#' @include theme-defaults.r
#' @include theme-elements.r
NULL

#' Get, set, and modify the active theme
#'
#' The current/active theme (see [theme()]) is automatically applied to every
#' plot you draw. Use `theme_get` to get the current theme, and `theme_set` to
#' completely override it. `theme_update` and `theme_replace` are shorthands for
#' changing individual elements.
#'
#' @section Adding on to a theme:
#'
#'   `+` and `%+replace%` can be used to modify elements in themes.
#'
#'   `+` updates the elements of e1 that differ from elements specified (not
#'   NULL) in e2. Thus this operator can be used to incrementally add or modify
#'   attributes of a ggplot theme.
#'
#'   In contrast, `%+replace%` replaces the entire element; any element of a
#'   theme not specified in e2 will not be present in the resulting theme (i.e.
#'   NULL). Thus this operator can be used to overwrite an entire theme.
#'
#'   `theme_update` uses the `+` operator, so that any unspecified values in the
#'   theme element will default to the values they are set in the theme.
#'   `theme_replace` uses `%+replace%` to completely replace the element, so any
#'   unspecified values will overwrite the current value in the theme with
#'   `NULL`.
#'
#'   In summary, the main differences between `theme_set()`, `theme_update()`,
#'   and `theme_replace()` are:
#'   * `theme_set()` completely overrides the current theme.
#'   * `theme_update()` modifies a particular element of the current theme
#'   using the `+` operator.
#'   * `theme_replace()` modifies a particular element of the current theme
#'   using the `%+replace%` operator.
#'
#' @param ... named list of theme settings
#' @param e1,e2 Theme and element to combine
#' @return `theme_set`, `theme_update`, and `theme_replace`
#'   invisibly return the previous theme so you can easily save it, then
#'   later restore it.
#' @seealso [+.gg()]
#' @export
#' @examples
#' p <- ggplot(mtcars, aes(mpg, wt)) +
#'   geom_point()
#' p
#'
#' # Use theme_set() to completely override the current theme.
#' # theme_update() and theme_replace() are similar except they
#' # apply directly to the current/active theme.
#' # theme_update() modifies a particular element of the current theme.
#' # Here we have the old theme so we can later restore it.
#' # Note that the theme is applied when the plot is drawn, not
#' # when it is created.
#' old <- theme_set(theme_bw())
#' p
#'
#' theme_set(old)
#' theme_update(panel.grid.minor = element_line(colour = "red"))
#' p
#'
#' theme_set(old)
#' theme_replace(panel.grid.minor = element_line(colour = "red"))
#' p
#'
#' theme_set(old)
#' p
#'
#'
#' # Modifying theme objects -----------------------------------------
#' # You can use + and %+replace% to modify a theme object.
#' # They differ in how they deal with missing arguments in
#' # the theme elements.
#'
#' add_el <- theme_grey() +
#'   theme(text = element_text(family = "Times"))
#' add_el$text
#'
#' rep_el <- theme_grey() %+replace%
#'   theme(text = element_text(family = "Times"))
#' rep_el$text
#'
theme_get <- function() {
  ggplot_global$theme_current
}

#' @rdname theme_get
#' @param new new theme (a list of theme elements)
#' @export
theme_set <- function(new) {
  old <- ggplot_global$theme_current
  ggplot_global$theme_current <- new
  invisible(old)
}

#' @rdname theme_get
#' @export
theme_update <- function(...) {
  theme_set(theme_get() + theme(...))
}

#' @rdname theme_get
#' @export
theme_replace <- function(...) {
  theme_set(theme_get() %+replace% theme(...))
}

#' @rdname theme_get
#' @export
"%+replace%" <- function(e1, e2) {
  if (!is.theme(e1) || !is.theme(e2)) {
    abort("%+replace% requires two theme objects")
  }

  # Can't use modifyList here since it works recursively and drops NULLs
  e1[names(e2)] <- e2

  # comment by @clauswilke:
  # `complete` and `validate` are currently ignored,
  # which means they are taken from e1. Is this correct?
  # I'm not sure how `%+replace%` should handle them.

  e1
}

