added a tech note explaining how to write a new XRC handler

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48141 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

diff --git a/docs/tech/index.txt b/docs/tech/index.txt
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--- a/docs/tech/index.txt
+++ b/docs/tech/index.txt
@@ -24,6 +24,6 @@ tn0020.txt   Binary compatibility and wxWidgets
 tn0021.txt   How to add a new wxUSE_XXX preprocessor constant
 tn0022.txt   Working with the wxWidgets release system
 tn0023.txt   Adding a new app/screenshot to the wxWidgets.org front page
-
+tn0024.txt   How to add a new XRC handler
 
 Version: $Id$

diff --git a/docs/tech/tn0024.txt b/docs/tech/tn0024.txt
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@@ -0,0 +1,55 @@
+                         How to add a new XRC handler\r
+                         ============================\r
+\r
+0. Purpose\r
+----------\r
+\r
+This note describes what needs to be done to add a new XRC handler, i.e. add\r
+support for loading the objects of some class wxFoo from XRC.\r
+\r
+\r
+1. Implement the handler\r
+------------------------\r
+\r
+By convention, the XRC handler for a class wxFoo declared in wx/foo.h is called\r
+wxFooXmlHandler and is declared in the file wx/xrc/xh_foo.h (this last rule\r
+wasn't always respected in the past, however it's not a reason to not respect\r
+it in the future). The steps for adding a new handler are:\r
+\r
+a) Add handler declaration in include/wx/xrc/xh_foo.h, it will usually be the\r
+   same as in the other files so you can get inspiration for your brand new\r
+   handler from e.g. wx/xrc/xh_srchctrl.h. Notice the use of wxUSE_FOO if wxFoo\r
+   is guarded by this symbol.\r
+\r
+b) Add implementation in src/xrc/xh_foo.cpp: again, it will be almost always\r
+   very similar to the existing controls. You will need to add support for\r
+   the control-specific styles.\r
+\r
+\r
+2. Update the other files\r
+-------------------------\r
+\r
+There are a few other files to update to make wxWidgets aware of the new\r
+handler:\r
+\r
+a) Add the new files created above to build/bakefiles/files.bkl: search for\r
+   "xh_srchctrl" to see where you need to add them\r
+\r
+b) Add #include "wx/xrc/xh_foo.h" to wx/xrc/xh_all.h.\r
+\r
+c) Register the new handler in wxXmlResource::InitAllHandlers() in\r
+   src/xrc/xmlrsall.cpp\r
+\r
+\r
+3. Update the sample\r
+--------------------\r
+\r
+Demonstrate that the new handler works by adding a control using it to\r
+samples/xrc/rc/controls.xrc.\r
+\r
+\r
+=== EOF ===\r
+\r
+Author:  VZ\r
+Version: $Id: tn0021.txt 47865 2007-08-03 20:00:32Z VZ $\r
+\r