Consider making `add_ggplot` as an S3 generic

[emitanaka]

Would it be possible to make add_ggplot as an S3 generic?
Currently we have to resort to overwriting "+.gg" if we want to adopt a different method based on the class of e1, e.g. GGally is a prime example of this. This allows developers to fix issues like #4304 .

The fix, as below, will be relatively simple, but the downside is that you do have to export another function.

#' @export
add_ggplot <- function(p, object, objectname) {
  UseMethod("add_ggplot")
}

#' @export
add_ggplot.ggplot <- function(p, object, objectname) {
  if (is.null(object)) return(p)
  
  p <- plot_clone(p)
  p <- ggplot_add(object, p, objectname)
  set_last_plot(p)
  p
}

[yutannihilation]

Just a quick note. We had some discussions on this topic, but couldn't reach a consensus, if I remember correctly.

• Make add_ggplot an S3 method #3815
• Add add_ggplot method #3818
• Missing generic + method #3986

I still believe this should be solved by utilizing double dispatch (probably with vctrs integration), but, considering I (or we) have failed to implement it for this 2 years, we might need to rethink.

[hadley]

I think @emitanaka's proposal is a reasonable fix. R7 should eventually get to the point where the double dispatch will just work but that's at least a few years away, and using vctrs would be a bit of a hack since ggplot2 objects aren't vectors. @schloerke's PR looks good to be, so I'll going to close this issue as a duplicate of #3986.