Terminate and nil OmniThread task when form closes? - delphi

Question

This is a sample code for a stopwatch I have implemented as a separate thread with the OmniThread library.
This is my question: Do I have to terminate and nil the task when the form closes or will it be destroyed automatically when the form closes?
uses
System.SysUtils, System.Classes,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls,
OtlComm, OtlTask, OtlTaskControl, OtlEventMonitor;
type
TForm1 = class(TForm)
OTLMonitor: TOmniEventMonitor;
btnStartClock: TButton;
btnStopClock: TButton;
procedure btnStartClockClick(Sender: TObject);
procedure btnStopClockClick(Sender: TObject);
procedure FormClose(Sender: TObject; var Action: TCloseAction);
procedure OTLMonitorTaskMessage(const task: IOmniTaskControl; const msg: TOmniMessage);
procedure OTLMonitorTaskTerminated(const task: IOmniTaskControl);
private
{ Private-Deklarationen }
FClockTask: IOmniTaskControl;
public
{ Public-Deklarationen }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
{ Place a TOmniEventMonitor component on the form,
name it OTLMonitor,
implement the OnTaskTerminated event-handler: OTLMonitorTaskTerminated
and implement the OnTaskmessage event-handler: OTLMonitorTaskMessage }
var
StopMessage: string;
procedure ShowElapsedSeconds(const ATask: IOmniTask);
var
ElapsedSeconds: Integer;
begin
ElapsedSeconds := 0;
while not ATask.Terminated do
begin
// stop after 10 seconds:
if ElapsedSeconds >= 10 then BREAK;
Inc(ElapsedSeconds);
ATask.Comm.Send(ElapsedSeconds);
Sleep(1000);
end;
end;
procedure TForm1.OTLMonitorTaskMessage(const task: IOmniTaskControl; const msg: TOmniMessage);
begin
// show elapsed seconds:
Self.Caption := IntToStr(msg.MsgID);
end;
procedure TForm1.OTLMonitorTaskTerminated(const task: IOmniTaskControl);
begin
FClockTask := nil;
Self.Caption := StopMessage;
end;
procedure TForm1.btnStartClockClick(Sender: TObject);
begin
if not Assigned(FClockTask) then // prevent multiple clock-tasks
begin
StopMessage := 'Automatically stopped after 10 seconds';
FClockTask := CreateTask(ShowElapsedSeconds, 'ShowElapsedSeconds').MonitorWith(OTLMonitor).Run;
end
else
begin
MessageDlg('Clock is already running!', mtInformation, [mbOK], 0);
{ Nice: The clock continues to run even while this message dialog is displayed! }
end;
end;
procedure TForm1.btnStopClockClick(Sender: TObject);
begin
if Assigned(FClockTask) then
begin
StopMessage := 'Stopped by the user';
FClockTask.Terminate;
FClockTask := nil;
end
else
MessageDlg('Clock is not running!', mtInformation, [mbOK], 0);
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
if Assigned(FClockTask) then
begin
{ Do I need to terminate and nil the clock-task here?
Or will it be destroyed autmatically when the form closes? }
end;
end;

Answer 1

Primož Gabrijelčič, the author of "Parallel Programming with OmniThreadLibrary" writes:
"We should also handle the possibility of user closing the program by
clicking the ‘X’ button while the background scanner is active. We
must catch the OnFormCloseQuery event and tell the task to terminate.
procedure TfrmBackgroundFileSearchDemo.FormCloseQuery(Sender: TObject;
var CanClose: boolean);
begin
if assigned(FScanTask) then
begin
FScanTask.Terminate;
FScanTask := nil;
CanClose := true;
end;
end;"
This book is for sale at http://leanpub.com/omnithreadlibrary