Using TTask from the Parallel Programming Library

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The Parallel Programming Library (PPL) provides a TTask class to run one task or multiple tasks in parallel. A Task is a unit of work you need to get done. The PPL does the association between the task and the thread that performs the task so you can run several tasks in parallel without having to create and manage your own custom threads.

TTask creates and manages interaction with instances of ITask. ITask is an interface that provides a range of methods and properties to Start, Wait, Cancel a thread, and also a property for Status (Created, WaitingToRun, Running, Completed, WaitingForChildren, Canceled, Exception).

TTask provides WaitForAll and WaitForAny to wait for the completion of all or any tasks. WaitForAll returns when all of the tasks are completed, whereas WaitForAny tells you the first one that is completed. For example, if you have two tasks A and B that take 3 and 5 seconds respectively, the time for you to get a result is:

The following example uses the WaitForAll method:

Delphi:

 
procedure TFormThreading.MyButtonClick(Sender: TObject);
var 
 tasks: array of ITask; 
 value: Integer; 
begin 
 Setlength (tasks ,2); 
 value := 0; 

 tasks[0] := TTask.Create (procedure () 
   begin 
   sleep (3000); // 3 seconds 
   TInterlocked.Add (value, 3000); 
  end); 
 tasks[0].Start; 

 tasks[1] := TTask.Create (procedure () 
   begin 
   sleep (5000); // 5 seconds 
   TInterlocked.Add (value, 5000);
 end); 
 tasks[1].Start; 
 
 TTask.WaitForAll(tasks); 
 ShowMessage ('All done: ' + value.ToString); 
end;

C++:

void __fastcall TFormThreading::MyButtonClick(TObject *Sender)
{
   _di_ITask tasks[2];

   tasks[0] = TTask::Create(_di_TProc(new TCppTask(lvalue, 3000, Label1)));
   tasks[0]->Start());
   
   tasks[1] = TTask::Create(_di_TProc(new TCppTask(lvalue, 5000, Label1)));
   tasks[1]->Start());

   TTask::WaitForAll(tasks,(sizeof(tasks)/sizeof(tasks[0])-1));
   ShowMessage("All done! "+IntToStr(lvalue));
}

Another functionality of TTask is to prevent the user interface locking up if you want to start something in the background. The following code example shows how can you run a single task and start it after a short delay:

Delphi:

 
procedure TFormThreading.Button1Click(Sender: TObject);
var
 aTask: ITask;
begin
 aTask := TTask.Create (procedure ()
   begin
     sleep (3000); // 3 seconds
     ShowMessage ('Hello');
   end);
 aTask.Start;
end;

C++:

void __fastcall TFormThreading::Button1Click(TObject *Sender)
{
  Label1->Caption = "--";
  lvalue = 0;
  _di_ITask aTask = TTask::Create(_di_TProc(new TCppTask(lvalue,3000,Label1)));
  aTask-> Start();
  Label1->Caption =InToStr(lvalue);
}

See Also