cmake does not copy file in custom command - cmake

Question

I want to copy qml files from source directory to build directory. Following script works fine only first time. When I change any of the *.qml files and run make, they are not copied to build folder, they are not updated. What am I doing wrong?
file(GLOB_RECURSE SOURCES *.cpp)
file(GLOB_RECURSE QMLS *.qml)
add_library(MyLib SHARED ${SOURCES} ${QMLS})
foreach(QmlFile ${QMLS})
add_custom_command(TARGET MyLib POST_BUILD
COMMAND ${CMAKE_COMMAND} -E copy_if_different
${QmlFile} $<TARGET_FILE_DIR:MyLib>)
endforeach()

Answer 1

While Angew's answer is good, it is possible to eliminate usage of additional target. For doing this, add_library call should use copied .qml files (instead of original ones, like in the script in the question post):
# This part is same as in Angew's answer
set(copiedQmls "")
foreach(QmlFile ${QMLS})
get_filename_component(nam ${QmlFile} NAME)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${nam}
COMMAND ${CMAKE_COMMAND} -E copy_if_different ${QmlFile} ${CMAKE_CURRENT_BINARY_DIR}
DEPENDS ${QmlFile}
COMMENT "Copying ${QmlFile}"
VERBATIM
)
list(APPEND copiedQmls ${CMAKE_CURRENT_BINARY_DIR}/${nam})
endforeach()
# But instead of creating new target, we reuse library one.
add_library(MyLib SHARED ${SOURCES} ${copiedQmls})
When library target is built, it triggers non-sources files in add_library call to be updated (if there is corresponded add_custom_command call), but updating non-source files doesn't force library file to be rebuilt. This is why your original code doesn't work as expected.
Note, because .qml files are not recognized by CMake as sources, you doesn't need to set GENERATED property for them, as stated here.

Answer 2

Your are using the TARGET signature of add_custom_command, which means the commands are executed as part of building the TARGET. In your case, POST_BUILD, which means the commands will be run after the build of MyLib finishes. If MyLib is up to date and does not need to be re-built, the commands will not run.
You might want to use the output-generating signature (OUTPUT) instead. Something like this:
set(copiedQmls "")
foreach(QmlFile ${QMLS})
get_filename_component(nam ${QmlFile} NAME)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${nam}
COMMAND ${CMAKE_COMMAND} -E copy_if_different ${QmlFile} ${CMAKE_CURRENT_BINARY_DIR}
DEPENDS ${QmlFile}
COMMENT "Copying ${QmlFile}"
VERBATIM
)
list(APPEND copiedQmls ${CMAKE_CURRENT_BINARY_DIR}/${nam})
endforeach()
add_custom_target(
CopyQMLs ALL
DEPENDS ${copiedQmls}
)
Note that unfortunately, the OUTPUT argument of add_custom_command does not support generator expressions, so $<TARGET_FILE_DIR> cannot be used there. I used ${CMAKE_CURRENT_BINARY_DIR} in the example above, you might need to customise this to suit your needs. ${CMAKE_CFG_INTDIR} can be used to specify the per-configuration directory for multiconfiguration generators.
Note the presence of the additional custom target in my code above. That's necessary, because CMake will only execute an output-producing custom command if something depends on that output. The custom command does that, by listing the outputs in its DEPENDS section.