AS3: EventListener callback overrides function's default parameters?

Question

I discovered encountered something weird today. I have a function with a Boolean default parameter:

function f(boolean:Boolean=false) {
    trace(boolean);
}

Calling it normally gives what you would expect:

f(); //traces false

But now if I make my function the callback for an Event Listener, something weird happens:

addEventListener("test",f);
dispatchEvent(new Event("test")); //traces true

The same thing happens if I make it a callback for clicking on something:

mySprite.addEventListener(MouseEvent.CLICK,f); //Traces true on click

Can anyone explain what is going on? What is happening to my default value of false? When the event listener calls my function, it's not passing any parameters.

score 1 · Answer 1 · answered Jan 17 '15 at 17:04

I don't think the listener will use the default parameter. You could use a small inline function to wrap the function you want to get called:

function f(boolean:Boolean=false) {
    trace(boolean);
}

addEventListener("test", function(e:Event):void { f() } );
dispatchEvent(new Event("test")); //traces false

score 0 · Answer 2 · answered Jan 17 '15 at 16:52

0

when u dispatch a event, your listener function would accept parameter from upstream, then the parameter convert to boolean

answered Jan 17 '15 at 16:52

ssskip

259
1
6

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. – arulmr Jan 18 '15 at 06:04
@arulmr just Actionscript basic, it's clean for this answer i think. – ssskip Jan 18 '15 at 06:08
This does look like an answer - not a comment. – Danny Staple Jan 18 '15 at 11:50

score 0 · Accepted Answer · answered Jan 17 '15 at 17:17

As Adobe said about the listener function :

... This function must accept an Event object as its only parameter and must return nothing

So in your case, if you do :

function f(boolean:Boolean = false) {
    trace(boolean);
}

stage.addEventListener(MouseEvent.CLICK, f);    // gives : true

OR

var event:MouseEvent = new MouseEvent(MouseEvent.CLICK);

trace(Boolean(event));                      // gives : true

f(event);                                   // gives : true

You will get always the same result : true, because every time, your event var pass automatically to your f function and converted to Boolean and because it's not null you will get true :

trace(Boolean(!null))                       // gives : true

AS3: EventListener callback overrides function's default parameters?

3 Answers3