Allow addition of custom objects

NAMESPACE

@@ -6,6 +6,7 @@ S3method("+",gg)
 S3method("[",uneval)
 S3method("[[",ggproto)
 S3method(.DollarNames,ggproto)
+S3method(add_to_ggplot,default)
 S3method(as.character,uneval)
 S3method(as.list,ggproto)
 S3method(autolayer,default)
@@ -201,6 +202,7 @@ export(StatSummaryBin)
 export(StatSummaryHex)
 export(StatUnique)
 export(StatYdensity)
+export(add_to_ggplot)
 export(aes)
 export(aes_)
 export(aes_all)

NEWS.md

@@ -1,5 +1,8 @@
 # ggplot2 2.2.1.9000
 
+* Custom objects can now be added using `+` if a `add_to_ggplot` method has been
+  defined for the class of the object (@thomasp85).
+
 * Fixed bug when setting strips to `element_blank()` (@thomasp85). 
 
 * Strips gain margins on all sides by default. This means that to fully justify

R/plot-construction.r

@@ -84,7 +84,7 @@ add_ggplot <- function(p, object, objectname) {
   } else if (is.facet(object)) {
       p$facet <- object
       p
-  } else if (is.list(object)) {
+  } else if (is.list(object) && is.null(attr(object, 'class'))) {
     for (o in object) {
       p <- p %+% o
     }
@@ -97,9 +97,28 @@ add_ggplot <- function(p, object, objectname) {
     new_labels <- defaults(mapping, default)
     p$labels <- defaults(p$labels, new_labels)
   } else {
-    stop("Don't know how to add ", objectname, " to a plot",
-      call. = FALSE)
+    p <- add_to_ggplot(object, p, objectname)
   }
   set_last_plot(p)
   p
 }
+#' Add custom objects to ggplot
+#'
+#' This generic allows you to add your own methods for adding custom objects to
+#' a ggplot with [+.gg].
+#'
+#' @param object An object to add to the plot
+#' @param p The ggplot object to add `object` to
+#' @param objectname The name of the object to add
+#'
+#' @return A modified ggplot object
+#'
+#' @keywords internal
+#' @export
+add_to_ggplot <- function(object, p, objectname) {
+  UseMethod("add_to_ggplot")
+}
+#' @export
+add_to_ggplot.default <- function(object, p, objectname) {
+  stop("Don't know how to add ", objectname, " to a plot", call. = FALSE)
+}

tests/testthat/test-plot-construction.R

@@ -0,0 +1,16 @@
+context("plot-construction")
+
+test_that("Custom object cannot be added without generic", {
+  p <- ggplot()
+  custom_object <- structure(list(), class = 'test_object')
+  expect_error(p + custom_object, "Don't know how to add custom_object to a plot")
+})
+
+test_that("Methods can be defined for adding custom objects", {
+  p <- ggplot()
+  custom_object <- structure(list(), class = 'test_object')
+  add_to_ggplot.test_object <- function(object, p, objectname) {
+    10
+  }
+  expect_equal(p + custom_object, 10)
+})