When to Free a Thread manually - delphi

Question

If I create a (suspended) thread from the main thread as such:
with TMyThread.Create(True) do
begin
OnTerminate := ThreadTerminated;
FreeOnTerminate := False;
Start;
end;
How do I go about freeing that instance once it's completed? (ie the Execute procedure has finished executing - assume I've captured exceptions).
This Proper way of destroying a tthread object link shows a way (via the PostMessage procedure) which works fine and makes sense. However, what if I create the thread and I don't have a handle to a form or something where I can invoke the PostMessage procedure. eg I create the thread within a class descended directly from TObject?
TMyClass = class
public
procedure DoSomething;
end;
TMyClass.DoSomething;
begin
with TMyThread.Create(True) do
begin
OnTerminate := ThreadTerminated;
FreeOnTerminate := False;
Start;
end;
end;
So, I guess, how do I free a thread without access to a form handle?
Thanks

Answer 1

Obviously, somewhere there has to be a reference to the instantiated thread. But I can relate to your wish: you want a always-done-never-care solution.
I suggest you manage the thread's existence by a separate ThreadController class:
unit Unit2;
interface
uses
Classes, SysUtils, Forms, Windows, Messages;
type
TMyThreadProgressEvent = procedure(Value: Integer;
Proceed: Boolean) of object;
procedure RunMyThread(StartValue: Integer; OnProgress: TMyThreadProgressEvent);
implementation
type
TMyThread = class(TThread)
private
FException: Exception;
FOnProgress: TMyThreadProgressEvent;
FProceed: Boolean;
FValue: Integer;
procedure DoProgress;
procedure HandleException;
procedure ShowException;
protected
procedure Execute; override;
end;
TMyThreadController = class(TObject)
private
FThreads: TList;
procedure StartThread(StartValue: Integer;
OnProgress: TMyThreadProgressEvent);
procedure ThreadTerminate(Sender: TObject);
public
constructor Create;
destructor Destroy; override;
end;
var
FMyThreadController: TMyThreadController;
function MyThreadController: TMyThreadController;
begin
if not Assigned(FMyThreadController) then
FMyThreadController := TMyThreadController.Create;
Result := FMyThreadController
end;
procedure RunMyThread(StartValue: Integer; OnProgress: TMyThreadProgressEvent);
begin
MyThreadController.StartThread(StartValue, OnProgress);
end;
{ TMyThreadController }
constructor TMyThreadController.Create;
begin
inherited;
FThreads := TList.Create;
end;
destructor TMyThreadController.Destroy;
var
Thread: TThread;
begin
while FThreads.Count > 0 do
begin
Thread := FThreads[0]; //Save reference because Terminate indirectly
//extracts the list entry in OnTerminate!
Thread.Terminate; //Indirectly decreases FThreads.Count
Thread.Free;
end;
FThreads.Free;
inherited Destroy;
end;
procedure TMyThreadController.StartThread(StartValue: Integer;
OnProgress: TMyThreadProgressEvent);
var
Thread: TMyThread;
begin
Thread := TMyThread.Create(True);
FThreads.Add(Thread); //Add to list before a call to Resume because once
//resumed, the thread might be gone already!
Thread.FValue := StartValue;
Thread.FOnProgress := OnProgress;
Thread.OnTerminate := ThreadTerminate;
Thread.Resume;
end;
procedure TMyThreadController.ThreadTerminate(Sender: TObject);
begin
FThreads.Extract(Sender);
end;
{ TMyThread }
procedure TMyThread.DoProgress;
begin
if (not Application.Terminated) and Assigned(FOnProgress) then
FOnProgress(FValue, FProceed);
end;
procedure TMyThread.Execute;
begin
try
FProceed := True;
while (not Terminated) and (not Application.Terminated) and FProceed and
(FValue < 20) do
begin
Synchronize(DoProgress);
if not FProceed then
Break;
Inc(FValue);
Sleep(2000);
end;
//In case of normal execution ending, the thread may free itself. Otherwise,
//the thread controller object frees the thread.
if not Terminated then
FreeOnTerminate := True;
except
HandleException;
end;
end;
procedure TMyThread.HandleException;
begin
FException := Exception(ExceptObject);
try
if not (FException is EAbort) then
Synchronize(ShowException);
finally
FException := nil;
end;
end;
procedure TMyThread.ShowException;
begin
if GetCapture <> 0 then
SendMessage(GetCapture, WM_CANCELMODE, 0, 0);
if (FException is Exception) and (not Application.Terminated) then
Application.ShowException(FException)
else
SysUtils.ShowException(FException, nil);
end;
initialization
finalization
FreeAndNil(FMyThreadController);
end.
To run this sample thread which counts from 5 to 19 in 2 second intervals and provides feedback and an opportunity to a premature termination, call from the main thread:
type
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
procedure MyThreadProgress(Value: Integer; Proceed: Boolean);
end;
...
procedure TForm1.Button1Click(Sender: TObject);
begin
RunMyThread(5, MyThreadProgress);
end;
procedure TForm1.MyThreadProgress(Value: Integer; Proceed: Boolean);
begin
Caption := IntToStr(Value);
end;
This thread automatically kills itself on either thread's or application's termination.
Maybe this unit is a little overkill for your situation because it is capable of handling multiple threads (of the same type), but I think it answers your question. Adjust to your liking.
Partial origin of this answer: NLDelphi.com.