Flash - Calling DispatchEvents Without Using Xml Schema In Flex?

I wanted to know if there are ways to use dispatchEvents with custom event function calls without actually instantiate the function in an xml form but doing so in the script. What I am trying to do is, have a developer use my custom component button, but add his/her own function to what is already added to my own click event call. so just as a case he/she wants to add there own validation function of that click event. One way i have it is where i create an instance in my component within the xml schema.i have a mxml component called DialogWindow.mxml and inside I have

<mx:Metadata>
[Event(name="cancBtnClick", type="flash.events.Event")]
[Event(name="okBtnClick", type="flash.events.Event")]
</mx:Metadata>

and in my script in DialogWindow.mxml i created a button with a click event called cancelAction(event);

_buttonCancel = new Button();
_buttonCancel.label = "Cancel";
_buttonCancel.addEventListener("click", cancelAction);
_buttonCancel.enabled = true;

In the "cancelAction" function I add the dispatchEvent to cancBtnClick as this will be the userdefined function.

protected function cancelAction(event:Event):void {
dispatchEvent(new Event("cancBtnClick"));
PopUpManager.removePopUp(this);
}

When deriving the component DialogWindow.mxml from another component, I can use this statement where myFunction is taken place of cancBtnClick in cancelAction function.

// from TestButton.mxml
<cmp:DialogWindow cancBthClick="myFunction()" />

The above works fine, but what if i want to call it using derived class call in a script. such as

public var dialogW:IFlexDisplayObject = null;
(dialogW as DialogWindow).cancBtnClick = "myFunction"; // this is an instance of DialogWindow

When trying this, of course there is no cancBtnClick. So i know i am not implementing this correctly.