AS3 [Event(name="", type="")], what is the significance?

Question

I develop with FlashDevelop3 R2 and the Flex 3.3 SDK and there are many occasions where I must use the embed metadata tag as such:

  [Embed(source="path/to/file")]
  private var Asset:Class;

I understand the above all well and good, and I am thankful it exists because I do not like to open the flash IDE too often.

When I am going through other authors classes lately, I have found an interesting metadata tag that I do not understand:

[Event(name="", type="")]

I have yet to see a situation where I require this, and furthermore I really just do not understand what it is for.

Thank in advance for your help.

Brian Hodge
blog.hodgedev.com hodgedev.com

score 20 · Answer 1 · answered May 09 '09 at 13:20

These [Event(name, type)] declarations describe which events a class instance is likely to dispatch.

They are actually useful for code completion - for instance when you type: mySprite.addEventListener(, your code editor (Flex Builder or FlashDevelop) will display a meaningful list of events that this object can dispatch.

So you can add these declarations in your code and benefit from a richer code completion.

Also note that this works with custom Event classes (see FlashDevelop's new Event class template).

package mycomp {
    import flash.events.Event;

    public class MyEvent extends Event {
         public const SOME_EVENT:String = "someEvent";
         // required Event type declarations
    }
}

package mycomp {
     [Event(name="someEvent", type="mycomp.MyEvent")]
     public class MyComp extends Sprite {
     }
}

package myproject {
     import mycomp.MyComp;

     public class MyProject {
          function MyProject() {
                var mc:MyComp = new MyComp();
                mc.addEventLister( //completes: SOME_EVENT + Sprite events
          }
     }
}

Exactly the answer I was looking for! – Jorjon Feb 29 '12 at 01:24 — Jorjon, Feb 29 '12 at 01:24

score 12 · Accepted Answer · edited Jun 20 '20 at 09:12

12

We use it for binding custom events to our custom MXML components. This tag allows you to reference it from MXML. See documentation:

[Event(name="enableChanged", type="flash.events.Event")]

class ModalText extends TextArea { ... }

<MyComp:ModalText enableChanged="handleEnableChangeEvent(event);"/>

The compiler will complain, however, if you try to refer to an event on an mxml tag that was not declared with an event metatag.

edited Jun 20 '20 at 09:12

Community

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answered Mar 23 '09 at 22:18

dirkgently

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Interesting, I have been finding this in AS3 projects that include no MXML, do you think this is because this class may be imported into and used in a project that utilizes MXML? – Brian Hodge Mar 23 '09 at 22:39
Yes, custom components are typically coded in pure AS3 to avoid performance issues. – dirkgently Mar 24 '09 at 07:04

AS3 [Event(name="", type="")], what is the significance?

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