RE: [boost] Re: C++ GUI - another view

From: Reece Dunn (msclrhd_at_[hidden])
Date: 2004-09-22 06:41:54

Matt S Trentini wrote:
>John Torjo wrote:
>
>>Could you elaborate a bit? What do you think is the window handle limit 
>>under win32?
>
>I think that perhaps Mathew is talking about the GDI object limit.  At 
>least under Windows 2000 and earlier versions (I'm not sure about XP) there 
>is a firm limit of 16K handles that can be allocated system wide - 
>regardless of your particular specs (ie adding memory does not help). When 
>this limit is reached weird stuff [1] happens which can only be fixed by a 
>reboot.
>
>Any object that allocates a handle under Windows (pens, brushes, dc's, etc) 
>consume those resources and if you've got a graphically intense GUI using a 
>greedy GUI library (ie one that holds it's handles for a long duration) 
>that limit isn't too hard to break.

Wouldn't you process the pen objects, etc. when handling the drawing event. 
Instead of storing a pen handle for each line (which IMHO is overkill), you 
would do something like:

namespace gdi = boost::gdi;
namespace colors = boost::gdi::colors;
namespace units = boost::units;

void draw( gdi::canvas & c )
{
   gdi::pen mypen( colors::lightskyblue );
   gdi::font myfont( "Times New Roman", units::picas( 12 ));

   c.set_units( units::picas );
   c.attach( mypen );
   c.attach( myfont );
   c.set_color( colors::red );

   c.draw_string( 1, 1, "Hello World!" ); // uses myfont + colors::red
   c.line( 10, 10, 10, 15 ); // uses mypen
   c.line( 10, 15, 15, 15 ); // uses mypen
   c.line( 10, 10, 15, 15 ); // uses mypen

   c.attach( gdi::default_pen( colors::red ));
   // ...
}

Likewise, with the component framework (windows, widgets), it should be 
possible to implement windowless components that do not have a handle 
attached to them. This would be useful when implementing the layout 
managers.

Handling an event should not be bound to the component that generates them. 
For example, MFC provides handling the exit message on the application 
object and not the main window object. This would then allow you to bind 
events to a graphical object without using a window handle.

IMHO, line information should be separate from its colour, thickness, etc. 
Otherwise, you would need to do something like:

class line_object
{
   private:
      unit x1, y1, x2, y2;
      gdi::pen * pen;
   public:
      void draw( gdi::canvas & c )
      {
         c.attach( *p );
         c.draw_line( x1, y1, x2, y2 );
      }
};

which wastes a call to attach the pen each time. This is especially bad if 
you have 1000s of lines (e.g. drawing a hilbert curve or serpinski's 
triangle fractal with a fractal depth of 4 or more).

Regards,
Reece

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