad()` to dynamically register
#' a method only if the generic's package is loaded.
#'
#' For R 3.5.0 and later, `s3_register()` is also useful when demonstrating
#' class creation in a vignette, since method lookup no longer always involves
#' the lexical scope. For R 3.6.0 and later, you can achieve a similar effect
#' by using "delayed method registration", i.e. placing the following in your
#' `NAMESPACE` file:
#'
#' ```
#' if (getRversion() >= "3.6.0") {
#'   S3method(package::generic, class)
#' }
#' ```
#'
#' @section Usage in other packages:
#' To avoid taking a dependency on vctrs, you copy the source of
#' [`s3_register()`](https://github.com/r-lib/rlang/blob/master/R/compat-register.R)
#' into your own package. It is licensed under the permissive
#' [unlicense](https://choosealicense.com/licenses/unlicense/) to make it
#' crystal clear that we're happy for you to do this. There's no need to include
#' the license or even credit us when using this function.
#'
#' @param generic Name of the generic in the form `"pkg::generic"`.
#' @param class Name of the class
#' @param method Optionally, the implementation of the method. By default,
#'   this will be found by looking for a function called `generic.class`
#'   in the package environment.
#' @examples
#' # A typical use case is to dynamically register tibble/pillar methods
#' # for your class. That way you avoid creating a hard dependency on packages
#' # that are not essential, while still providing finer control over
#' # printing when they are used.
#'
#' .onLoad <- function(...) {
#'   s3_register("pillar::pillar_shaft", "vctrs_vctr")
#'   s3_register("tibble::type_sum", "vctrs_vctr")
#' }
#' @keywords internal
#' @noRd
s3_register <- function(generic, class, method = NULL) {
  stopifnot(is.character(generic), length(generic) == 1)
  stopifnot(is.character(class), length(class) == 1)

  pieces <- strsplit(generic, "::")[[1]]
  stopifnot(length(pieces) == 2)
  package <- pieces[[1]]
  generic <- pieces[[2]]

  caller <- parent.frame()

  get_method_env <- function() {
    top <- topenv(caller)
    if (isNamespace(top)) {
      asNamespace(environmentName(top))
    } else {
      caller
    }
  }
  get_method <- function(method, env) {
    if (is.null(method)) {
      get(paste0(generic, ".", class), envir = get_method_env())
    } else {
      method
    }
  }

  register <- function(...) {
    envir <- asNamespace(package)

    # Refresh the method each time, it might have been updated by
    # `devtools::load_all()`
    method_fn <- get_method(method)
    stopifnot(is.function(method_fn))


    # Only register if generic can be accessed
    if (exists(generic, envir)) {
      registerS3method(generic, class, method_fn, envir = envir)
    } else if (identical(Sys.getenv("NOT_CRAN"), "true")) {
      warn <- .rlang_s3_register_compat("warn")

      warn(c(
        sprintf(
          "Can't find generic `%s` in package %s to register S3 method.",
          generic,
          package
        ),
        "i" = "This message is only shown to developers using devtools.",
        "i" = sprintf("Do you need to update %s to the latest version?", package)
      ))
    }
  }

  # Always register hook in case package is later unloaded & reloaded
  setHook(packageEvent(package, "onLoad"), function(...) {
    register()
  })

  # Avoid registration failures during loading (pkgload or regular).
  # Check that environment is locked because the registering package
  # might be a dependency of the package that exports the generic. In
  # that case, the exports (and the generic) might not be populated
  # yet (#1225).
  if (isNamespaceLoaded(package) && environmentIsLocked(asNamespace(package))) {
    register()
  }

  invisible()
}

.rlang_s3_register_compat <- function(fn, try_rlang = TRUE) {
  # Compats that behave the same independently of rlang's presence
  out <- switch(fn,
    is_installed = return(function(pkg) requireNamespace(pkg, quietly = TRUE))
  )

  if (try_rlang && requireNamespace("rlang", quietly = TRUE)) {
    # Don't use `::` because this is also called from rlang's onLoad
    # hook and exports are not initialised at this point
    ns <- asNamespace("rlang")

    switch(fn,
      is_interactive = return(get("is_interactive", envir = ns))
    )

    # Make sure rlang knows about "x" and "i" bullets
    if (utils::packageVersion("rlang") >= "0.4.2") {
      switch(fn,
        abort = return(get("abort", envir = ns)),
        warn = return(get("warn", envir = ns)),
        inform = return(get("inform", envir = ns))
      )
    }
  }

  # Fall back to base compats

  is_interactive_compat <- function() {
    opt <- getOption("rlang_interactive")
    if (!is.null(opt)) {
      opt
    } else {
      interactive()
    }
  }

  format_msg <- function(x) paste(x, collapse = "\n")
  switch(fn,
    is_interactive = return(is_interactive_compat),
    abort = return(function(msg) stop(format_msg(msg), call. = FALSE)),
    warn = return(function(msg) warning(format_msg(msg), call. = FALSE)),
    inform = return(function(msg) message(format_msg(msg)))
  )

  stop(sprintf("Internal error in rlang shims: Unknown function `%s()`.", fn))
}

# nocov end