cms.combiner.beforePrepare

Triggered in System\Classes\CombineAssets.

Provides an opportunity to interact with the asset combiner before assets are combined

Example usage:

Event::listen('cms.combiner.beforePrepare', function ((\System\Classes\CombineAssets) $assetCombiner, (array) $assets) {
    $assetCombiner->registerFilter(...)
});