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 your own custom threads and managing them.
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 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 which take 3 and 5 seconds respectively, the time for you to get a result is:
- Sequentially (not using TTask/ITask) = 8 seconds
- TTask.WaitForAny = 3 seconds
- TTask.WaitForAll = 5 seconds
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 to run a single task and start it:
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);
}