Is there a tutorial to create a visual component for Lazarus?

[garlar27]

Hi,
   I'm trying to create a simple component to display a number and I don't don't have pretty clear which methods i have to override in TGraphicControl or TWinControl to paint my control.

[Leledumbo]

http://wiki.lazarus.freepascal.org/How_To_Write_Lazarus_Component

[garlar27]

Thank you very much!!

Another question:
   Do you know how to redefine the paint method of a TWinControl descendant?

   The component I'm creating contains a TLabel, and some drawings (lines, polygons, and hopefully a glyph). Also, some component's areas must remain transparent.

[Leledumbo]

Do you know how to redefine the paint method of a TWinControl descendant?

Code: [Select]

TMyControl = class
public
  procedure EraseBackground(DC: HDC); override;
  procedure Paint; override;
end;

...

procedure EraseBackground(DC: HDC); override;
begin
  inherited EraseBackground(DC); // default, change as necessary
end;

procedure TMyControl.Paint;
begin
  ... // you control drawing code
  inherited Paint;
end;


[felipemdc]

More generally this explained here:

http://wiki.freepascal.org/Developing_with_Graphics#Create_a_custom_control_which_draws_itself

[garlar27]

I can't find the Paint method in TWinControl what I found where:

Code: [Select]

  TWinControl = class(TControl)
  protected
    procedure PaintControls(DC: HDC; First: TControl); // apparently draws child controls..
    procedure PaintHandler(var TheMessage: TLMPaint);
    procedure PaintWindow(DC: HDC); virtual;
  public
    procedure EraseBackground(DC: HDC); virtual;
    procedure PaintTo(DC: HDC; X, Y: Integer); virtual; overload;
    procedure PaintTo(ACanvas: TCanvas; X, Y: Integer); overload;
  end;
I think that PaintWindow is the one I need to override. Is that true?

[Leledumbo]

Sorry, you should override if your control descends from TCustomControl. TWinControl itself doesn't have any paint method and relies on the built in widgetset functionality to draw itself.

[garlar27]

   Thank you!! Now I can draw and paint in my control!!

   But I can't make it with transparent background... I wonder what am I missing? I've overridden the EraseBackground method but some how it keeps drawing a gray rectangle in the background..

[Leledumbo]

But I can't make it with transparent background... I wonder what am I missing?

I'm not sure about how it's implemented in LCL. Perhaps you can open forms.pp and look for AlphaBlendValue. See how it's used.

I've overridden the EraseBackground method but some how it keeps drawing a gray rectangle in the background..

Did you still call inherited EraseBackground? Where? Before or after your own?

[garlar27]

I'm not sure about how it's implemented in LCL. Perhaps you can open forms.pp and look for AlphaBlendValue. See how it's used.

I will take a look at it..

Did you still call inherited EraseBackground? Where? Before or after your own?

Nope, EraseBackground is empty and I'm not calling it, never the less it is called in somewhere else.

The code for EraseBackground:

Code: [Select]

procedure TNewCmpnt.EraseBackground(DC: HDC);
begin
   //WriteLn('TNewCmpnt.EraseBackground(DC: HDC);'); // To see in console if it's called...
   //inherited EraseBackground(DC);
end;


[Leledumbo]

A quick forum search leads me to this:
http://www.lazarus.freepascal.org/index.php?topic=11019.0

[garlar27]

Thanks again,

TCustomControl currently does no support transparency, you just have to draw the entire area in which it is located. You can, however:

* immitate the color of the parent control.
and
* Set an irregulare shape for the control, using TWinControl.SetShape(TRegion)

I was thinking to do a CopyRect of the area beneath the control and paint it in the control's background. Although, I have doubts about it, I will try to do it if I get the Parent's Canvas or image ...