web\AssetManager $beforeCopy

A PHP callback that is called before copying each sub-directory or file. This option is used only when publishing a directory. If the callback returns false, the copy operation for the sub-directory or file will be cancelled.

The signature of the callback should be: function ($from, $to), where $from is the sub-directory or file to be copied from, while $to is the copy target.

This is passed as a parameter beforeCopy to yii\helpers\FileHelper::copyDirectory().