Adding news item and tests. Closes tidyverse#2540.

Author: clauswilke

NEWS.md

@@ -14,6 +14,10 @@
 * `Geom` now gains a `setup_params()` method in line with the other ggproto
   classes (@thomasp85, #3509)
 
+* Themes can now modify the theme element tree, via the
+  `element_tree` argument. This allows extension packages to add functionality that
+  alters the element tree (@clauswilke, #2540).
+
 * `element_text()` now issues a warning when vectorized arguments are provided, as in
   `colour = c("red", "green", "blue")`. Such use is discouraged and not officially supported
    (@clauswilke, #3492).

tests/testthat/test-theme.r

@@ -196,6 +196,31 @@ test_that("theme(validate=FALSE) means do not validate_element", {
   expect_equal(red.before$theme$animint.width, 500)
 })
 
+test_that("theme validation happens at build stage", {
+  # adding a non-valid theme element to a theme is no problem
+  expect_silent(theme_gray() + theme(text = 0))
+
+  # the error occurs when we try to render the plot
+  p <- ggplot() + theme(text = 0)
+  expect_error(print(p), "must be an `element_text`")
+
+  # without validation, the error occurs when the element is accessed
+  p <- ggplot() + theme(text = 0, validate = FALSE)
+  expect_error(print(p), "text should have class element_text")
+})
+
+test_that("element tree can be modified", {
+  # we cannot add a new theme element without modifying the element tree
+  p <- ggplot() + theme(blablabla = element_text())
+  expect_error(print(p), "Theme element `blablabla` is not defined in the element hierarchy")
+
+  # things work once we add a new element to the element tree
+  q <- p + theme(
+    element_tree = list(blablabla = el_def("element_text", "text"))
+  )
+  expect_silent(print(q))
+})
+
 test_that("all elements in complete themes have inherit.blank=TRUE", {
   inherit_blanks <- function(theme) {
     all(vapply(theme, function(el) {