[fpc-pascal] Abstract class implementation on different platforms

[Graeme Geldenhuys]

Hi,

How do I do the following using FPC with Makefile's or a Lazarus Package?

I have a TGfxCanvas abstract class (something like the TFPCustomCanvas
class in the graphics.pp unit) and it gets implemented differently for
different platforms. Currently Win32 and X11, but others will follow
later.

How do I structure my source directories and units so that I can
compile my library under both platforms with the same Makefile or
Lazarus Package.

At the moment I have two different Lazarus Packages. One for each
platform, but this is really not ideal, because if an application uses
the Linux package, all is fine.  Now if you want to compile that
project under Windows, you need to change the Project Requirement, to
now include the Windows package.  :-(

I don't want to use IFDEF's in my code if possible.  A bad example is
the gfxbase.pp unit in Lazarus's LCL, which I think is a nightmare...
It is full of {$IFDEF Win32} ... {$else} .... statements.  What
happens to all that code, when more platforms get added?

At the moment my library is in the following directory structure:

src/
   /common         <- some common units for all platforms.
   /x11                 <- X11 implementation of TGfxBase
      /gfximpl.pp
      /gfx_x11.pp
   /win32gdi          <- win32 implementation of TGfxBase
      /gfximpl.pp
      /gfx_gdi.pp
   /gfxbase.pp       <- initial library unit (start of project)


The library includes a unit called "gfximpl.pp" which gets set
differently for each platform, aliasing the classes to a common
classname.

example "gfximpl.pp" for X11:
  type
    TDefWindow = TXWindow;
    TDefDisplay = TXDisplay;

example "gfximpl.pp" for Win32 GDI:
  type
    TDefWindow = TGDIWindow;
    TDefDisplay = TGDIDisplay;


So now any project using this library can refer to TDefWindow, and
depending on which platform it was compiled, it will include the
correct implementation of TGfxCanvas.
I still have the issue with the Lazarus Packages though. Each platform
needs a different search path to find the "gfximpl" unit, hence the
two packages (one for each platform).

Is there a better way to do this?  Makefiles or Lazarus Packages?
Surely it is done somehow in FPC considering it supports so many
platforms...

Many thanks for any help.

Regards,
  - Graeme -


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