Triggering rebuild after changing a *.json file

[Milian Wolff]

On Sunday 23 November 2014 10:39:02 Andreas Pakulat wrote:
> Hi,
> 
> On Sun, Nov 23, 2014 at 4:36 AM, Milian Wolff <mail at milianw.de> wrote:
> > Hey all,
> > 
> > in my quest for better *.json support in KF5 based applications, I noticed
> > that we currently do not rebuild properly on changes to the *.desktop or
> > *.json files.
> > 
> > For KDevelop, I'm thus playing around with something like this currently:
> > 
> > ~~~~~~~~~~~~~~~+
> > function(kdevplatform_add_plugin plugin)
> > 
> >     set(options )
> >     set(oneValueArgs JSON)
> >     set(multiValueArgs SOURCES)
> >     cmake_parse_arguments(KDEV_ADD_PLUGIN "${options}" "${oneValueArgs}"
> > 
> > "${multiValueArgs}" ${ARGN})
> > 
> >     string(REGEX REPLACE "\\.cmake$" "" json_out ${KDEV_ADD_PLUGIN_JSON})
> >     configure_file(${KDEV_ADD_PLUGIN_JSON}
> > 
> > ${CMAKE_CURRENT_BINARY_DIR}/${json_out})
> > 
> >     # ensure we recompile the corresponding object files when the json
> >     file
> > 
> > changes
> > 
> >     set(dependent_sources )
> >     foreach(header ${KDEV_ADD_PLUGIN_SOURCES})
> >     
> >         file(STRINGS "${header}" match REGEX "K_PLUGIN_FACTORY_WITH_JSON")
> >         if(match)
> >         
> >             list(APPEND dependent_sources "${header}")
> >         
> >         endif()
> >     
> >     endforeach()
> >     if(NOT dependent_sources)
> >     
> >         # fallback to all sources - better safe than sorry...
> >         set(dependent_sources ${KDEV_ADD_PLUGIN_SOURCES})
> >     
> >     endif()
> >     set_property(SOURCE ${dependent_sources} APPEND PROPERTY
> >     OBJECT_DEPENDS
> > 
> > ${CMAKE_CURRENT_BINARY_DIR}/${json_out})
> > 
> >     add_library(${plugin} MODULE ${KDEV_ADD_PLUGIN_SOURCES})
> >     set_property(TARGET ${plugin} APPEND PROPERTY AUTOGEN_TARGET_DEPENDS
> > 
> > ${CMAKE_CURRENT_BINARY_DIR}/${json_out})
> > endfunction()
> > ~~~~~~~~~~~~~~~+
> > 
> > 
> > To be used like this:
> > 
> > kdevplatform_add_plugin(kdevgit JSON kdevgit.json.cmake SOURCES
> > ${kdevgit_PART_SRCS})
> > 
> > This does trigger a rebuild, but the strings are still not updated
> > properly.
> > I'm quite confused actually, does anyone know where the code comes from
> > that
> > is embedded into the *.o that uses Q_PLUGIN_METADATA? I suspect CMake
> > AUTOGEN?
> > But where does that put its generated binary JSON representation? How can
> > I
> > make sure that it gets updated properly when the source file changes?
> > 
> > To reproduce this, you can just change any *.desktop file that is piped
> > through desktop_to_json. The change will be picked up by CMake and a
> > reconfigure is triggered, which is pretty slow as well. But nothing is
> > rebuilt. With the macro above, I trigger the build but still, the *.o file
> > that uses K_PLUGIN_FACTORY_WITH_JSON is still containing the "old"
> > strings...
> > I'm at loss - can someone help me please? <http://milianw.de>
> 
> Is there a particular reason why you run the to-json part during cmake
> time? If you setup a simple cmake script you could easily switch the
> generation to be a custom target and then have the plugin depend on that
> custom target (or rather its json output file).

That was apparently a side-effect of KDevelop using configure_file to embed 
the version number in the *.desktop file. This is not required anymore with 
the JSON-based plugin loading mechanism, as we install plugins into a 
versioned directory path. I've changed this now and the CMake configure step 
when changing a file is gone. Note that desktop_to_json already uses a custom 
target internally.

> I also think you're misunderstanding the AUTOGEN_TARGET_DEPENDS property,
> according to the documentation it is to be set on an auto-generator target
> and not on a 'standard' one like the library. As far as I can see you know
> the input and output files of the desktop-to-json conversion so a custom
> target should be easily doable (unlike automoc).

This might be true, yes. Note that I just copied that line from 
desktop_to_json. But note that in the ideal case, we wouldn't have any 
desktop-to-json. Rather, we just have a *.json file, a *.cpp file that uses 
K_PLUGIN_FACTORY_WITH_JSON. Currently, when the *.json file is changed, the 
*.cpp.o is not updated, nor the plugin *.so rebuilt and thus the new strings 
are not available at runtime.

This is what we need to fix somehow, and I still don't know how. Will we have 
to fix this inside CMake?

-- 
Milian Wolff
mail at milianw.de
http://milianw.de