show serve url as result of quarto_preview

NEWS.md

@@ -1,5 +1,7 @@
 # quarto (development version)
 
+- `quarto_preview()` now explicitly returns the preview server URL (invisibly) and documents this behavior. This enables programmatic workflows such as taking screenshots with **webshot2** or passing the URL to other automation tools (thanks, @cwickham, #233).
+
 - Added NA value detection in YAML processing to prevent silent failures when passing R's `NA` values to Quarto CLI. Functions `as_yaml()` and `write_yaml()` now validate for NA values and provide clear error messages with actionable suggestions. This addresses issues where R's `NA` values get converted to YAML strings (like `.na.real`) that Quarto doesn't recognize as missing values, because they are not supported in YAML 1.2 spec. This is to help users handle missing data appropriately before passing to Quarto (#168).
 
 - Added `add_spin_preamble()` function to add YAML preambles to R scripts for use with Quarto Script rendering support. The function automatically detects existing preambles and provides flexible customization options through `title` and `preamble` parameters (#164).

R/daemon.R

@@ -62,7 +62,10 @@
 
   # port and host
   args <- c(args, "--port", port)
-  if (!identical(host, "127.0.0.1")) {
+  if (identical(host, "127.0.0.1")) {
+    host <- "localhost" # use localhost for consistency
+  }
+  if (!identical(host, "localhost")) {
     args <- c(args, "--host", host)
   }
 
@@ -148,12 +151,14 @@
   }
   poll_process()
 
+  serve_url <- quarto[[url_key]] %||% sprintf("http://%s:%i", host, port)
+
   # indicate server is running
   if (isFALSE(quiet)) {
-    cli::cli
     cli::cli_inform(c(
       "",
-      i = "Stop the preview with {.code quarto::quarto_{command}_stop()}"
+      # "i" = "Preview server running at {.url {serve_url}}",
+      ">" = "Stop the preview with {.code quarto_{command}_stop()}"
     ))
   }
 
@@ -166,11 +171,10 @@
         utils::browseURL
       )
     }
-    serve_url <- quarto[[url_key]] %||% paste0("http://localhost:", port)
     browse(serve_url)
   }
 
-  invisible()
+  invisible(serve_url)
 }
 
 stop_serve_daemon <- function(command) {

R/preview.R

@@ -24,6 +24,10 @@
 #' @param quiet Suppress warning and other messages, from R and also Quarto CLI
 #'   (i.e `--quiet` is passed as command line)
 #'
+#' @return The URL of the preview server (invisibly). This can be used to 
+#'   programmatically access the server location, for example to take screenshots
+#'   with webshot2 or pass to other automation tools.
+#'
 #' @importFrom processx process
 #' @importFrom rstudioapi isAvailable
 #' @importFrom rstudioapi viewer
@@ -42,6 +46,16 @@
 #' # (rather than RStudio Viewer)
 #' quarto_preview("myproj", open = utils::browseURL)
 #'
+#' # Capture the preview URL for programmatic use
+#' preview_url <- quarto_preview("document.qmd", browse = FALSE)
+#' cat("Preview available at:", preview_url, "\n")
+#'
+#' # Take a screenshot of the preview using webshot2
+#' if (require(webshot2)) {
+#'   preview_url <- quarto_preview("document.qmd", browse = FALSE)
+#'   webshot2::webshot(preview_url, "preview.png")
+#' }
+#'
 #' # Stop any running quarto preview
 #' quarto_preview_stop()
 #' }
@@ -71,7 +85,7 @@ quarto_preview <- function(
     args <- c("--no-navigate")
   }
 
-  # serve
+  # serve (return serve_url)
   run_serve_daemon(
     "preview",
     file,

tests/testthat/test-preview.R

@@ -0,0 +1,44 @@
+test_that("quarto_preview default functionality", {
+  skip_if_no_quarto()
+  skip_on_cran()
+
+  tmp_dir <- withr::local_tempdir()
+  withr::local_dir(tmp_dir)
+  xfun::write_utf8(c("---", "title: Test", "---", "", "# Hello"), "test.qmd")
+
+  expect_no_error({
+    url <- withr::with_dir(tmp_dir, {
+      quarto_preview("test.qmd", browse = FALSE, quiet = TRUE)
+    })
+  })
+
+  # Always clean up
+  withr::defer(quarto_preview_stop())
+
+  if (exists("url")) {
+    expect_true(grepl("^https?://", url))
+  }
+})
+
+test_that("quarto_preview can change port", {
+  skip_if_no_quarto()
+  skip_on_cran()
+
+  tmp_dir <- withr::local_tempdir()
+  withr::local_dir(tmp_dir)
+  xfun::write_utf8(c("---", "title: Test", "---", "", "# Hello"), "test.qmd")
+
+  expect_no_error({
+    url <- withr::with_dir(tmp_dir, {
+      quarto_preview("test.qmd", port = 8888, browse = FALSE, quiet = TRUE)
+    })
+  })
+
+  # Always clean up
+  withr::defer(quarto_preview_stop())
+
+  if (exists("url")) {
+    expect_true(grepl("^https?://", url))
+    expect_true(grepl(":8888", url))
+  }
+})