Element subclass inheritance

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 (development version)
 
+* In the theme element hierarchy, parent elements that are a strict subclass
+  of child elements now confer their subclass upon the children (#5457).
+
 * `ggsave()` no longer sometimes creates new directories, which is now 
   controlled by the new `create.dir` argument (#5489).
 

R/theme.R

@@ -783,9 +783,21 @@ combine_elements <- function(e1, e2) {
     e1$linewidth <- e2$linewidth * unclass(e1$linewidth)
   }
 
+  # If e2 is 'richer' than e1, fill e2 with e1 parameters
+  if (is.subclass(e2, e1)) {
+    new <- defaults(e1, e2)
+    e2[names(new)] <- new
+    return(e2)
+  }
+
   e1
 }
 
+is.subclass <- function(x, y) {
+  inheritance <- inherits(x, class(y), which = TRUE)
+  !any(inheritance == 0) && length(setdiff(class(x), class(y))) > 0
+}
+
 #' Reports whether x is a theme object
 #' @param x An object to test
 #' @export

tests/testthat/test-theme.R

@@ -509,6 +509,50 @@ test_that("Theme validation behaves as expected", {
   expect_snapshot_error(validate_element("A", "aspect.ratio", tree))
 })
 
+test_that("Element subclasses are inherited", {
+
+  # `rich` is subclass of `poor`
+  poor <- element_line(colour = "red", linetype = 3)
+  rich <- element_line(linetype = 2, linewidth = 2)
+  class(rich) <- c("element_rich", class(rich))
+
+  # `poor` should acquire `rich`
+  test <- combine_elements(poor, rich)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 3, linewidth = 2)
+  )
+
+  # `rich` should stay `rich`
+  test <- combine_elements(rich, poor)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 2, linewidth = 2)
+  )
+
+  # `sibling` is not strict subclass of `rich`
+  sibling <- poor
+  class(sibling) <- c("element_sibling", class(sibling))
+
+  # `sibling` should stay `sibling`
+  test <- combine_elements(sibling, rich)
+  expect_s3_class(test, "element_sibling")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 3, linewidth = 2)
+  )
+
+  # `rich` should stay `rich`
+  test <- combine_elements(rich, sibling)
+  expect_s3_class(test, "element_rich")
+  expect_equal(
+    test[c("colour", "linetype", "linewidth")],
+    list(colour = "red", linetype = 2, linewidth = 2)
+  )
+})
+
 test_that("Minor tick length supports biparental inheritance", {
   my_theme <- theme_gray() + theme(
     axis.ticks.length = unit(1, "cm"),