Internal “Periodical” functions in version 7

Question

In version 7 there exist these Internal` context functions:

?Internal`*Periodical*

AddPeriodical   Periodicals   RemovePeriodical   $ThisPeriodical

In a comment to this answer Szabolcs stated that these "seem to provide very similar (the same?) functionality" to ScheduledTasks in version 8.

• How do I use them?

• To what extent is is possible to mimic version 8 functionality using these?

Answer 1

As a starting point I'll write up what I found about these functions before. I'm hoping someone will take a better look at them and will write a more complete answer.

---

Spelunking in version 8,

Internal`AddPeriodical[Print["boo!"], 3]

Now you get a boo! every 3 seconds.

Internal`Periodicals[]

(* ==> {Print["boo!"]} *)

Now do

Internal`RemovePeriodical[Print["boo!"]]

to stop it.

Internal`Periodicals[]

(* ==> {} *)

Note that AddPeriodical and RemovePeriodical have HoldFirst.

Internal`$ThisPeriodical seems to be undefined during normal evaluations, while it is set to the currently executing periodical (wrapped in HoldForm) when a periodical is being executed.

Most (all?) of the version 8 functionality seems to be implementable in terms of these, provided that they work the same way in version 7 as in 8 (which I cannot test).

Answer 2

You can turn this then into a package, and change the lower-case to uppercase... Hope it works well enough for your goals. If not, we'll improve it in time

AppendTo[$ContextPath, "Internal`"];

ClearAll["`private`*"];
SetAttributes[`private`count, HoldRest];
`private`count[_, expr_, count_Symbol][id_, ___] /; 
  Block[{$scheduledTask = `private`getST[id]}, expr; ++count]/;False := Null
e : `private`count[i_, _, count_Symbol][id_, endFun : _ : (Null &)] /;
   count === i := (Remove[count]; RemovePeriodical[e]; endFun[])
`private`count[___][___] := Null

`private`getST[id_] /; 
  MemberQ[scheduledTasks[], scheduledTaskObject[id, ___]] := 
 First@Cases[scheduledTasks[], scheduledTaskObject[id, ___]]
`private`getST[_] := $Failed;

`private`STQ[scheduledTaskObject[id_, expr_, time_, start_, _]] := 
 Length[Cases[scheduledTasks[]/. HoldPattern@\[Infinity] -> \[Infinity], 
    scheduledTaskObject[id, expr, time, start, _]/. HoldPattern@\[Infinity] -> \[Infinity]]] >= 1
`private`STQ[___] := False;

`private`startScheduledTask[
   st : scheduledTaskObject[id_, 
     expr_, {time_, count_}, (start_ /; start <= AbsoluteTime[]) | 
      Automatic, _]] := 
  Module[{counter = 0}, ClearAttributes[counter, Temporary];
   AddPeriodical[`private`count[count, expr, counter][
     id, `private`startStop[id, False] &], time]];

`private`startScheduledTask[
  st : scheduledTaskObject[id_, expr_, {time_, count_}, start_, _]] :=
  start - AbsoluteTime[] /. 
  remaining_ :> 
   With[{pst := `private`startScheduledTask[
       scheduledTaskObject[
        "", `private`startScheduledTask[
         scheduledTaskObject[id, expr, {time, count}, Automatic, 
          False]], {remaining, 1}, Automatic, 
        False]]}, `private`extraTasks[id] = Hold[pst]; pst]

ClearAll[createScheduledTask, startScheduledTask, 
  stopScheduledTask, $scheduledTask, removeScheduledTask, 
      scheduledTasks, scheduledTaskObject, runScheduledTask];
    $scheduledTask::usage = ""; scheduledTaskObject::usage = "";
SetAttributes[{createScheduledTask, runScheduledTask}, HoldFirst];
SetAttributes[scheduledTaskObject, HoldAll];
SetAttributes[{stopScheduledTask, removeScheduledTask}, Listable];


scheduledTasks[] = {}; `private`idCounter = 0;
`private`startStop[st : (sto : scheduledTaskObject)[__], 
   b : True | False] := st /. sto[rest__, _] :> sto[rest, b];
`private`startStop[id_Integer, b : True | False] := 
  scheduledTasks[] = 
   scheduledTasks[] /. 
    e : scheduledTaskObject[id, rest__] :> `private`startStop[e, b];

createScheduledTask[expr_, {time_, count_: 1}, 
   start_: Automatic] := ++`private`idCounter /. 
    c_ :> scheduledTaskObject[c, expr, {time, count}, start, 
      False] /. sto_ :> (AppendTo[scheduledTasks[], sto]; sto);

createScheduledTask[expr_, time_: 1, rest___] := 
  createScheduledTask[expr, {time, \[Infinity]}, rest];

startScheduledTask[
  st : scheduledTaskObject[
     id_, __]?`private`STQ] := (`private`startStop[id, 
   True]; `private`startScheduledTask[st]; Null /; False)
startScheduledTask[s_] := s;

stopScheduledTask[
  st : scheduledTaskObject[id_, 
     expr_, {time_, 
      count_}, __]?`private`STQ] := (Periodicals[] /. {HoldForm[
      i : `private`count[sth__][id, __]] :> 
     Quiet@RemovePeriodical[i]}; `private`extraTasks[id] /. 
   Hold[`private`startScheduledTask[
      scheduledTaskObject[_, e_, ___]]] :> Quiet@RemovePeriodical[e];
  Quiet[`private`extraTasks[id] =.];
  `private`startStop[id, False]; Null /; False)

stopScheduledTask[st_] := st;

runScheduledTask[stuff___] := 
  startScheduledTask@createScheduledTask@stuff~`private`startStop~True;

removeScheduledTask[
  st : scheduledTaskObject[
     id_, __]?`private`STQ] := (stopScheduledTask[st]; 
  scheduledTasks[] = (DeleteCases[scheduledTasks[], 
     scheduledTaskObject[id, __]]); st)
removeScheduledTask[_] := $Failed;

In case you are interested to copy or as reference, here go the usage messages

ToExpression[#, InputForm, Function[name, name::usage, HoldAll]] & /@ 
 Names["System`*Sche*"]

returns, copied as plain text

{CreateScheduledTask[expr] creates a task that will repeatedly evaluate expr once per second.
CreateScheduledTask[expr,time] creates a task that will repeatedly evaluate expr every time seconds.
CreateScheduledTask[expr,{time}] creates a task that will evaluate expr once after time seconds.
CreateScheduledTask[expr,{time,count}] creates a task that will try evaluating expr once every time seconds up to count times total.
CreateScheduledTask[expr,timespec,start] creates a task that will evaluate expr according to timespec starting at start time.,RemoveScheduledTask[obj] remove the obj from the list of currently set tasks.,ResetScheduledTask[obj] resets scheduled task object obj to the original parameter values.
ResetScheduledTask[obj,timespec]  resets scheduled task timing to timespec.
ResetScheduledTask[obj,timespec,offset] resets scheduled task time offset to offset.,RunScheduledTask[expr] schedules and starts a task that will repeatedly evaluate expr once per second.
RunScheduledTask[expr,time] schedules and starts a task that will repeatedly evaluate expr every time seconds.
RunScheduledTask[expr,{time}] schedules and starts a task that will evaluate expr once after time seconds.
RunScheduledTask[expr,{time,count}] schedules and starts a task that will try evaluating expr once every time seconds up to count times.
RunScheduledTask[expr,timespec,start] schedules a task that will automatically start at start time.,ScheduledTaskObject[id,expr,spec,...]  is a task object specifying future evaluation of expr according to spec.,ScheduledTasks[]  returns a list of ScheduledTaskObject expressions that represent current tasks.,StartScheduledTask[obj] starts the task represented by obj.,StopScheduledTask[obj] deactivates the task obj.,$ScheduledTask returns the current ScheduledTaskObject. }