Add isolate function for accessing reactives non-reactively

R/reactives.R

@@ -392,3 +392,48 @@ invalidateLater <- function(millis) {
   })
   invisible()
 }
+
+#' Create a non-reactive scope for an expression
+#' 
+#' Executes the given expression in a scope where reactive values or functions 
+#' can be read, but they cannot cause the reactive scope of the caller to be 
+#' re-evaluated when they change.
+#' 
+#' Ordinarily, the simple act of reading a reactive value causes a relationship 
+#' to be established between the caller and the reactive value, where a change 
+#' to the reactive value will cause the caller to re-execute. (The same applies 
+#' for the act of getting a reactive function's value.) The \code{isolate} 
+#' function lets you read a reactive value or function without establishing this
+#' relationship.
+#' 
+#' @param expr An expression that can access reactive values or functions.
+#' 
+#' @examples
+#' \dontrun{
+#' observer(function() {
+#'   input$saveButton  # Do take a dependency on input$saveButton
+#'   
+#'   # isolate a simple expression
+#'   data <- get(isolate(input$dataset))  # No dependency on input$dataset
+#'   writeToDatabase(data)
+#' })
+#' 
+#' observer(function() {
+#'   input$saveButton  # Do take a dependency on input$saveButton
+#'   
+#'   # isolate a whole block
+#'   data <- isolate({
+#'     a <- input$valueA   # No dependency on input$valueA or input$valueB
+#'     b <- input$valueB
+#'     c(a=a, b=b)
+#'   })
+#'   writeToDatabase(data)
+#' })
+#' }
+#' @export
+isolate <- function(expr) {
+  ctx <- Context$new()
+  ctx$run(function() {
+    eval.parent(expr)
+  })
+}