Fix S3 method registration

Previously, the register_s3_method function called asNamespace(pkg), which tries
to load the package, but the purpose of the function is to not load the package
immediately.

Author: wch

R/zzz.r

@@ -10,32 +10,31 @@
 
 .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.
+  # Imports of ggvis, 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())
+  } else {
+    stopifnot(is.function(fun))
   }
-  stopifnot(is.function(fun))
-
 
   if (pkg %in% loadedNamespaces()) {
-    registerS3method(generic, class, fun, envir = envir)
+    registerS3method(generic, class, fun, envir = asNamespace(pkg))
   }
 
   # Always register hook in case package is later unloaded & reloaded
   setHook(
     packageEvent(pkg, "onLoad"),
     function(...) {
-      registerS3method(generic, class, fun, envir = envir)
+      registerS3method(generic, class, fun, envir = asNamespace(pkg))
     }
   )
 }