Flex adding event listeners with callback functions which have no arguments

Question

fileReference.addEventListener(Event.COMPLETE, uploadCompleteHandler);

  private function uploadCompleteHandler(event:Event):void {}

Above is one way to add an event listener in Actionscript. By default the callback function needs to have an argument with name event and type Event. Is there a way to declare this function without any arguments :

  private function uploadCompleteHandler():void {}

Edit : It's possible to add an event handler without any arguments in mxml. So one student wanted to know, why isn't it possible to do the same in actionscript?

Answer 1

No, you'll get a runtime exception when the event fires if you'll try to register it as event listener.

You may use the following syntax to allow calling your handler directly without event object:

private function uploadCompleteHandler(event:Event = null):void {}

Answer 2

The reason is that in mxml what you write isn't actually the handler, is what gets executed in the handler. If you compile with -keep-generated-actionscript flag (To set it in Flex Builder right click to open the project Properties, select Flex Compiler, and add -keep-generated-actionscript to the Additional compiler arguments), you can see in the generated source for your component, that the compiler created a handler for that event, and the body is composed by that you wrote in mxml.

So if you have something like:

click="doSomething();"

You can already notice that you're actually giving an instruction there, that's not a method reference you're passing like when you use addEventHandler.

Then you'll have in the generated file something like:

private function myComponent_Click(evt : MouseEvent) : void
{
    doSomething();
}

And somewhere else in the same file the adding of the event listener:

this.addEventListener(MouseEvent.CLICK, myComponent_Click);

Notice the second parameter is not a function result, it's a function reference, because the parenthesizes that denote a function call are missing, and our particular function is not a getter either.

You can also specify more calls in mxml, like:

click="doSomething(); doSomethingElse();"

You can even pass the event parameter to your method:

click="doSomething(event);"

Whatever you write in the value of the mxml event (not sure it's the right term to use) will become the body of the generated handler for the actionscript event.