[gentoo-dev] Future EAPI Idea: post source hook for eclasses

[Kent Fredric]

I often see one of two patterns:

Pattern A)  Code Before `inherit` to drive behaviour

Example A.1

SOME_VALUE="Foo"
inherit someclass
OTHERVALUE=Bar"
inherit someotherclass

This is used heavily in things using perl-module.eclass

Pattern B)  Require either function calls or have a finalising function
call to apply settings.

Example B.1

inherit someclass
SOME_VALUE="Foo"
SOME_OTHER_VALUE="Bar"
someclass_post_src;

Example B.2

inherit someclass
someclass_set_some_value "Foo"
someclass_set_some_other_value "Foo"


Examples A.1 and B.1 essentially share the same logic: define a value, can
call some function that augments that value into other values ( and in the
case of A.1, the function is called "inherit" )

A.1 is the least-worst here, its just ugly and you have to remember to put
the inherit *after* the variable or the variable will behave as if it
wasn't there.

And B.1 has the convenient defect that if you forget to call the finalising
function at the end, the ebuild may be radically different.

B.2 Seems to make things much more complicated for eclass authors.

<< TO THE POINT >>

What I keep wanting to have is a hook that triggers after the ebuild is
loaded, defined in the eclass.

> source Foo.ebuild
>> see inhert bar
>> source bar.eclass
>>> bar.eclass registers a hook bar_mauve
>>> returns to processing Foo.ebuild
>> Process code in Foo.ebuild
> return to eclass handeler
> trigger bar_mauve

Note I've conveniently called it "bar_mauve" here because I have no idea
what a good name will be, and establishing if the concept is useful or not
should be done before we decide what we'll call it.

The hook should be able to see and modify any variables that are to be
expected visible to the ebuild itself, and the primary expected use would
be augmenting exported variables based on other variables, like we
presently do during ebuild sourcing in perl-module.eclass ( which is mostly
computing SRC_URI from other variables ), essentially making it easier to
write variable declaration based ebuilds.





-- 
Kent