Change ggplot_build and ggplot_gtable to generics (tidyverse#2370)

NAMESPACE

@@ -50,6 +50,8 @@ S3method(ggplot_add,labels)
 S3method(ggplot_add,list)
 S3method(ggplot_add,theme)
 S3method(ggplot_add,uneval)
+S3method(ggplot_build,ggplot)
+S3method(ggplot_gtable,ggplot_built)
 S3method(grid.draw,absoluteGrob)
 S3method(grid.draw,ggplot)
 S3method(grobHeight,absoluteGrob)

NEWS.md

@@ -129,6 +129,9 @@ up correct aspect ratio, and draws a graticule.
   `panel_params` (#1311). These are parameters of the Coord that vary from
   panel to panel.
 
+* `ggplot_build` and `ggplot_gtable` are now generics so ggplot-subclasses can
+  define additional behavior during the build stage.
+
 ## Minor bug fixes and improvements
 
 ### Facetting

R/plot-build.r

@@ -16,6 +16,11 @@
 #' @keywords internal
 #' @export
 ggplot_build <- function(plot) {
+  UseMethod('ggplot_build')
+}
+
+#' @export
+ggplot_build.ggplot <- function(plot) {
   plot <- plot_clone(plot)
   if (length(plot$layers) == 0) {
     plot <- plot + geom_blank()
@@ -141,8 +146,11 @@ layer_grob <- function(plot, i = 1L) {
 #' @param data plot data generated by [ggplot_build()]
 #' @export
 ggplot_gtable <- function(data) {
-  stopifnot(inherits(data, "ggplot_built"))
+  UseMethod('ggplot_gtable')
+}
 
+#' @export
+ggplot_gtable.ggplot_built <- function(data) {
   plot <- data$plot
   layout <- data$layout
   data <- data$data