get_layer_data(), get_layer_grob() can use layer names (#6724)

* get_layer_data(), get_layer_grob() accept layer names

* Add layer name tests for get_layer_data(), get_layer_grob()

* Man

* Fix tests

* Fix tests

* Revert formatting changes

* get_layer_data(), get_layer_grob() now accept integers or characters

* remove error snapshots

* ensure differences between layers, so test fails when appropriate

* add news bullet

---------

Co-authored-by: Teun van den Brand <tahvdbrand@gmail.com>

Author: lgaborini

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* `get_layer_data()` and `get_layer_grob()` now accept layer names as index 
+  (@lgaborini, #6724)
+
 # ggplot2 4.0.1
 
 This is a smaller patch release focussed on fixing regressions from 4.0.0 and 

R/plot-build.R

@@ -11,9 +11,9 @@
 #' layer. These are useful for tests.
 #'
 #' @param plot ggplot object
-#' @param i An integer. In `get_layer_data()`, the data to return (in the order added to the
+#' @param i An integer or a name of a layer. In `get_layer_data()`, the data to return (in the order added to the
 #'   plot). In `get_layer_grob()`, the grob to return (in the order added to the
-#'   plot). In `get_panel_scales()`, the row of a facet to return scales for.
+#'   plot). In `get_panel_scales()` (only integers allowed), the row of a facet to return scales for.
 #' @param j An integer. In `get_panel_scales()`, the column of a facet to return
 #'   scales for.
 #' @param ... Not currently in use.
@@ -142,8 +142,15 @@ build_ggplot <- S7::method(ggplot_build, class_ggplot) <- function(plot, ...) {
 #' @export
 #' @rdname ggplot_build
 get_layer_data <- function(plot = get_last_plot(), i = 1L) {
-  ggplot_build(plot)@data[[i]]
+  b <- ggplot_build(plot)
+  idx <- vec_as_location2(
+    i = i,
+    n = vec_size(b@plot@layers),
+    names = names(b@plot@layers)
+  )
+  b@data[[idx]]
 }
+
 #' @export
 #' @rdname ggplot_build
 layer_data <- get_layer_data
@@ -171,7 +178,12 @@ layer_scales <- get_panel_scales
 get_layer_grob <- function(plot = get_last_plot(), i = 1L) {
   b <- ggplot_build(plot)
 
-  b@plot@layers[[i]]$draw_geom(b@data[[i]], b@layout)
+  idx <- vec_as_location2(
+    i = i,
+    n = vec_size(b@plot@layers),
+    names = names(b@plot@layers)
+  )
+  b@plot@layers[[idx]]$draw_geom(b@data[[idx]], b@layout)
 }
 
 #' @export

man/ggplot_build.Rd

@@ -226,6 +226,30 @@ test_that("layer_data returns a data.frame", {
   expect_snapshot_error(l$layer_data(mtcars))
 })
 
+test_that("get_layer_data works with layer names", {
+  p <- ggplot() +
+    annotate("point", x = 1, y = 1, name = "foo") +
+    annotate("line", x = 1:2, y = 1:2, name = "bar")
+
+  # name has higher precedence than index
+  expect_identical(
+    get_layer_data(p, i = "bar"),
+    get_layer_data(p, i = 2L)
+  )
+})
+
+test_that("get_layer_grob works with layer names", {
+  p <- ggplot() +
+    annotate("point", x = 1, y = 1, name = "foo") +
+    annotate("line", x = 1:2, y = 1:2, name = "bar")
+
+  # name has higher precedence than index
+  named <- get_layer_grob(p, i = "bar")
+  nummed <- get_layer_grob(p, i = 2L)
+  named[[1]]$name <- nummed[[1]]$name <- NULL # ignore grid's unique names
+  expect_identical(named, nummed)
+})
+
 test_that("data.frames and matrix aesthetics survive the build stage", {
   df <- data_frame0(
     x = 1:2,