How to know when a draggable is ready to receive setOffset()

I have a container with a draggable declared like this:

Code:

Ext.define("App.view.FaderControl", {
    extend: 'Ext.Container',
    xtype: 'fadercontrol',
    config: {
        ...
        items: [
            {
                xtype: 'container',
                ...
            },
            {
                xtype: 'container',
                cls: 'fader-knob',
                width: 50,
                height: 80,
                draggable: {
                    direction: 'vertical'
                },
                zIndex: 10
            }
        ],
        listeners: {
            painted: 'updateSizes'
        }
    }

If in that updateSizes method I try to set its initial position (from a dynamic data source) like this,

Code:

var drag = me.down('container[cls=fader-knob]').getDraggable();
drag.setOffset(0, some_val);

I find that the draggable does not change its position. setInitialOffset() doesn't do anything either.

In fact, there appears to be a random amount of "loading" time after the component is painted and before the draggable starts responding to setOffset(). I have quite a few of these things so the delay is in the order of 0.5-1 second on my iPad 3 using the home-screen browser. Is there some sort of event I can hook in to know when it's ready to be updated? The draggable itself only has the drag* events. Thanks.