Deprecation for ggproto classes

[teunbrand]

This now has come up in #3798 and #5961 and the gist of it is that we have a procedure for managing lifecycles of functions, but we don't have an equivalent for ggproto classes.

We don't use GeomCol and maybe soon GeomErrorbarh anymore in ggplot2, but they are still in use by extensions. We can see for example that GeomCol is used in {ichimoku}, {entropart}, {ggiraph}, {ggside}. or that GeomErrorbarh is used in {errors} and {ggiraph}. Having a deprecation mechanism in place allows us to discourage the use of these classes while not breaking other packages.

Generally speaking, I think there are 3 ways in which ggproto classes are used in other packages::

1. They may be subclassed in an extension class.
2. They may be provided as arguments in e.g. layer(geom = GeomCol, ...).
3. They're used for accessing fields, .e.g. GeomCol$default_aes.

The question of this issue is whether we can find a satisfactory way to deprecate these old classes. I think it would be relatively straightforward to throw deprecation messages in cases (1) and (2) via either ggproto() or check_subclass(). I'm not sure (3) can be caught in an easy fashion.

[teunbrand]

Just to jot down some notes regarding topics discussed off-github;

We should ensure symmetry at the check_subclass() (unsed in layer()) level, so that it can find classes based on constructors even if the class itself is absent. This would ensure that layer(geom = "col") will find GeomBar and layer(geom = "errorbarh") will find GeomErrorbar.

Finding a class based on the constructor name is much slower than based on the class name, so probably we should only use it as a fallback.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

check_subclass1 <- function(x, subclass, env = parent.frame()) {
  name <- paste0(subclass, camelize(x, first = TRUE))
  obj <- find_global(name, env = env)
  obj
}

check_subclass2 <- function(x, subclass, env = parent.frame()) {
  name <- paste0(to_lower_ascii(subclass), "_", x)
  obj <- find_global(name, env, mode = "function")()
  switch(
    subclass,
    Geom = obj$geom,
    Stat = obj$stat
  )
}

bench::mark(
  check_subclass1("point", "Geom"),
  check_subclass2("point", "Geom")
)
#> # A tibble: 2 × 6
#>   expression                            min  median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr>                        <bch:t> <bch:t>     <dbl> <bch:byt>    <dbl>
#> 1 "check_subclass1(\"point\", \"Ge…  79.7µs  82.2µs    11734.        0B     2.04
#> 2 "check_subclass2(\"point\", \"Ge… 758.2µs 810.4µs     1216.     101KB    11.2

^{Created on 2024-08-27 with reprex v2.1.1}