cmake add_custom_command and DEPENDS/TARGET - cmake

Question

I have this in my toplevel CMakeLists.txt:
add_subdirectory (src) # add_executable (${PROJECT_NAME} ${_SOURCES})
add_subdirectory (data)
In data subdirectory I want to create a file, when ${PROJECT_NAME} is build. The following doesn't work, returns target "foo" does not exist:
add_custom_command(
OUTPUT "${PROJECT_BINARY_DIR}/${PROJECT_NAME}.desktop"
WORKING_DIRECTORY "${PROJECT_BINARY_DIR}"
COMMAND ${INTLTOOLMERGE} -d -u ../po ${PROJECT_NAME}.desktop.in "${PROJECT_NAME}.desktop"
COMMENT "Creating desktop file"
DEPENDS ${PROJECT_NAME}
)
This also doesn't work. Returns: The target name "foo" is unknown in this context
add_custom_command(
TARGET ${PROJECT_NAME}
WORKING_DIRECTORY "${PROJECT_BINARY_DIR}"
COMMAND ${INTLTOOLMERGE} -d -u ../po ${PROJECT_NAME}.desktop.in "${PROJECT_NAME}.desktop"
COMMENT "Creating desktop file"
)
But this works as expected:
add_custom_command(
OUTPUT "${PROJECT_BINARY_DIR}/${PROJECT_NAME}.desktop"
WORKING_DIRECTORY "${PROJECT_BINARY_DIR}"
COMMAND ${INTLTOOLMERGE} -d -u ../po ${PROJECT_NAME}.desktop.in "${PROJECT_NAME}.desktop"
COMMENT "Creating desktop file"
)
add_custom_target (desktopfile DEPENDS "${PROJECT_BINARY_DIR}/${PROJECT_NAME}.desktop")
add_dependencies (${PROJECT_NAME} desktopfile)
Question:
1.- How can I use add_custom_command to run the command when "foo" is build without using a target?
2.- How come add_dependencies knows about "foo" but pure add_custom_command doesn't?
update #1: Simple code:
# /CMakeLists.txt
cmake_minimum_required (VERSION 3.0)
project ("foo")
add_subdirectory (src)
add_subdirectory (data)
# : EOF~
# src/CMakeLists.txt
add_executable (${PROJECT_NAME} main.c)
# : EOF:~
# data/CMakeLists.txt
add_custom_command (
TARGET ${PROJECT_NAME}
POST_BUILD
COMMAND ${CMAKE_COMMAND} -E sleep 5
COMMENT "We're going to try to pause here for 5 seconds"
)
# EOF:~
# src/main.c
#include <stdio.h>
int main () {
printf ("Hello world");
return 0;
}

Answer 1

Please use POST_BUILD option.
From the cmake documentation:
add_custom_command(TARGET target
PRE_BUILD | PRE_LINK | POST_BUILD
COMMAND command1 [ARGS] [args1...]
[COMMAND command2 [ARGS] [args2...] ...]
[WORKING_DIRECTORY dir]
[COMMENT comment] [VERBATIM])
This defines a new command that will be associated with building the specified target. When the command will happen is determined by which of the following is specified:
PRE_BUILD - run before all other dependencies
PRE_LINK - run after other dependencies
POST_BUILD - run after the target has been built
Note that the PRE_BUILD option is only supported on Visual Studio 7 or later. For all other generators PRE_BUILD will be treated as PRE_LINK.