Provide support for no_update in Dash for R

NAMESPACE

@@ -2,6 +2,7 @@
 
 S3method(print,dash_component)
 export(Dash)
+export(dashNoUpdate)
 export(input)
 export(output)
 export(state)

R/dash.R

@@ -287,11 +287,17 @@ Dash <- R6::R6Class(
         output_value <- getStackTrace(do.call(callback, callback_args),
                                       debug = private$debug,
                                       pruned_errors = private$pruned_errors)
-
+        
         # reset callback context
         private$callback_context_ <- NULL
 
-        if (is.null(private$stack_message)) {
+        # inspect the output_value to determine whether any outputs have no_update
+        # objects within them; these should not be updated
+        if (class(output_value) == "no_update") {
+          response$body <- character(1) # return empty string
+          response$status <- 204L
+        }
+        else if (is.null(private$stack_message)) {
           # pass on output_value to encode_plotly in case there are dccGraph
           # components which include Plotly.js figures for which we'll need to 
           # run plotly_build from the plotly package

R/dependencies.R

@@ -5,11 +5,15 @@
 #' Use in conjunction with the `callback()` method from the [dash::Dash] class
 #' to define the update logic in your application.
 #'
+#' The `dashNoUpdate()` function permits application developers to prevent a
+#' single output from updating the layout. It has no formal arguments, but
+#' returns an object of class `no_update`, for which Dash will return a `204`
+#' status code and empty response body.
+#' 
 #' @name dependencies
 #' @param id a component id
 #' @param property the component property to use
 
-
 #' @rdname dependencies
 #' @export
 output <- function(id, property) {
@@ -44,3 +48,11 @@ dependency <- function(id = NULL, property = NULL) {
     property = property
   )
 }
+
+#' @rdname dependencies
+#' @export
+dashNoUpdate <- function() {
+  x <- list(NULL)
+  class(x) <- "no_update"
+  return(x)
+}