I still dont quite understand how to implement multi-threading with TThreads

Question

For this question please refer to a previous question I asked a while back: Do I need TThreads? If so can I pause, resume and stop them? LU RD answered the question with a provided demo and some comments.

I stopped using Delphi for quite a long time but now I am getting back into it and redoing a project. This project has time consuming operations such as opening a Gif, extracting the frames and then adding those frames (bitmaps) into a TImageList and TListView. This time I actually add the bitmaps directly into a TObjectList, as seen here: How to add and retrieve Bitmaps to and from a TList?

Typically nothing special needs doing here to speed it up as most Gif animations are small, but with medium to large Gifs the application can hang. This is going to get worse though as those bitmaps are going to be modified at runtime using various imaging filters such as grayscale, change hue etc. So I am sure I need multi-threading for this otherwise accessing each bitmap and then manipulating is going to be very slow (as I found out before).

---

So with that said, I am foolishly trying to adapt (without a clue of what I am doing) some of my procedures to work with the TThread example posted by LU RD I linked to at the top.

I wish I spent a bit more time with the original question to ask for more information but I guess I got sidetracked and moved onto something else, which meant I learned nothing.

Take this snippet from the threading example:

const
  cWorkLoopMax = 500;

function TForm1.HeavyWork: boolean; // True when ready
var
  i, j: integer;
begin
  j := 0;
  for i := 0 to 10000000 do
    Inc(j);
  Inc(workLoopIx);
  Result := (workLoopIx >= cWorkLoopMax);
end;

For a start I have no idea what cWorkLoopMax is for, and why its value is set to 500?

Secondly I guess the HeavyWork procedure is just a sample, which runs in a loop a 10000000 times whilst incrementing the j variable?

Then we have the workLoopIx which I am unsure what is for? Maybe something to do with the position within the thread maybe?

---

So, here I have my current code (no threading) which handles opening the Gif and adding to the TListView and TImageList. The procedures I use are in another unit, if needed I will post it also, but this is what I use inside a TAction (actOpen):

if OpenPictureDialog.Execute then
begin
  Screen.Cursor := crHourGlass;
  try
    BitmapCollection.AddFromGif(OpenPictureDialog.FileName, ImageList1);

    ListView1.Items.BeginUpdate;
    try
      ListView1.Items.Clear;

      for I := 0 to BitmapCollection.BitmapList.Count - 1 do
      begin
        with ListView1.Items.Add do
        begin
          Caption := 'bitmap' + IntToStr(I+1);
          ImageIndex := I;
        end;
      end;
    finally
      ListView1.Items.EndUpdate;
    end;
  finally
    Screen.Cursor := crDefault;
  end;
end;

What I dont understand is how to put that into a thread procedure, such as HeavyWork? I just created a new one called Job_Open and did this:

procedure TForm1.actOpenExecute(Sender: TObject);
begin
  if OpenPictureDialog.Execute then
  begin
    if not Assigned(MyThread) then
    begin
      workLoopIx := 0;
      btnStartTask.Enabled := false;
      btnPauseResume.Enabled := true;
      btnCancelTask.Enabled := true;
      MyThread := TWorkerThread.Create(Self.Handle, WM_MyProgress, Job_Open);
    end;
  end;
end;

function TForm1.Job_Open: boolean;
var
  I: Integer;
begin
  BitmapCollection.AddFromGif(OpenPictureDialog.FileName, ImageList1);

  for I := 0 to BitmapCollection.BitmapList.Count - 1 do
  begin
    with ListView1.Items.Add do
    begin
      Caption := 'bitmap' + IntToStr(I+1);
      ImageIndex := I;
    end;
    Inc(workLoopIx);
  end;

  Result := (workLoopIx >= BitmapCollection.BitmapList.Count);// cWorkLoopMax);
end;

This is clearly not right, the performance is slower and I am getting all kind of errors such as Invalid Handle.

I would be extremely grateful if someone could take some time to explain my comments, what I am doing wrong and what I should be doing instead, updated code and comments in source are welcome but I am hoping to learn a bit more of what is going on with the code ideally.

In a perfect world if there is a library of sorts that exists out there that is easy to use then that would be a massive help if I cannot understand what is happening above. Is there such a library that can do something like:

procedure DoSomething;
begin
  BeginThreading();
    HeavyWork;
  StopThreading();
end;

Thanks in advance, and apologies for the lengthy post.

Answer 1

DISCLAIMER: While my answer is not an answer directly to your question it is a psobile solution to your problem.

After reading your questions I have one question to you:

Are you applying same image graphical effects on all ImageList images?

If you are then I must say that you started on working on your problem from wrong approach.

First you need to know that Imagelist doesen't store all those images seperately but that it is storing them all in same verry wide image. So when you read any ImageList image internally ImageList creates output bitmap and then uses Canvas.CopyRect which is quite. When you save image to image list it internally uses Canvas.Draw.

So when you do this many times you create lots of unnecessary data movment.

So instead of your approach where you work on seperate images I recomend you work on ImageLists internal image whose handle you can get using ImageList.GetImageBitmap. This will alow you to apply same graphical effect on all ImageList images at once. And if you don't need to apply graphical effects to all ImageList images I bet you can modify your mage processing method to work only on parts of the image.

In order to learn more about ImageList I recomed you read its documentation:

Image list explanation: http://docwiki.embarcadero.com/Libraries/XE6/en/Vcl.Controls.TImageList

Image list GetImageBitmap explanation http://docwiki.embarcadero.com/Libraries/XE6/en/Vcl.ImgList.TCustomImageList.GetImageBitmap