[cmake-developers] Package Config files with COMPONENTS

[Brad King]

On 3/10/2012 1:15 PM, Alexander Neundorf wrote:
> I pushed a branch ImprovedCOMPONENTSSupportInFindPackage to stage.
>
> It is not finished (documentation and an updated fphsa() are missing at
> least), but you may want to have a look at it.
>
> It adds
> * OPTIONAL_COMPONENTS

Will this interact well when a Find module forwards with NO_MODULE?

> * CMP0018: if a listed component is not found, Foo_FOUND is set to false in
> Config mode

I do not think CMP0018 can work quite as we need.  If a project sets
the behavior to NEW how does it know the package configuration file
it finds will be aware that it needs to set the Foo_<component>_FOUND
variables?  That is not something that cmake_minimum_required(VERSION)
can know when it sets the policy to NEW.

We don't know how packages out there interpret components.  For some
it may just be a list which can always be satisfied.  Others may
already provide a value equivalent to Foo_<component>_FOUND but with
a different name that makes more sense for their use case.

The relationship of <pkg>_FOUND and <pkg>'s components is specific to
each package.  I think it should simply be up to the package config
file for the package to set <pkg>_FOUND based on its ability to
satisfy the requested components in whatever sense is meaningful
for the given package.  This will be possible after the topic
find_package_set_FOUND_VariableInConfigFile is done.

I think OPTIONAL_COMPONENTS may be worthwhile but that is independent
from the proposed policy.

-Brad