Move flush_console() to its own file

* Make it more self-contained
* Add a test

R/flush-console.R

@@ -0,0 +1,33 @@
+#' An emulation of flush.console() in evaluate()
+#'
+#' When [evaluate()] is evaluating code, the text output is diverted into
+#' an internal connection, and there is no way to flush that connection. This
+#' function provides a way to "flush" the connection so that any text output can
+#' be immediately written out, and more importantly, the `text` handler
+#' (specified in the `output_handler` argument of `evaluate()`) will
+#' be called, which makes it possible for users to know it when the code
+#' produces text output using the handler.
+#' @note This function is supposed to be called inside `evaluate()` (e.g.
+#'   either a direct `evaluate()` call or in \pkg{knitr} code chunks).
+#' @export
+flush_console = function() {
+  if (!is.null(.env$output_handler)) {
+    .env$output_handler()
+  }
+  invisible()
+}
+
+.env = new.env()
+.env$output_handler <- NULL
+
+set_output_handler <- function(handler) {
+  old <- .env$output_handler
+  .env$output_handler <- handler
+  invisible(old)
+} 
+
+local_output_handler <- function(handler, frame = parent.frame()) {
+  old <- set_output_handler(handler)
+  defer(set_output_handler(old), frame)
+  invisible()
+}

R/utils.R

@@ -0,0 +1,4 @@
+defer <- function(expr, frame = parent.frame(), after = FALSE) {
+  thunk <- as.call(list(function() expr))
+  do.call(on.exit, list(thunk, TRUE, after), envir = frame)
+}

tests/testthat/test-flush-console.R

@@ -0,0 +1,15 @@
+test_that("flush_console() is a null op at top-level", {
+  expect_no_error(flush_console())
+
+})
+
+test_that("can use flush_console() inside evaluate", {
+  test <- function() {
+    cat("hi")
+    flush_console()
+    cat("bye")
+  }
+  ev <- evaluate("test()")
+  expect_equal(ev[[2]], "hi")
+  expect_equal(ev[[3]], "bye")
+})