Allow for custom headers on initial (GET, non-WS) response

[mmuurr]

I've recently migrated a Shiny application to using the UI-as-a-function approach, allowing for increased control on the initial request (to be able to serve a non-Shiny-app response). This is combined with Shiny's HTML template system, also allowing for finer control of the app's returned page (on the initial, non-websocket, request). The general structure looks like so:

ui <- function(req) {
  custom_header_value(req)
  if (some_test(req)) {
    # return the static, non-app response
    shiny::httpResponse(
      content = nonapp_content(),
      headers = list("x-custom-header" = custom_header_value(req))
    )
  } else {
    # return the shiny app response
    shiny::htmlTemplate(
      text_ = the_template(),
      document_ = TRUE,
      named, template, variables, here
    )
  }
}

This works great until I also want to include that x-custom-header in the Shiny app response.

My initial thought was to simply set htmlTemplate()'s return value as the content of httpResponse():

## does NOT work:
shiny::httpResponse(
  content = shiny::htmlTemplate(...),
  headers = list("x-custom-header" = custom_header_value(req)
)

... but I quickly realized httpResponse() wants a fully-baked, i.e. rendered, HTML document, which htmlTemplate() does not provide.

That rendering step (going from an "html_document" and "shiny.tag.list" to a fully-baked HTML string happens here, in shiny:::uiHttpHandler() (called via shiny::shinyApp()):

shiny/R/shinyui.R

Lines 256 to 261 in e3cf4fb

	if (inherits(uiValue, "httpResponse")) {
	return(uiValue)
	} else {
	html <- renderPage(uiValue, showcaseMode, testMode)
	return(httpResponse(200, content=html))
	}

A small chunk of uiHttpHandler() is used for determining the values for the other two arguments to renderPage(): showcase and testMode.
In my particular case, those two will always be disabled/FALSE, and so I was able to complete my initial goal like so:

ui <- function(req) {
  custom_header_value(req)
  if (some_test(req)) {
    # return the static, non-app response
    shiny::httpResponse(
      content = nonapp_content(),
      headers = list("x-custom-header" = custom_header_value(req, app = FALSE))
    )
  } else {
    # return the shiny app response
    shiny::httpResponse(
      content =
        shiny::htmlTemplate(
          text_ = the_template(),
          document_ = TRUE,
          named, template, variables, here
        ) |>
        shiny:::renderPage(),  ## now we have a fully-baked HTML string
      headers = list("x-custom-header" = custom_header_value(req, app =TRUE))
    )
  }
}

This required accessing the non-exported shiny:::renderPage() function, which never feels great to put into production code, but I was otherwise pretty stumped for a way to add a header to the app's initial (pre-websocket) response.

Perhaps there's another way to do this?

If not, maybe the showcase- and testMode-determining logic can be 'officially' factored-out of uiHttpResponse() along with making renderPage() exported?
This way one could use the (working) pattern above while also retaining the potential use of those two flags (without simply re-implementing that flag-determining logic themselves ... i.e. don't repeat yourself).