Drawing the Component Image

The essential element of a graphic control is the way it paints its image on the screen. The abstract type TGraphicControl defines a method called Paint that you override to paint the image you want on your control.

The Paint method for the shape control needs to do several things:

Use the pen and brush selected by the user.
Use the selected shape.
Adjust coordinates so that squares and circles use the same width and height.

Overriding the Paint method requires two steps:

Add Paint to the component's declaration.
Write the Paint method in the implementation part of the unit.

For the shape control, add the following declaration to the class declaration:

Copy Code

type
  TSampleShape = class(TGraphicControl)
  .
  .
  .
  protected
    procedure Paint; override;
  .
  .
  .
  end;

Copy Code

class PACKAGE TSampleShape : public TGraphicControl
{
  .
  .
  .
protected:
    virtual void __fastcall Paint();   
    .
    .
    .
};

Then write the method in the implementation part of the unit:

Copy Code

procedure TSampleShape.Paint;
begin
  with Canvas do
  begin
    Pen := FPen;                                              { copy the component's pen }
    Brush := FBrush;                                        { copy the component's brush }
    case FShape of
      sstRectangle, sstSquare:
        Rectangle(0, 0, Width, Height);                    { draw rectangles and squares }
      sstRoundRect, sstRoundSquare:
        RoundRect(0, 0, Width, Height, Width div 4, Height div 4); { draw rounded shapes }
      sstCircle, sstEllipse:
        Ellipse(0, 0, Width, Height);                                { draw round shapes }
    end;
  end;
end;

Copy Code

void __fastcall TSampleShape::Paint()
{
  int X,Y,W,H,S;
  Canvas->Pen = FPen;                                // copy the component's pen
  Canvas->Brush = FBrush;                            // copy the component's brush
  W=Width;                                           // use the component width
  H=Height;                                          // use the component height
  X=Y=0;                                             // save smallest for circles/squares
  if( W<H )
    S=W;
  else
    S=H;
  switch(FShape)
  {
    case sstRectangle:                                // draw rectangles and squares
    case sstSquare:
      Canvas->Rectangle(X,Y,X+W,Y+H);
      break;
    case sstRoundRect:                                // draw rounded rectangles and squares
    case sstRoundSquare:
      Canvas->RoundRect(X,Y,X+W,Y+H,S/4,S/4);
      break;
    case sstCircle:                                   // draw circles and ellipses
    case sstEllipse:
      Canvas->Ellipse(X,Y,X+W,Y+H);
      break;
    default:
      break;
  }
}

Paint is called whenever the control needs to update its image. Controls are painted when they first appear or when a window in front of them goes away. In addition, you can force repainting by calling Invalidate, as the StyleChanged method does.

What do you think about this topic? Send feedback!