[CMake] Generate a file with a custom command as part of build

[doug livesey]

Hi -- sorry about the really late reply, I've been away. Up to Hadrian's
Wall, and then for a Dark Skies observatory evening.
It was ace. :)
I've added the following to my CMakeLists.txt file:

> set(WEBPACK ${CMAKE_SOURCE_DIR}/node_modules/.bin/webpack)
> configure_file(webpack.config.js webpack.config.js)
> add_custom_target(public/bundle.js)
> add_custom_command(OUTPUT public/bundle.js COMMAND ${WEBPACK})

However, when I try to run `cmake ..` (I'm using a build subdirectory), I
get the following error:

> CMake Error at CMakeLists.txt:30 (add_custom_target):
>   add_custom_target called with invalid target name "public/bundle.js".
>   Target names may not contain a slash.  Use ADD_CUSTOM_COMMAND to
generate
>   files.

I'm guessing that this means that I should be running some of those
commands in a CMakeLists.txt file in a subdirectory (maybe public/) and
calling `add_subdirectory(public)` from my top-level file?
I've tried this a couple of ways, but so far haven't got it working.
Am I on the right sort of approach, or have I wandered far from the beaten
track?
Thanks again for your help,
   Doug.

On 22 January 2017 at 00:16, Michael Ellery <mellery451 at gmail.com> wrote:

> add_custom_command is the probably the right thing…but it needs to be
> triggered by a dependency relationship. You will specify public/bundle.js
> as the OUTPUT parameter of the add_custom_command and then some other
> target or install command needs to have that file listed in its sources,
> and then CMAKE will understand that it needs to run the custom command to
> generate that OUTPUT file before it runs the dependent target. I think
> add_custom_command can also be explicitly linked to a specific target
> (there are two forms of the command) - and in that case I think it always
> runs (?) when that target rebuilds…I’m not completely sure about that, so
> you might need to test.
>
> Alternatively, if you want to run the generation at the time of makefile
> generation (when CMAKE is run…), then have a look at execute_process. The
> downside of that is that it will generally always run when CMAKE is run,
> but never during the make process so it doesn’t really handle updates to
> the source (input) files very well. I’m not sure which technique applies
> best to your situation here.
>
> HTH,
> Mike
>
> > On Jan 21, 2017, at 4:01 PM, doug livesey <biot023 at gmail.com> wrote:
> >
> > Hi, I want to call the following command as part of a build:
> >
> > $ ./node_modules/.bin/webpack
> >
> > This should generate a file public/bundle.js
> >
> > I'm really struggling with this. I guess there's something fundamental
> that I'm not understanding. `add_custom_command` doesn't seem to be doing
> anything.
> > I would anticipate that it would put the command I want to call in the
> makefile, but it hasn't.
> > If anyone could advise me on what I'm not getting here, that would be
> brilliant.
> > Cheers,
> >    Doug.
> > --
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