Cannot terminate threads - multithreading

Question

I use threads in my project. And I wanna kill and terminate a thread immediately.
sample:
type
test = class(TThread)
private
{ Private declarations }
protected
procedure Execute; override;
end;
var
Form1: TForm1;
a:tthread;
implementation
{$R *.dfm}
procedure test.Execute;
begin
Synchronize(procedure begin
form1.ProgressBar1.position := 0;
sleep(5000);
form1.ProgressBar1.position := 100;
end
);
end;
procedure TForm1.btn_startClick(Sender: TObject);
begin
a:=test.Create(false);
end;
procedure TForm1.btn_stopClick(Sender: TObject);
begin
terminatethread(a.ThreadID,1); //Force Terminate
end;
But when I click on the btn_stop (after clicking on btn_start), the thread won't stop. So how can stop this thread immediately?
BTW a.terminate; didn't work too.
Thanks.

Answer 1

This is a complete misuse of a worker thread. You are delegating all of the thread's work to the main thread, rendering the worker thread useless. You could have used a simple timer instead.
The correct use of a worker thread would look more like this instead:
type
test = class(TThread)
private
{ Private declarations }
protected
procedure Execute; override;
end;
var
Form1: TForm1;
a: test = nil;
implementation
{$R *.dfm}
procedure test.Execute;
var
I: integer
begin
Synchronize(
procedure begin
form1.ProgressBar1.Position := 0;
end
);
for I := 1 to 5 do
begin
if Terminated then Exit;
Sleep(1000);
if Terminated then Exit;
Synchronize(
procedure begin
Form1.ProgressBar1.Position := I * 20;
end
);
end;
Synchronize(
procedure begin
form1.ProgressBar1.Position := 100;
end
);
end;
procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
btn_stopClick(nil);
end;
procedure TForm1.btn_startClick(Sender: TObject);
begin
if a = nil then
a := test.Create(False);
end;
procedure TForm1.btn_stopClick(Sender: TObject);
begin
if a = nil then Exit;
a.Terminate;
a.WaitFor;
FreeAndNil(a);
end;

Answer 2

The problem is the thread waits by using Sleep. This method will keep the thread sleeping for the specified time, no matter what happens around it. In order to be able to "break sleep" you should use an event. The code should be changed to this:
procedure test.Execute;
begin
Synchronize(procedure begin
form1.ProgressBar1.position := 0;
end);
Event.WaitFor(5000);
if not IsTerminated then
Synchronize(procedure begin
form1.ProgressBar1.position := 100;
end);
end;
The event should be created and destroyed like this:
constructor test.Create(aCreateSuspended: Boolean);
begin
inherited;
Event := TSimpleEvent.Create;
end;
destructor test.Destroy;
begin
FreeAndNil(Event);
inherited;
end;
In order to stop the thread, the code is:
procedure TForm1.btn_stopClick(Sender: TObject);
begin
a.Terminate;
end;
But simply calling Terminate won´t signal the Event, so we have to reimplement Terminate:
procedure test.Terminate;
begin
inherited;
Event.SetEvent;
end;
Calling SetEvent will signal the event, so it will wake the thread up. The execution continues in the next line, that tests for thread termination and decides to execute the second part of the code or not.