How to make CMake target executed whether specified file was changed?

Question

I'm trying to use ANTLR in my C++ project. I made a target for running ANTLR generator for specified grammar and made main prjct dependent from it.

ADD_CUSTOM_TARGET(GenerateParser
    COMMAND ${ANTLR_COMMAND} ${PROJECT_SOURCE_DIR}/src/MyGrammar.g 
                             -o ${PROJECT_SOURCE_DIR}/src/MyGrammar
)

ADD_LIBRARY(MainProject ${LIBRARY_TYPE} ${TARGET_SOURCES} ${TARGET_OPTIONS})
ADD_DEPENDENCIES(MainProject GenerateParser)

The problem is that ANTLR generator running every time I build project and consumes enough time. How can I make it run only whether my grammar has been changed? Or may be it is possible to make ANTLR somehow generate parser only for out of date grammar.

Answer 1

add_custom_command will do the trick, if you construct the call to it correctly.

Something like this should work:

ADD_CUSTOM_COMMAND(OUTPUT ${PROJECT_SOURCE_DIR}/src/MyGrammar
  COMMAND ${ANTLR_COMMAND} ${PROJECT_SOURCE_DIR}/src/MyGrammar.g
    -o ${PROJECT_SOURCE_DIR}/src/MyGrammar
  DEPENDS ${PROJECT_SOURCE_DIR}/src/MyGrammar.g
)

ADD_CUSTOM_TARGET(GenerateParser ALL
   DEPENDS ${PROJECT_SOURCE_DIR}/src/MyGrammar
)

ADD_LIBRARY(MainProject ${LIBRARY_TYPE} ${TARGET_SOURCES} ${TARGET_OPTIONS})
ADD_DEPENDENCIES(MainProject GenerateParser)

Here, the custom target will "build" every time, but the only thing it will do is build the custom command on whose output it depends, but if and only if the output of the custom command is out of date with respect to its DEPENDS file(s).