[CMake] Deep dependency checking for custom commands?

[Brad King]

Chris Green wrote:
> If I have a custom command and I declare a header file as a dependency, 
> is there any easy way to tap into CMake's deep dependency checking? ie 
> if a header included by that explicitly declared dependency changes, can 
> I arrange to have the target regenerated?

Unfortunately there is no easy way to do this now.  Having customized 
dependency scanning would be very helpful.  Something like

ADD_CUSTOM_COMMAND(
   OUTPUT myout.txt
   COMMAND do_something
   DEPENDS_SCANNER my_custom_depends_scanner myinput.txt
)

and then the CMake scanner for C could be exposed so that it could be 
used as a DEPENDS_SCANNER.

Please submit a feature request here:

http://www.cmake.org/Bug

...but it looks like a non-trivial feature so it may not get done for a 
while.

Meanwhile you might be able to hack something similar to what is done in 
ITK for custom dependencies.  See the source here:

http://www.itk.org/cgi-bin/viewcvs.cgi/Wrapping/CSwig/CMakeLists.txt?rev=1.55&root=Insight&view=auto

Basically the custom command runs the first time because the output is 
not present.  The custom command generates its output AND a list of 
dependencies.  The list is dumped via copy-if-different to a file that 
is read by CMake.  This causes CMake to re-run and include the list of 
dependencies in the DEPENDS argument of the custom command.  It is an 
ultra-hack that seems to work most of the time.  If you code something 
similar it will probably get you by until a real feature can be implemented.

-Brad