[cmake-developers] CMAKE_TOOLCHAIN_FILE and ExternalProject_Add

[Andrey Pokrovskiy]

Hi,

I'm using cmake and CMAKE_TOOLCHAIN_FILE to build the project and it's
dependencies. Some toolchain files have additional options (like API
version, target architecture, linker type, etc.) which you need to
pass to cmake command with "-D" in addition to the
"-DCMAKE_TOOLCHAIN_FILE=" itself.

So when you want to build some dependency with ExternalProject_Add()
it's not enough to pass the same CMAKE_TOOLCHAIN_FILE. You also need
to explicitly specify all other toolchain variables that you care
about. That means that each time you start to override another
toolchain variables or add a new toolchain, you need to carefully add
proper variable forwardings to each ExternalProject_Add() call. This
is not convenient and error prone.

What do you think about formalizing toolchains a bit further? For
example, it would be nice to have a way to get all variables (with
their values) that are relevant for the current toolchain. Consider:

    ExternalProject_Add(websockets_ep
        CMAKE_ARGS
            -DCMAKE_TOOLCHAIN_FILE:string=${CMAKE_TOOLCHAIN_FILE}
            -DCMAKE_TOOLCHAIN_ARGS:string=${CMAKE_TOOLCHAIN_ARGS})

Where ${CMAKE_TOOLCHAIN_ARGS} expands to "VAR1=VAL1;VAR2=VAL2;" (I
realize that will not work like that, but conceptually) which defines
toolchain configuration.

For example, well behaved toolchain file could be asked to mark each
configuration variable:

    toolchain_option(ANDROID_NATIVE_API_LEVEL)

so CMake after processing of a toolchain file can have a list of all
relevant variables. Or maybe CMake can track all variables "touched"
by the toolchain file.

What do you think about that?