tagAddRenderHook: Modify a tag prior to rendering

View source: R/tags.R

tagAddRenderHookR Documentation

Modify a tag prior to rendering


Adds a hook to call on a tag() object when it is is rendered as HTML (with, for example, print(), renderTags(), as.tags(), etc).


tagAddRenderHook(tag, func, replace = FALSE)



A tag() object.


A function (hook) to call when the tag is rendered. This function should have at least one argument (the tag) and return anything that can be converted into tags via as.tags().


If TRUE, the previous hooks will be removed. If FALSE, func is appended to the previous hooks.


The primary motivation for tagAddRenderHook() is to create tags that can change their attributes (e.g., change CSS classes) depending upon the context in which they're rendered (e.g., use one set of CSS classes in one a page layout, but a different set in another page layout). In this situation, tagAddRenderHook() is preferable to tagFunction() since the latter is more a "black box" in the sense that you don't know anything about the tag structure until it's rendered.


A tag() object with a .renderHooks field containing a list of functions (e.g. func). When the return value is rendered (such as with as.tags()), these functions will be called just prior to writing the HTML.

See Also



# Have a place holder div and return a span instead
obj <- div("example", .renderHook = function(x) {
  x$name <- "span"
obj$name # "div"
print(obj) # Prints as a `span`

# Add a class to the tag
# Should print a `span` with class `"extra"`
spanExtra <- tagAddRenderHook(obj, function(x) {
  tagAppendAttributes(x, class = "extra")

# Replace the previous render method
# Should print a `div` with class `"extra"`
divExtra <- tagAddRenderHook(obj, replace = TRUE, function(x) {
  tagAppendAttributes(x, class = "extra")

# Add more child tags
spanExtended <- tagAddRenderHook(obj, function(x) {
  tagAppendChildren(x, " ", tags$strong("bold text"))

# Add a new html dependency
newDep <- tagAddRenderHook(obj, function(x) {
  fa <- htmlDependency(
    "font-awesome", "4.5.0", c(href="shared/font-awesome"),
    stylesheet = "css/font-awesome.min.css")
  attachDependencies(x, fa, append = TRUE)
# Also add a jqueryui html dependency
htmlDependencies(newDep) <- htmlDependency(
  "jqueryui", "1.11.4", c(href="shared/jqueryui"),
  script = "jquery-ui.min.js")
# At render time, both dependencies will be found

# Ignore the original tag and return something completely new.
newObj <- tagAddRenderHook(obj, function(x) {
  tags$p("Something else")

htmltools documentation built on March 31, 2023, 11:47 p.m.