[SOLVED] How to Draw a TCanvas on a GTK2 Window

[aydın]

Hi everyone!

For one of my projects, I'm not using the TForm in Lazarus LCL. Instead, I'm writing my own widgetset using pure API.

On Windows, I capture and draw the painting (WM_PAINT) event as follows:

Code: Pascal  [Select][+][-]

1. Procedure Form_Paint(hwnd: THandle);
2. Var
3.   Context: HDC;
4.   BufCanvas: TCanvas;
5. Begin
6.   Context := GetDC(hwnd);
7.  
8.   BufCanvas := TCanvas.Create;
9.   BufCanvas.Handle := Context;
10.  
11.   {I make my drawings}
12.  
13.   //FIA(BufCanvas);
14.   BufCanvas.Free;
15.  
16.   ReleaseDC(hwnd, Context);
17. End;

To achieve the same in GTK2, you need to set up a drawing trigger as follows:

Code: Pascal  [Select][+][-]

1. gtk_signal_connect(GTK_OBJECT(window), 'expose-event', G_CALLBACK(@Form_Paint), nil);

My question is, how can I draw the TCanvas I created on the GTK2 window?

Thank you in advance.

[aydın]

Hello again.

I've examined Lcl a bit, and although I don't fully understand its logic, I managed to create a working system.

After creating my window, I set up the trigger as follows:

Code: Pascal  [Select][+][-]

1. gtk_signal_connect(GTK_OBJECT(PGtkWidget(Result)), 'expose-event', G_CALLBACK(@onDrawEvent),

And here is my drawing code:

Code: Pascal  [Select][+][-]

1. function onDrawEvent(widget: PGtkWidget; event: PGdkEvent; user_data: gpointer): gboolean; cdecl;
2. var
3.   Bmp: TCanvas;
4.   Context: HDC;
5. begin
6.   Context:= InterfaceBase.WidgetSet.GetDC(THandle(widget));
7.  
8.   Bmp:= TCanvas.Create;
9.   Bmp.Handle:= Context;
10.  
11.   // Sample drawings
12.   Bmp.Pen.Color:= clBlue;
13.   Bmp.Pen.Width:= 2;
14.   Bmp.Line(10, 10, 100, 100);
15.  
16.   // FIA(Bmp);
17.   Bmp.Free;
18.  
19.   InterfaceBase.WidgetSet.ReleaseDC(THandle(widget), Context);
20.  
21.   Result:= True;
22. end;

In fact, it's almost identical; I hadn't thought of converting PGtkWidget to THandle.

I hope it's useful to someone.