I'm having a rough time with Threads.

When my app starts, it creates an instance of a TThread that loads a bunch of images. That TThread instance can be destroyed in two ways:

1) When the loading finishes.
2) When the user closes the app while loading.

In case 2, I destroy the thread using this code:

  if Assigned(FMyThread) then
  begin
    FMyThread.Terminate;
    while not FMyThread.Finished do
      CheckSynchronize(100);
    FreeAndNil(FMyThread);
  end;

This works ok.

Now, in case 1, I thought that I should assign an OnTerminate event to the thread, then destroy it, but it doesn't work as expected. The app gets locked on my thread destroy event:

  procedure TForm1.Createthread;
  begin
    FMyThread := TMyThread.Create;
    FMyThread.OnTerminate := @ThreadDestroyEvent;
    FMyThread.Start;
  end;
  procedure TForm1.ThreadDestroyEvent(Sender: TObject);
  begin
    FMyThread.Free;
  end;
  destructor TMyThread.destroy;
  begin
    // destroy internal objects
    inherited destroy; // <-- The app gets locked here!!
  end;

Why is this happening?, am I doing something wrong?.

I don't know whether this will solve your error, but two things of which I don't know whether they are known to you:
1. Calling "Terminate" will not kill the thread, it just sets the "Terminated" property to true, so you must check for yourself inside "Execute" whether your Thread should stop
2. The event you assigned to "OnTerminate" will only be called if you call "DoTerminate" inside the thread (the event will be called using Synchronize).

You might also want to take a look at the property "FreeOnTerminate" which tells the thread to free itself once "Execute" returns.

Regards,
Sven

Sven, the event OnTerminate is called. What's wrong is the destruction of the thread.

FreeOnTerminate is False. I don't want the thread to be destroyed automatically.

try
   FMyThread.Terminate;
   FMyThread.WaitFor;
....

Zahker, using WaitFor blocks the app.

Also, as I said in the original post, that part is working ok.

Your thread calls OnTerminate via Synchronize and waits until the callback finishes. In the callback you call Thread.Destroy which waits until the thread finishes. It is a deadlock.

Other than using FreeOnTerminate=True, maybe periodically check from the main thread the status of the thread. If it is finished then destroy, but don't do it from OnTerminate.

Well, the "best" way I found is to create a Timer that checks if the thread is terminated, then destroy it.

I don't like this. It would be better if the thread can communicate the main form that it has finished, then the main form can destroy it, without using a Timer.

Why must the main thread destroy it? The thread can destroy itself. Just add a Synchronize call in the destructor or at the end of Execute to let the main thread clear the references.

Mattias

So, you say that I should use FreeOnTerminate := true, and set MyThread
:= nil on the main thread?.

Yes.

Mattias

Would the attached example work for you?

-Fl?vio
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Leonardo M. Ram? wrote

Zahker, using WaitFor blocks the app.

Block the whole application or the current thread that run WaitFor? Example: Listener thread WaitFor Connection Thread.

Thanks

Zaher Dirkey
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How you found a way of resolving your problem?

I suspect I will be interested in this issue soon.