Dynamically register knit_print.ggvis method

Author: wch

NAMESPACE

@@ -257,7 +257,6 @@ export(is.prop_constant)
 export(is.prop_reactive)
 export(is.prop_variable)
 export(is.scaled_value)
-export(knit_print.ggvis)
 export(layer_arcs)
 export(layer_bars)
 export(layer_boxplots)

R/print.R

@@ -106,10 +106,11 @@ view_dynamic <- function(x, plot_id = rand_id("plot_"), port = NULL,
   )
 }
 
+
+# Dynamically registered. See zzz.R.
 #' Knit print method for ggvis plots.
 #'
 #' @keywords internal
-#' @export
 knit_print.ggvis <- function(x, options = list(), inline = FALSE, ...) {
   # Set height and width from knitr chunk options
   knitr_opts <- list(

R/zzz.r

@@ -7,3 +7,35 @@
     "For questions and other discussion, please use ",
     "https://groups.google.com/group/ggvis.")
 }
+
+.onLoad <- function(libname, pkgname) {
+  # ggvis provides methods for knitr::knit_print, but knitr isn't a Depends or
+  # Imports of htmltools, only a Suggests. This code snippet manually
+  # registers our method(s) with S3 once both ggvis and knitr are loaded.
+  register_s3_method("knitr", "knit_print", "ggvis")
+}
+
+register_s3_method <- function(pkg, generic, class, fun = NULL) {
+  stopifnot(is.character(pkg), length(pkg) == 1)
+  envir <- asNamespace(pkg)
+
+  stopifnot(is.character(generic), length(generic) == 1)
+  stopifnot(is.character(class), length(class) == 1)
+  if (is.null(fun)) {
+    fun <- get(paste0(generic, ".", class), envir = parent.frame())
+  }
+  stopifnot(is.function(fun))
+
+
+  if (pkg %in% loadedNamespaces()) {
+    registerS3method(generic, class, fun, envir = envir)
+  }
+
+  # Always register hook in case package is later unloaded & reloaded
+  setHook(
+    packageEvent(pkg, "onLoad"),
+    function(...) {
+      registerS3method(generic, class, fun, envir = envir)
+    }
+  )
+}