Manage padding for a NSTextField

I've create a subclass of NSTextField to manage its background color, text color, insertion point color and so on ...

I'd like to improve customized aspect of the objects from this class, but i can't understand how to manage size. If i change frame.size.height property the field grows from bottom but text开发者_运维技巧's still placed at the top of the field.

Here a screen to describe what i mean:

I'd like to be able to manage a sort of "padding" and maintain text on the center of the field .