[CMake] Adding non-file dependencies to a custom target

[Giampiero Salvi]

Perhaps simplifying the code would make it clearer:

This is my CMakeLists.txt file (I modified the code from a post in August):

cmake_minimum_required(VERSION 2.6)
PROJECT(TEST)

ADD_CUSTOM_TARGET(foo
  COMMAND echo "foo"
  DEPENDS foo.txt
  COMMENT "Fooing .."
)

ADD_CUSTOM_TARGET(bar ALL
  COMMAND echo "bar"
  COMMENT "Bar-tender .."
)
ADD_DEPENDENCIES(bar foo)

ADD_CUSTOM_COMMAND(
   OUTPUT ${CMAKE_SOURCE_DIR}/bar2.txt
   COMMAND echo bar2 > ${CMAKE_SOURCE_DIR}/bar2.txt
   WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
)
ADD_CUSTOM_TARGET(bar2 ALL
   DEPENDS ${CMAKE_SOURCE_DIR}/bar2.txt
)
ADD_DEPENDENCIES(bar2 foo)


In the case of target bar everything works because bar is built every 
time, but this is not what I want. I want to build the target if and 
*only* if the dependencies have changed. So I use the form in bar2 
instead, as suggested by the documentation. However, in this case if I 
change foo.txt, target foo is built again (as a matter of fact it is 
always built), but target bar2 is not rebuilt.

Can anyone explain why? And tell me how to implement this right?

Thank you,
Giampiero