Get the package url before adding a block

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How can i retrive full package url while adding a block to the page i.e. controller->add function?
I'm trying to do something like this:
$pkg = Package::getByHandle( 'my_package' );
$blockURL = $pkg->getRelativePath();   
$fullURL =  BASE_URL . $blockURL . '/blocks/my_package'

But it's not the best way to get the correct URL

vr5