dialogDefinition.onShow adding event handler

I am trying to add a event handler to all dialogs onShow to get their height/width.
The problem is that when I say objDialogDefinition.onShow = function(){...} It appears I overwrite the function onShow from the dialog... What happens for example, I insert an image and all is well.. I right-click the image and choose properties, none of image dialog fields gets populated..

How can I add my handler alongside the one for the dialog?

The code is in my editor.htm script tag:

CKEDITOR.on('dialogDefinition', function (e) {
    var dialogName = e.data.name;
    var dialogDefinition = e.data.definition;
    dialogDefinition.onShow = function () {
        var h = this.getSize().height;
        var w = this.getSize().width;
    };
    dialogDefinition.onHide = function () { alert('dialog closed'); }
});