How to completely terminate Parallel task from main thread?

Question

How can I completely terminate this background thread from the main thread, i.e. that not even the code in OnTerminated will be executed?

uses
  CodeSiteLogging,
  OtlParallel, OtlTaskControl, OtlTask;

procedure TForm2.btnParallelAsyncClick(Sender: TObject);
begin
  CodeSite.Send('btnParallelAsyncClick 1');

  Parallel.Async(
    procedure(const task: IOmniTask)
    var
      a: Integer;
    begin
      // executed in background thread:
      a := 1 + 1;
      Sleep(2000);
      CodeSite.Send('Executed in Async Thread', a);

      task.Invoke( // used to execute code in main thread
        procedure
        begin
          CodeSite.Send('task.Invoke executed in main thread', a);
        end);

      Sleep(2000);
      CodeSite.Send('Again executed in Async Thread', a);
    end,
    Parallel.TaskConfig.OnTerminated(
    procedure(const task: IOmniTaskControl)
    begin
      // executed in main thread:
      CodeSite.Send('After background thread termination: Executed in Main Thread');
    end
    )
    );

  CodeSite.Send('btnParallelAsyncClick 2');
end;

Is this going to be one of those questions where you desire to terminate the task without its co-operation? — David Heffernan, Dec 30 '16 at 15:38

score 3 · Accepted Answer · answered Dec 30 '16 at 15:38

3

You can't do that (apart from killing the thread, but please don't do that).

Add a global flag shouldSkipTerminate, set it to false before creating the task, set it to true before you terminate the task and check if this flag is set in OnTerminated.

answered Dec 30 '16 at 15:38

gabr

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Thanks, Gabriel. I've tried this, but it doesn't seem to work: http://i.imgur.com/Xam1273.png – user1580348 Dec 30 '16 at 16:08
This one works: http://i.imgur.com/UnhfHDZ.png But this seems to be a primitive solution. Isn't it possible to actively terminate the `Async` thread with something like `task.Terminate` which doesn't work for me in the above example? – user1580348 Dec 30 '16 at 17:00
3

@user1580348 no. `Terminate` simply signals the task to stop running, but the task itself needs to cooperate by exiting itself when signaled. – Remy Lebeau Dec 30 '16 at 19:22
1

To help speed up terminating use TEvent.WaitFor instead of Sleep. Then when you want to terminate, signal the Event and check your TWaitResult: If (MyEvent.Waitfor(2000) <> System.Types.wrTimeout) or Terminated then Exit; – FredS Dec 30 '16 at 20:29
Thank you for your very useful comments, Gabriel! BTW, is it possible to set a priority with `SetPriority` to the `Parallel.Async` background task? – user1580348 Dec 30 '16 at 20:48
@user1580348 That's a separate question. [Ask a new question](http://stackoverflow.com/questions/ask) – Disillusioned Dec 31 '16 at 08:37

How to completely terminate Parallel task from main thread?

1 Answers1