close #160: implement when_activated convenience function

NAMESPACE

@@ -147,6 +147,7 @@ export(trajectory)
 export(trap)
 export(untrap)
 export(wait)
+export(when_activated)
 export(wrap)
 importFrom(R6,R6Class)
 importFrom(Rcpp,evalCpp)

R/generators.R

@@ -159,3 +159,32 @@ from_to <- function(start_time, stop_time, dist, arrive=TRUE, every=NULL) {
  }
  }
 }
+
+#' @rdname generators
+#' @param n number of arrivals to generate when activated.
+#'
+#' @details \code{\link{when_activated}} sets up an initially inactive generator
+#' which generates \code{n} arrivals each time it is activated from any
+#' trajectory using the activity \code{\link{activate}}.
+#' @export
+#' @examples
+#' # triggering arrivals on demand from a trajectory
+#' t1 <- trajectory() %>%
+#' activate("dummy")
+#'
+#' simmer() %>%
+#' add_generator("dummy", t0, when_activated()) %>%
+#' add_generator("trigger", t1, at(2)) %>%
+#' run() %>%
+#' get_mon_arrivals()
+#'
+when_activated <- function(n=1) {
+ first <- TRUE
+ function() {
+ if (first) {
+ first <<- FALSE
+ return(-1)
+ }
+ c(rep(0, n), -1)
+ }
+}