Function "before" for attribute not working

[distroid]

Hi, how can i use method "before" for custom methods? Exapmle:

$mapper->string('role')->before("\User::getLabel");

I have error

Call to undefined function \User::getLabel() 

but if i add hardcode function in your ListBuilder->build()

if ($clone->hasBefore()) {
    $before = $clone->getBefore();
    $value  = \User::getLabel($value);
}

all works, what am I doing wrong?