tclTask: Schedule and manage delayed tasks

Description

Tcl allows fo scheduling execution of code on the next event loop or after a given time (after Tcl command). tclTaskXxx() functions use it to schedule execution of R code with much control from within R (central management of scheduled tasks, possibility to define redoable tasks, use of S3 objects to keep track of tasks information. The tclAfterXxx() functions are low-level access to the Tcl after command.

Usage

## Convenient tclTask objects management
tclTaskSchedule(wait, expr, id = "task#", redo = FALSE)
tclTaskRun(id)
tclTaskGet(id = NULL, all = FALSE)
tclTaskChange(id, expr, wait, redo)
tclTaskDelete(id)
## Low-level Tcl functions
tclAfter(wait, fun)
tclAfterCancel(task)
tclAfterInfo(task = NULL)

Value

The tclAfterXxx() functions return a 'tclObj' with the result of the corresponding Tcl function. tclAfter() returns the created Tcl timer in this object. If 'task' does not ecxists, tclAfterInfo() returns

NULL.

tclTaskGet() returns a 'tclTask' object, a list of such objects, or

NULL if not found.

The four remaining tclTaskXxx() functions return invisibly TRUE

if the process is done successfully, FALSE otherwise.

tclTaskRun() forces running a task now, even if it is scheduled later.

Arguments

wait: time in ms to delay the task (take care: approximative value, depends on when event loops are triggered). Using a value lower or equal to zero, the task is scheduled on the next event loop.
fun: name of the R function to run (you may not supply arguments to this function, otherwise it is not scheduled properly; take care of scoping, since a copy of the function will be run from within Tcl).
expr: an expression to run after 'wait'.
id: the R identifier of the task to schedule, if this id contains #, then, it is replaced by next available number, but you cannot schedule more than a thousand tasks with the same name (the system will give up well before, anyway). If NULL in tclTaskGet(), retrieve the list of all existing tasks.
all: if id = NULL, all = TRUE indicate to list all tasks, including hidden ones (with id starting with a dot).
redo: should the task be rescheduled n times, indefinitely (redo = TRUE) or not (redo = FALSE, default, or a value <= 0).
task: a Tcl task timer, or its name in Tcl (in the form of 'after#xxx').

Author

Philippe Grosjean

Examples

Run this code

if (FALSE) {
## These cannot be run by examples() but should be OK when pasted
## into an interactive R session with the tcltk package loaded

## Run just once, after 1 sec
test <- function () cat("==== Hello from Tcl! ====\n")
tclTaskSchedule(1000, test())
Sys.sleep(2)

## Run ten times a task with a specified id
test2 <- function () cat("==== Hello again from Tcl! ====\n")
tclTaskSchedule(1000, test2(), id = "test2", redo = 10)
Sys.sleep(1)

## Run a function with arguments (will be evaluated in global environment)
test3 <- function (txt) cat(txt, "\n")
msg <- "==== First message ===="
tclTaskSchedule(1000, test3(msg), id = "test3", redo = TRUE)
Sys.sleep(2)
msg <- "==== Second message ===="
Sys.sleep(2)

## Get info on pending tasks
tclTaskGet() # List all (non hidden) tasks
tclTaskGet("test2")
## List all active Tcl timers
tclAfterInfo()

## Change a task (run 'test3' only once more, after 60 sec)
tclTaskChange("test3", wait = 60000, redo = 1)
Sys.sleep(1)
## ... but don't wait so long and force running 'test3' right now
tclTaskRun("test3")

Sys.sleep(3)
## finally, delete all pending tasks
tclTaskDelete(NULL)
}