Simplify zeroGrob() (#6391)

* cleanup branch

* For backward compatibility, `zeroGrob()` returns a `nullGrob()` subclass

* adjust tests

* add news bullet

* migrate to grob utilities

* avoid including fragile grobName in snapshot

* use `add_class()` in other places too

* fix misresolution of merge conflict

Author: teunbrand

DESCRIPTION

@@ -177,7 +177,6 @@ Collate:
     'ggplot2-package.R'
     'grob-absolute.R'
     'grob-dotstack.R'
-    'grob-null.R'
     'grouping.R'
     'properties.R'
     'margins.R'

NAMESPACE

@@ -20,7 +20,6 @@ S3method(as.list,ggproto)
 S3method(autolayer,default)
 S3method(autoplot,default)
 S3method(c,mapped_discrete)
-S3method(drawDetails,zeroGrob)
 S3method(format,ggproto)
 S3method(format,ggproto_method)
 S3method(format,rd_section_aesthetics)
@@ -52,9 +51,7 @@ S3method(ggplot,default)
 S3method(ggplot_add,default)
 S3method(grid.draw,absoluteGrob)
 S3method(grobHeight,absoluteGrob)
-S3method(grobHeight,zeroGrob)
 S3method(grobWidth,absoluteGrob)
-S3method(grobWidth,zeroGrob)
 S3method(grobX,absoluteGrob)
 S3method(grobY,absoluteGrob)
 S3method(guide_gengrob,default)
@@ -63,7 +60,6 @@ S3method(guide_merge,default)
 S3method(guide_train,default)
 S3method(guide_transform,default)
 S3method(heightDetails,titleGrob)
-S3method(heightDetails,zeroGrob)
 S3method(limits,Date)
 S3method(limits,POSIXct)
 S3method(limits,POSIXlt)
@@ -117,7 +113,6 @@ S3method(vec_ptype2,mapped_discrete.factor)
 S3method(vec_ptype2,mapped_discrete.integer)
 S3method(vec_ptype2,mapped_discrete.mapped_discrete)
 S3method(widthDetails,titleGrob)
-S3method(widthDetails,zeroGrob)
 export("%+%")
 export("%+replace%")
 export(.data)

NEWS.md

@@ -1,5 +1,6 @@
 # ggplot2 (development version)
 
+* (internal) `zeroGrob()` now returns a `grid::nullGrob()` (#6390).
 * `stat_ydensity()` now only requires the `x` or `y` aesthetic. The other will
   be populated with 0, similar to `stat_boxplot()` (@teunbrand, #6600)
 * Implemented `as.list()` and `S7::convert()` methods for lists and classes in ggplot2 

R/aes-variants.R

@@ -113,8 +113,7 @@ aes_all <- function(vars) {
   # Quosure the symbols in the empty environment because they can only
   # refer to the data mask
   x <- class_mapping(lapply(vars, function(x) new_quosure(as.name(x), emptyenv())))
-  class(x) <- union("unlabelled", class(x))
-  x
+  add_class(x, "unlabelled")
 }
 
 #' Automatic aesthetic mapping

R/grob-null.R

@@ -501,7 +501,7 @@ theme <- function(...,
   }
 
   t <- class_theme(elements, complete = complete, validate = validate)
-  class(t) <- union("theme", class(t))
+  t <- add_class(t, "theme")
   t
 }
 

R/theme.R

@@ -53,6 +53,17 @@ gg_par <- function(..., stroke = NULL, pointsize = NULL) {
   inject(gpar(!!!args))
 }
 
+#' The zero grob draws nothing and has zero size.
+#'
+#' @keywords internal
+#' @export
+zeroGrob <- function() .zeroGrob
+
+.zeroGrob <- NULL
+on_load(.zeroGrob <- add_class(nullGrob(), "zeroGrob"))
+
+is_zero <- function(x) is.null(x) || inherits(x, "null")
+
 width_cm <- function(x) {
   if (is.grob(x)) {
     convertWidth(grobWidth(x), "cm", TRUE)

R/utilities-grid.R

@@ -942,6 +942,15 @@ compute_data_size <- function(data, size, default = 0.9,
   data
 }
 
+add_class <- function(x, new_class) {
+  new_class <- setdiff(new_class, class(x))
+  if (length(new_class) < 1) {
+    return(x)
+  }
+  class(x) <- union(new_class, class(x))
+  x
+}
+
 try_prop <- function(object, name, default = NULL) {
   if (!S7::S7_inherits(object)) {
     return(default)

R/utilities.R

@@ -64,10 +64,8 @@
 ---
 
     Code
-      element_grob(el, label = element_blank())
+      x <- element_grob(el, label = element_blank())
     Condition
       Warning:
       `label` cannot be a <ggplot2::element_blank> object.
-    Output
-      zeroGrob[NULL]