chore: Restore legacy autoreload watcher if {watcher} not installed

Author: gadenbuie

DESCRIPTION

@@ -93,8 +93,7 @@ Imports:
     glue (>= 1.3.2),
     bslib (>= 0.6.0),
     cachem (>= 1.1.0),
-    lifecycle (>= 0.2.0),
-    watcher 
+    lifecycle (>= 0.2.0)
 Suggests:
     coro (>= 1.1.0),
     datasets,
@@ -112,7 +111,8 @@ Suggests:
     dygraphs,
     ragg,
     showtext,
-    sass
+    sass,
+    watcher 
 URL: https://shiny.posit.co/,
     https://github.com/rstudio/shiny
 BugReports: https://github.com/rstudio/shiny/issues

R/shinyapp.R

@@ -313,32 +313,72 @@ initAutoReloadMonitor <- function(dir) {
     ".*\\.(r|html?|js|css|png|jpe?g|gif)$"
   )
 
-  lastValue <- NULL
-  check_for_update <- function(paths) {
-    paths <- grep(
-      filePattern,
-      paths,
-      ignore.case = TRUE,
-      value = TRUE
-    )
-
-    if (length(paths) == 0) {
-      return()
+  
+  if (is_installed("watcher")) {
+    check_for_update <- function(paths) {
+      paths <- grep(
+        filePattern,
+        paths,
+        ignore.case = TRUE,
+        value = TRUE
+      )
+      
+      if (length(paths) == 0) {
+        return()
+      }
+      
+      cachedAutoReloadLastChanged$set()
+      autoReloadCallbacks$invoke()
+    }
+    
+    # [garrick, 2025-02-20] Shiny <= v1.10.0 used `invalidateLater()` with an
+    # autoreload.interval in ms. {watcher} instead uses a latency parameter in
+    # seconds, which serves a similar purpose and that I'm keeping for backcompat.
+    latency <- getOption("shiny.autoreload.interval", 250) / 1000
+    watcher <- watcher::watcher(dir, check_for_update, latency = latency)
+    watcher$start()
+    onStop(watcher$stop)
+  } else {
+    # Fall back to legacy observer behavior
+    if (!isFALSE(getOption("shiny.autoreload.legacy_warning", TRUE))) {
+      cli::cli_warn(
+        c(
+          "Using legacy autoreload file watching. Please install {.pkg watcher} for a more performant autoreload file watcher.",
+          "i" = "Set {.run options(shiny.autoreload.legacy_warning = FALSE)} to suppress this warning."
+        ),
+        .frequency = "regularly",
+        .frequency_id = "shiny.autoreload.legacy_warning"
+      )
     }
 
-    cachedAutoReloadLastChanged$set()
-    autoReloadCallbacks$invoke()
+    lastValue <- NULL
+    observeLabel <- paste0("File Auto-Reload - '", basename(dir), "'")
+    watcher <- observe(label = observeLabel, {
+      files <- sort_c(
+        list.files(dir, pattern = filePattern, recursive = TRUE, ignore.case = TRUE)
+      )
+      times <- file.info(files)$mtime
+      names(times) <- files
+  
+      if (is.null(lastValue)) {
+        # First run
+        lastValue <<- times
+      } else if (!identical(lastValue, times)) {
+        # We've changed!
+        lastValue <<- times
+        cachedAutoReloadLastChanged$set()
+        autoReloadCallbacks$invoke()
+      }
+  
+      invalidateLater(getOption("shiny.autoreload.interval", 500))
+    })
+  
+    onStop(watcher$destroy)
+  
+    watcher$destroy
   }
 
-  # [garrick, 2025-02-20] Shiny <= v1.10.0 used `invalidateLater()` with an
-  # autoreload.interval in ms. {watcher} instead uses a latency parameter in
-  # seconds, which serves a similar purpose and that I'm keeping for backcompat.
-  latency <- getOption("shiny.autoreload.interval", 250) / 1000
-  watcher <- watcher::watcher(dir, check_for_update, latency = latency)
-  watcher$start()
-  onStop(watcher$stop)
-
-  watcher
+  invisible(watcher)
 }
 
 #' Load an app's supporting R files