Facet: gain draw_panel_content method to draw grobs for each panel (#6421)

* Facet: gain draw_facet_panels method to draw grobs for each panel

* renmae `draw_facet_panels` to `draw_panel_content`

* rename `Facet$draw_facet_panels` to `Facet$draw_panel_content`

* Update R/facet-.R

Co-authored-by: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>

* add news bullet

---------

Co-authored-by: Teun van den Brand <49372158+teunbrand@users.noreply.github.com>
Co-authored-by: Teun van den Brand <tahvdbrand@gmail.com>

Author: Yunuuuu

NEWS.md

@@ -1,5 +1,7 @@
 # ggplot2 (development version)
 
+* (internal) New `Facet$draw_panel_content()` method for delegating panel 
+  assembly (@Yunuuuu, #6406).
 * Facet gains a new method `setup_panel_params` to interact with the
   panel_params setted by Coord object (@Yunuuuu, #6397, #6380)
 * `position_fill()` avoids stacking observations of zero (@teunbrand, #6338)

R/facet-.R

@@ -68,6 +68,10 @@ NULL
 #'   between the layer stack and the foreground defined by the Coord object
 #'   (usually empty). The default is, as above, to return an empty grob.
 #'
+#'   - `draw_panel_content`: Draws each panel for the facet. Should return a list
+#'     of grobs, one for each panel. The output is used by the `draw_panels`
+#'     method.
+#'
 #'   - `draw_labels`: Given the gtable returned by `draw_panels`,
 #'   add axis titles to the gtable. The default is to add one title at each side
 #'   depending on the position and existence of axes.
@@ -138,6 +142,34 @@ Facet <- ggproto("Facet", NULL,
   draw_front = function(data, layout, x_scales, y_scales, theme, params) {
     rep(list(zeroGrob()), vec_unique_count(layout$PANEL))
   },
+  draw_panel_content = function(self, panels, layout, x_scales, y_scales,
+                                ranges, coord, data, theme, params, ...) {
+    facet_bg <- self$draw_back(
+      data,
+      layout,
+      x_scales,
+      y_scales,
+      theme,
+      params
+    )
+    facet_fg <- self$draw_front(
+      data,
+      layout,
+      x_scales,
+      y_scales,
+      theme,
+      params
+    )
+
+    # Draw individual panels, then call `$draw_panels()` method to 
+    # assemble into gtable
+    lapply(seq_along(panels[[1]]), function(i) {
+      panel <- lapply(panels, `[[`, i)
+      panel <- c(facet_bg[i], panel, facet_fg[i])
+      panel <- coord$draw_panel(panel, ranges[[i]], theme)
+      ggname(paste("panel", i, sep = "-"), panel)
+    })
+  },
   draw_panels = function(self, panels, layout, x_scales = NULL, y_scales = NULL,
                          ranges, coord, data = NULL, theme, params) {
 

R/layout.R

@@ -60,29 +60,17 @@ Layout <- ggproto("Layout", NULL,
   # Assemble the facet fg & bg, the coord fg & bg, and the layers
   # Returns a gtable
   render = function(self, panels, data, theme, labels) {
-    facet_bg <- self$facet$draw_back(data,
+    panels <- self$facet$draw_panel_content(
+      panels,
       self$layout,
       self$panel_scales_x,
       self$panel_scales_y,
-      theme,
-      self$facet_params
-    )
-    facet_fg <- self$facet$draw_front(
+      self$panel_params,
+      self$coord,
       data,
-      self$layout,
-      self$panel_scales_x,
-      self$panel_scales_y,
       theme,
       self$facet_params
     )
-
-    # Draw individual panels, then assemble into gtable
-    panels <- lapply(seq_along(panels[[1]]), function(i) {
-      panel <- lapply(panels, `[[`, i)
-      panel <- c(facet_bg[i], panel, facet_fg[i])
-      panel <- self$coord$draw_panel(panel, self$panel_params[[i]], theme)
-      ggname(paste("panel", i, sep = "-"), panel)
-    })
     plot_table <- self$facet$draw_panels(
       panels,
       self$layout,