doneRendering can't call other functions in the controller?

Question

I wanted to call a function out of the controller "doneRendering"

CMP file:

<aura:handler event="aura:doneRendering" value="{!this}" action="{!c.doneRendering}"/>

Controller:

    doneRendering: function(component) {....}

But every time I include a function (that is not even in the helper file) I end up getting the function undefined error. Has anyone figured out a way around that?