Alt text label can be function

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* A function can be provided to `labs(alt = <...>)` that takes the plot as input
+  and returns text as output (@teunbrand, #4795).
 * Fixed spurious warning when `weight` aesthetic was used in `stat_smooth()` 
   (@teunbrand based on @clauswilke's suggestion, #5053).
 * The `lwd` alias now correctly replaced by `linewidth` instead of `size` 

R/labels.r

@@ -44,7 +44,9 @@ update_labels <- function(p, labels) {
 #' @param tag The text for the tag label which will be displayed at the
 #'        top-left of the plot by default.
 #' @param alt,alt_insight Text used for the generation of alt-text for the plot.
-#'        See [get_alt_text] for examples.
+#'        See [get_alt_text] for examples. `alt` can also be a function that
+#'        takes the plot as input and returns text as output. `alt` also accepts
+#'        rlang [lambda][rlang::as_function()] function notation.
 #' @param ... A list of new name-value pairs. The name should be an aesthetic.
 #' @export
 #' @examples
@@ -73,7 +75,8 @@ labs <- function(..., title = waiver(), subtitle = waiver(), caption = waiver(),
                  tag = waiver(), alt = waiver(), alt_insight = waiver()) {
   # .ignore_empty = "all" is needed to allow trailing commas, which is NOT a trailing comma for dots_list() as it's in ...
   args <- dots_list(..., title = title, subtitle = subtitle, caption = caption,
-    tag = tag, alt = alt, alt_insight = alt_insight, .ignore_empty = "all")
+    tag = tag, alt = allow_lambda(alt), alt_insight = alt_insight,
+    .ignore_empty = "all")
 
   is_waive <- vapply(args, is.waive, logical(1))
   args <- args[!is_waive]
@@ -137,11 +140,15 @@ get_alt_text <- function(p, ...) {
 }
 #' @export
 get_alt_text.ggplot <- function(p, ...) {
-  p$labels[["alt"]] %||% ""
+  alt <- p$labels[["alt"]] %||% ""
+  p$labels[["alt"]] <- NULL
+  if (is.function(alt)) alt(p) else alt
 }
 #' @export
 get_alt_text.ggplot_built <- function(p, ...) {
-  p$plot$labels[["alt"]] %||% ""
+  alt <- p$plot$labels[["alt"]] %||% ""
+  p$plot$labels[["alt"]] <- NULL
+  if (is.function(alt)) alt(p$plot) else alt
 }
 #' @export
 get_alt_text.gtable <- function(p, ...) {

tests/testthat/_snaps/labels.md

@@ -0,0 +1,7 @@
+# alt text can take a function
+
+    Code
+      get_alt_text(p)
+    Output
+      [1] "A plot showing class on the x-axis and count on the y-axis using a bar layer"
+

tests/testthat/test-labels.r

@@ -69,6 +69,12 @@ test_that("alt text is returned", {
   expect_equal(get_alt_text(p), "An alt text")
 })
 
+test_that("alt text can take a function", {
+  p <- ggplot(mpg, aes(class)) +
+    geom_bar() +
+    labs(alt = ~ generate_alt_text(.x))
+  expect_snapshot(get_alt_text(p))
+})
 
 # Visual tests ------------------------------------------------------------