Delphi ignores CS_OWNDC flag when creating TFrame component, wglMakeCurrent subsequently fails

I'm trying to put together some components for using OpenGL in RAD Studio 2009. I want to have multiple Rendering Contexts.

My idea is to have something like a "main OpenGL component", called GLMaster. It's a TFrame descendant so it should provide me with a DC. Also there's a component for a GLMonitor, acting as camera.

I create an OpenGL RC in the overriden GLMaster.Loaded inside an "if not (csDesigning in ComponentState) then"-clause. The DC for this I do not keep, it's stated in the RAD Studio help that you shouldn't: "TWinControl.Handle Do not refer to the Handle property during component creation or streaming. The underlying window does not exist until the first time the Handle property is referenced. When this occurs, the HandleNeeded method is called automatica开发者_如何学Golly." I handle this by function pointers in components using GLMaster pointing to GLMaster.GetCurrentDC (returns a HDC).

During destruction GLMonitor wants to clean up some render-to textures and other OpenGL resources. When retrieving the DC for a call to wglMakeActive the function pointer is followed and execution jumps to GLMaster.GetCurrentDC. Pointer(Self) tells me that it's the same GLMaster that we created the "master RC" in during the streaming of the components. The property [GLMaster.]Handle is now invalid! If I however free the GLMonitor in the app form's OnClose (using simply GLMonitor_1.Free;) the Handle inside GLMaster.GetCurrentDC is valid end everything works.

I managed to see that the handle used in GLMaster.Loaded is not the same (!?!?) as after the initialization is done and the app form is showed to the user. When I found this I googled my problem and added an overriden CreateParams to add CS_OWNDC to the GLMaster component. A breakpoint there tells me that it's being executed.

Why does the VCL/Delphi silently provides the newly created components with other Handles, and thereby indirectly other DCs, even if I provide the OWNDC flag? Is there someway to get an "ALL components are now loaded and their properties are read from the .dfm file and"-message so I can run GLMaster.InitEverything inside the component?

Right now I'm thinking the fastest way onward is to put this InitEverything in the main form's OnShow handler. And then a call to GLMatser.CleanAll in the main form's OnClose. But I don't want any code in the form, that's the reason I created the components in the first place!

P.S. Not that I think it matters for this particular problem but I use OpenGL 3.2 Core Profile.

I'm answering the "Why does the VCL/Delphi silently provides the newly created components with other Handles".

Why an control's Window Handle might change at run-time

Your TFrame (an TWinControl descendant) is placed on an other TWinControl, let's say a TForm. The parent container provides property wrappers around many settings, allowing us to easily make changes; Some of those changes can't be implemented without re-creating the windhow handle. You can get a long list of properties that cause this to happen by doing a search for RecreateWnd in Forms.pas, Controls.pas etc.

Examples of properties that call RecreateWnd to implement a change at run-time:

• TScollBox.BorderStyle
• TForm.BorderIcons (for MDI childs)

When your TFrame's parent needs to change Window Handle, your TFrame is going to have to change Window Handle as well. Your Delphi Control, no matter the descendense, needs to be handle this, and other controls have it worst: Controls that are implemented as wrappers around Windows controls need to save state and reload state (TEdit, TComboBox, ...)

What I think is going on

While streaming (loading your form) some control does something that needs a Window Handle before loading finishes. It's very likely your own code! After everything finishes loading the Form might need to re-create it's Handle, and this in turn causes your Frame's handle to be changed.

What you might want to override

Given the way the VCL works, you need to be prepared for your Window Handle to change. You should do a search in Controls.pas for the words Handle, HDC. If your control is so intimately linked to it's Window Handle and HDC, it's in your best interest to read up on this stuff.

Take a look at those routintes. Maybe you can sopot a better place to hook:

• CreateHandle
• CreateWnd
• DestroyHandle
• DestroyWnd
• GetDeviceContext

If the title is the question, then no, Delphi does not ignore CS_OWNDC when creating a frame. Just tested with the below procedure with a frame on a form. When the flag is not specified, the line is drawn on itself again and again, resulting with a line from (0,0) to (10,10). When the DC is owned, the line extends from (0,0) to (50,50), no need to tell but the retrieved DC is same at all times.

procedure TFrame2.WmPaint(var Msg: TWMPaint);
var
  i: Integer;
  DC: HDC;
  Pt: TPoint;
begin
  inherited;
  for i := 1 to 5 do begin
    DC := GetDC(Handle);
    GetCurrentPositionEx(DC, @Pt);
    LineTo(DC, Pt.X + 10, Pt.Y + 10);
    ReleaseDC(Handle, DC);
  end;
end;

if your question is this:

ALL components are now loaded and their properties are read from the .dfm file and

The answer is:

Yes, just override the Loaded method of your component. It is called when all the references are valid (after dfm reading is finished).