From 3c5d80cf7955eea92d90987c65161edecc401e4c Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Sat, 3 Dec 2022 13:41:37 +0100
Subject: [PATCH 1/6] Allow functions as `labs(alt = ...)` input

---
 R/labels.r  | 7 +++++--
 man/labs.Rd | 4 +++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/R/labels.r b/R/labels.r
index 669db6b5fa..1c716f74ba 100644
--- a/R/labels.r
+++ b/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]
diff --git a/man/labs.Rd b/man/labs.Rd
index d335f33f15..ff63ad7bd6 100644
--- a/man/labs.Rd
+++ b/man/labs.Rd
@@ -38,7 +38,9 @@ bottom-right of the plot by default.}
 top-left of the plot by default.}
 
 \item{alt, alt_insight}{Text used for the generation of alt-text for the plot.
-See \link{get_alt_text} for examples.}
+See \link{get_alt_text} for examples. \code{alt} can also be a function that
+takes the plot as input and returns text as output. \code{alt} also accepts
+rlang \link[rlang:as_function]{lambda} function notation.}
 
 \item{label}{The title of the respective axis (for \code{xlab()} or \code{ylab()}) or
 of the plot (for \code{ggtitle()}).}

From 3c029534c90dbf342c809e8b224cce394d9b275c Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Sat, 3 Dec 2022 13:42:09 +0100
Subject: [PATCH 2/6] Alt functions are called with plot as input

---
 R/labels.r | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/R/labels.r b/R/labels.r
index 1c716f74ba..35557dbb5a 100644
--- a/R/labels.r
+++ b/R/labels.r
@@ -140,11 +140,13 @@ get_alt_text <- function(p, ...) {
 }
 #' @export
 get_alt_text.ggplot <- function(p, ...) {
-  p$labels[["alt"]] %||% ""
+  alt <- p$labels[["alt"]] %||% ""
+  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"]] %||% ""
+  if (is.function(alt)) alt(p$plot) else alt
 }
 #' @export
 get_alt_text.gtable <- function(p, ...) {

From eb02c0eb0eefeb2f1386725f3d40df51ae900df5 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Sat, 3 Dec 2022 13:47:52 +0100
Subject: [PATCH 3/6] Add test for alt text functions

---
 tests/testthat/_snaps/labels.md | 7 +++++++
 tests/testthat/test-labels.r    | 6 ++++++
 2 files changed, 13 insertions(+)
 create mode 100644 tests/testthat/_snaps/labels.md

diff --git a/tests/testthat/_snaps/labels.md b/tests/testthat/_snaps/labels.md
new file mode 100644
index 0000000000..3d0d834223
--- /dev/null
+++ b/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"
+
diff --git a/tests/testthat/test-labels.r b/tests/testthat/test-labels.r
index 0e866fa2a4..dd2222fc69 100644
--- a/tests/testthat/test-labels.r
+++ b/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 ------------------------------------------------------------
 

From c1d38ea45c2d43bfe14994feb93918a3d3edcf11 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Sat, 3 Dec 2022 13:48:13 +0100
Subject: [PATCH 4/6] Add NEWS bullet

---
 NEWS.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/NEWS.md b/NEWS.md
index 1b4dc36ba0..2d5100de2a 100644
--- a/NEWS.md
+++ b/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` 

From 5c5dac0e79b10f913510a0257fa246724d0fbad3 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Date: Sat, 3 Dec 2022 14:11:47 +0100
Subject: [PATCH 5/6] Avoid recursion

---
 R/labels.r | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/R/labels.r b/R/labels.r
index 35557dbb5a..30d40e61c1 100644
--- a/R/labels.r
+++ b/R/labels.r
@@ -141,11 +141,13 @@ get_alt_text <- function(p, ...) {
 #' @export
 get_alt_text.ggplot <- function(p, ...) {
   alt <- p$labels[["alt"]] %||% ""
+  p$labels[["alt"]] <- NULL
   if (is.function(alt)) alt(p) else alt
 }
 #' @export
 get_alt_text.ggplot_built <- function(p, ...) {
   alt <- p$plot$labels[["alt"]] %||% ""
+  p$plot$labels[["alt"]] <- NULL
   if (is.function(alt)) alt(p$plot) else alt
 }
 #' @export

From 4910cc0e8f5e9946fde6b542341d1700f3595726 Mon Sep 17 00:00:00 2001
From: Teun van den Brand <tahvdbrand@gmail.com>
Date: Mon, 20 May 2024 10:13:54 +0200
Subject: [PATCH 6/6] deal with changes in {glue}

---
 R/labels.r | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/R/labels.r b/R/labels.r
index 30d40e61c1..4f5ade202b 100644
--- a/R/labels.r
+++ b/R/labels.r
@@ -201,11 +201,16 @@ get_alt_text.gtable <- function(p, ...) {
 #'
 generate_alt_text <- function(p) {
   # Combine titles
-  title <- glue(glue_collapse(
-    sub("\\.?$", "", c(p$labels$title, p$labels$subtitle)),
-    last = ": "
-  ), ". ")
-  title <- safe_string(title)
+  if (!is.null(p$label$title %||% p$labels$subtitle)) {
+    title <- glue(glue_collapse(
+      sub("\\.?$", "", c(p$labels$title, p$labels$subtitle)),
+      last = ": "
+    ), ". ")
+    title <- safe_string(title)
+  } else {
+    title <- ""
+  }
+
 
   # Get axes descriptions
   axes <- glue(" showing ", glue_collapse(
@@ -222,7 +227,7 @@ generate_alt_text <- function(p) {
     if (length(layers) == 1) "a " else "",
     glue_collapse(layers, sep = ", ", last = " and "),
     " layer",
-    if (length(layers) == 1) "" else "s",
+    if (length(layers) == 1) "" else "s"
   )
   layers <- safe_string(layers)