Threaded loading of icons in Delphi

Question

Using Delphi 2009, trying to make a launcher. In order to make it "snappy" I would really like to load icons in a background thread.

I have used the solution found here : Can 48x48 or 64x64 icons be obtained from the Vista Shell?

This works fine, if NOT run in a thread. As soon as I put it in a thread, some icons are not "fetched", or being some kind of generic icon. I even tried serializing the threads (making them obsolote, in effect) but it yields the same results.

So, the question is: How do I load icons (with the same available options as the linked example) in a thread?

/Lars

Edit: Added some very basic error-checking in GetIconFromFile

if SHGetFileInfo( PChar( aFile ),
                FILE_ATTRIBUTE_NORMAL,
                SFI,
                SizeOf( TSHFileInfo ),
                SHGFI_ICON
                  or SHGFI_LARGEICON
                  or SHGFI_SHELLICONSIZE
                  or SHGFI_SYSICONINDEX
                  or SHGFI_TYPENAME
                  or SHGFI_DISPLAYNAME ) <> 0 then
begin
  if not Assigned( aIcon ) then
    aIcon := TIcon.Create;
  aImgList := GetImageListSH( SHIL_FLAG );
  aIndex := SFI.iIcon;
  if aImgList <> 0 then
    aIcon.Handle := ImageList_GetIcon( aImgList, aIndex, ILD_NORMAL );
end;

This doesn't make any diffenrence. I am still getting some generic icons (only when this is called in a thread, though)

Edit2 : Threading-code (very simple) :

procedure TIconLoader.Execute;
var
  Item : TGridItem;
  I : TIcon;
begin
  inherited;

  while not terminated do
  begin
    Item := nil;
    if assigned(FOnNeedGridItem) then
    begin
      Synchronize(
          procedure
          begin
            FOnNeedGridItem(Self, Item);
          end
      );
    end;

    if assigned(Item) then
    begin
      GetIconFromFile(Item.FFilename, I, SHIL_EXTRALARGE);
      Synchronize(
          procedure
          begin
            Item.SetIcon(I);
          end
      );
//      I.Free;
    end
    else
      Terminate;
  end;
end;

Well, not much to show. I have the procedures from the linked example. I have tried different thread-approaches, but everyone gives the same (wrong) result. — Lars Bargmann, Jul 09 '12 at 12:22
What errors are returned by the api calls? Did you initialise com? — David Heffernan, Jul 09 '12 at 12:24
No errors returned, at all. I call CoInitialize, but it doesn't make a difference. — Lars Bargmann, Jul 09 '12 at 12:32
How did you check that there were no errors? The code that you link to has simply ignored error handling. Did you add error handling? — David Heffernan, Jul 09 '12 at 12:53
Oh, no, I haven't added any errorhandling to those procedures. I was (wrongfully) expecting exceptions. — Lars Bargmann, Jul 09 '12 at 12:56
You've added code, but you don't call `CoInitialize`. The documentation clearly states that you must. You said in a comment above that you had done so. Do you realise that the call to `CoInitialize` must be made from the thread which calls `SHGetFileInfo`? — David Heffernan, Jul 09 '12 at 21:02
This means you have to call CoInitialize in the execute part. — LU RD, Jul 09 '12 at 21:59
@LURD is right. You need to initialise com in the thread. That means inside Execute. — David Heffernan, Jul 09 '12 at 22:01
That was it! Thanks for pointing that out! ...though I feel stupid now... :-) — Lars Bargmann, Jul 10 '12 at 04:20
Please LURD or David or Lars, add this as an answer so it is more discoverable than in the comments. And @Lars: please accept it even if you add it yourself (http://meta.stackexchange.com/questions/118042/a-way-to-reduce-criticism-and-down-voting-for-answering-your-own-question) — Marjan Venema, Jul 10 '12 at 06:02

score 2 · Accepted Answer · answered Jul 10 '12 at 06:23

2

The documentation for SHGetFileInfo specifies that you must initialize COM before calling the function. Since COM initialization is per-thread, you need to initialise COM in the thread. That means calling CoInitialize or CoInitializeEx from the Execute method of the thread.

answered Jul 10 '12 at 06:23

David Heffernan

601,492
42
1,072
1,490

You accepted the answer. Then unaccepted. Is there a problem? – David Heffernan Jul 10 '12 at 06:48

score 0 · Answer 2 · answered Jul 09 '12 at 17:50

0

Maybe you can simplify your threading code by using the TThread.Queue methods introduced in Delphi 2009.

An article which gives some examples and background information is here:

Synchronize and Queue with Parameters

TThread.Queue is non-blocking. This allows to sequentially create background worker threads for all your icons and then let them load asynchronously.

answered Jul 09 '12 at 17:50

mjn

36,362
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378

I am practically 100% certain it's not a thread-code problem. I have a simple list of some simple objects, which holds a filename and a TIcon. I have tried making a thread per object, and the above where a thread is created, and being hand-fed the objects when needed. No matter how I call the GetIconFromFile from within a thread, it ends up giving me wrong icons. If I make the call from the main-thread, everything is fine... ?? – Lars Bargmann Jul 09 '12 at 18:20
Oh, and just for fun, I tried wrapping GetIconFromFile in a TCriticalSection. That didn't make any difference... – Lars Bargmann Jul 09 '12 at 18:25

score -1 · Answer 3 · answered Jul 10 '12 at 01:25

-1

Try moving this:

GetIconFromFile(Item.FFilename, I, SHIL_EXTRALARGE);

into synchronize block. And you should call DestroyIcon() after using SHGetFileInfo().

answered Jul 10 '12 at 01:25

Nahk Cogn

31
2

1

That would defeat the whole purpose of using a thread, wouldn't it? – Lars Bargmann Jul 10 '12 at 04:14

Threaded loading of icons in Delphi

3 Answers3

Linked