Explicitly register knit_print S3 methods

DESCRIPTION

@@ -1,8 +1,7 @@
 Package: htmltools
 Type: Package
 Title: Tools for HTML
-Version: 0.3.6.9001
-Date: 2017-04-26
+Version: 0.3.6.9002
 Author: RStudio, Inc.
 Maintainer: Joe Cheng <joe@rstudio.com>
 Description: Tools for HTML generation and output.

NEWS

@@ -1,6 +1,10 @@
-htmltools 0.3.6.9001
+htmltools 0.3.6.9002
 --------------------------------------------------------------------------------
 
+* The `knit_print` methods are explicitly registered as S3 methods when both
+  knitr and htmltools are loaded. This means that neither shiny nor htmltools
+  need to be attached for the `knit_print` methods to work. (#108)
+
 * Updated RcppExports for new version of Rcpp. (#93)
 
 * `as.character.shiny.tags()` will handle non-ASCII attributes correctly if they

R/zzz.R

@@ -0,0 +1,30 @@
+register_s3_method <- function(pkg, generic, class, fun = NULL) {
+  stopifnot(is.character(pkg), length(pkg) == 1)
+  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))
+  }
+
+  if (pkg %in% loadedNamespaces()) {
+    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 = asNamespace(pkg))
+    }
+  )
+}
+
+
+.onLoad <- function(...) {
+  register_s3_method("knitr", "knit_print", "shiny.tag")
+  register_s3_method("knitr", "knit_print", "html")
+  register_s3_method("knitr", "knit_print", "shiny.tag.list")
+}