How to add dependencies to a custom_target() after the fact? - cmake

Question

I'm trying to have a top level target, we'll call it sdk_extraction that is tested/built every time, and had depends on a bunch of generated files that are basically copies or minor transformations of files in the tree.
I build up the list of files to export as a property string/list, then when it's all said and done take that list and iterate through it, creating custom_commands for each one and then adding a dependency between the output file and the sdk_extraction target.
function(finalize_sdk_export destination)
file(REAL_PATH ${destination} destination )
add_custom_target(sdk_target ALL
COMMENT "Exporting SDK"
)
get_property(sdk_source GLOBAL PROPERTY SDK_SOURCE )
string(REPLACE " " ";" sdk_source "${sdk_source}")
foreach (item ${sdk_source})
string(REPLACE ${destination} ${destination}/sdk target "${item}")
add_custom_command(
OUTPUT ${target}
DEPENDS ${item}
COMMAND ${CMAKE_COMMAND} -E copy ${item} ${target}
VERBATIM
)
add_dependencies(sdk_target ${target})
endforeach()
endfunction()
This fails with an error:
CMake Error at <redacted>/build_scripts/CMakeLists.txt:209 (add_dependencies):
The dependency target
"<redacted>/sdk/CMakeLists.txt"
of target "sdk_target" does not exist.
Call Stack (most recent call first):
<redacted>/CMakeLists.txt:78 (finalize_sdk_export)
Does this not work? I WAS able to do this:
function(finalize_sdk_export destination)
file(REAL_PATH ${destination} destination )
get_property(sdk_source GLOBAL PROPERTY SDK_SOURCE )
string(REPLACE " " ";" sdk_source "${sdk_source}")
foreach (item ${sdk_source})
string(REPLACE ${destination} ${destination}/sdk target "${item}")
add_custom_command(
OUTPUT ${target}
DEPENDS ${item}
COMMAND ${CMAKE_COMMAND} -E copy ${item} ${target}
VERBATIM
)
list(APPEND sdk_files ${target})
endforeach()
add_custom_target(sdk_target ALL
COMMENT "Exporting SDK"
DEPENDS ${sdk_files}
)
endfunction()
Where I make all the custom_commands and keep a list of each ones output, then make the call to add_custom_target() with them all specified in the DEPENDS parameter.
This works for now, but I'm still just testing and I'm unsure it'll hold up if I end up having hundreds/thousands of files in the export.
So, how am I supposed to do this correctly?
(FYI, I gave up with the built in cmake install/export functionality because we really want to have control over the directory structure and look/feel of the CMake files)

Answer 1

All files, from which a custom target depends, should be specified in the DEPENDS option for add_custom_target() call. There is no way to add dependencies from the files afterwards.
The command add_dependencies can add only dependencies between targets.
Unlike to add_dependencies, which may connect targets creates in different directories, all generated files in DEPENDS clause should be created by add_custom_command in the same directory only. That is, it is usually not
a problem to postpone creation of the target unless all dependent files are calculated.