feat: add Less custom function extender, is-extension-enabled function

[davwheat]

Changes proposed in this pull request:

• create a new custom less function function on our Theme extender
• creates a new Less function for applying styles only when a specific extension is enabled

This prevents stylesheet bloats from extensions which extend multiple extensions, by only adding styles which are needed.

Extender usage:

    (new Extend\Theme())
        ->addCustomLessFunction('test', function () {
            return new \Less_Tree_Quoted('', 'true');
        })

Is ext enabled func usage:

// Is extension enabled function:
body when (is-extension-enabled('flarum-tags') = true) {
  // enabled
}
body when (is-extension-enabled('flarum-tags') = false) {
  // not enabled
}

Reviewers should focus on:

(no longer applicable)
Would a simpler syntax be better, such as:

.if-extension-enabled(flarum-tags, {
  // ...
});

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[askvortsov1]

Would a simpler syntax be better, such as:

I think so. What's the benefit of doing it the other way?

[davwheat]

Would a simpler syntax be better, such as:

I think so. What's the benefit of doing it the other way?

There's no inherent benefit of either way, I don't believe. Just syntactically different.

[davwheat]

So I've pushed an alternate syntax which I think may be better.

The original syntax required that the mixin be declared in the root scope, meaning that

// This would work
.if-extension-enabled(@ext) when (@ext = flarum-tags) {
  body { background: black; }
}

// ...but this would NOT
body {
  .if-extension-enabled(@ext) when (@ext = flarum-tags) {
    background: black;
  }
}

The new syntax, however, does support this:

// This would work
.when-extension-enabled(flarum-tags, {
  body { background: black; }
});

// ...and so does this
body {
  .when-extension-enabled(flarum-tags, {
    background: black;
  });
}

However, the newer method will be significantly less performant, since it performs a loop over all enabled extensions each time it is called, rather than only doing it once.

We could reduce that by setting a less variable per enabled extension (e.g. @extension-enabled--flarum-tags), then use Less's variable-variables support to check for these, but that seems a bit hacky, imo.

[SychO9]

What worries me about this is that, even without this, the Less compiler breaks the forum sometimes by running out of memory, it has been reported multiple times before, I've even experienced it locally, so I'm personally very reluctant to introducing anything that could reduce its performance even less.

How about registering a custom function instead ? I haven't actually tested the difference (yet) but I'm assuming it would be a more efficient solution. (tested locally and it works).

.App {
  & when(is-extension-enabled('flarum-tags') = true) {
    opacity: 0.3;
  }
}

$parser->registerFunction('is-extension-enabled', function (Less_Tree_Quoted $extensionId) {
    return new Less_Tree_Quoted('', in_array($extensionId->value, ['flarum-tags', 'flarum-blog']) ? 'true' : 'false');
});

[davwheat]

That does seem like a much better idea.

When I saw the documentation of less-php state that custom functions weren't supported, I assumed it meant completely, but if we can define them in PHP, that's perfect!

[davwheat]

@SychO9 Should be done now :)

[askvortsov1]

Overall makes sense, but is the = true / = false really necessary? Is there no "not" symbol

[SychO9]

LGTM!

just nitpicks, because they're fun :p