[CMake] Understanding dependencies with an external project

[Tom Davis]

Hi there, I only started using CMake a couple days ago so please pardon
the newbie questions. I'm having some trouble understanding the
relationships between add_dependencies, add_custom_command, and
add_custom_target as they relate to forming dependencies on external
project targets.

I am trying to write plugins for a CMake-enabled project. How one goes
about this is by getting the source and adding an optional cmake module
that calls what amounts to a custom externalproject_add() with no
build/install step; the external sources are just copied into the
framework's source tree. That way, when I call `make`, the framework
build includes the plugins. (This inflexible "linking" mechanism is
necessary due to language constraints that aren't relevant here)

The way I've integrated this into my own project is via my own
externalproject_add() that grabs the framework and, as a custom step,
copies over the module I mentioned:

    ExternalProject_Add(framework
      # ...
    )

    ExternalProject_Add_Step(framework copy-plugin-loader
      # First symlink so the framework will "download" from local fs
      COMMAND ${CMAKE_COMMAND} -E create_symlink "${CMAKE_CURRENT_SOURCE_DIR}" "${framework_EXTERNALS_DIR}/my-plugins"
      # Now link the custom module it needs to find in its modules dir
      COMMAND ${CMAKE_COMMAND} -E create_symlink "${CMAKE_CURRENT_BINARY_DIR}/CMake/plugin_loader.cmake" "${heka_SOURCE_DIR}/cmake/plugin_loader.cmake"
      DEPENDERS configure
      DEPENDEES download
    )

Later on in my CMakeLists I use add_test() and use `ctest` to run my
plugin tests (that only work once my source is grafted onto the
framework's tree; see weird language design choice mentioned above).

My question (finally) is this: how can I tell CMake that, if I change a
source or test file in *my* tree, the framework (specifically, a custom
target defined in _the framework's_ CMakeLists) needs to be rebuilt
before the tests can run? To illustrate, here's my current workflow:

    # Edit *my* source files, tests, whatever, then...
    % cd framework/binary_dir
    % make graft_plugin_sources
    % cd -
    % ctest .

Ideally, I could just run `ctest` and CMake will handle the rest. I
don't know if that's a thing it does, so, I'd settle for calling the
top-level `make` again. I just can't figure out the proper add_*() calls
to make to tell CMake "My project (and/or custom target/command of my
own) depends on this particular target of `framework` so whenever I
change a source file in my project, we gotta re-run it."

Thanks in advance for any insight,

Tom