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Tuesday, October 12, 2010

Understanding the AS3 Event System #2 - Custom Events

This post can be found on the new blog at Understanding the AS3 Event System #2 - Custom Events
This thread is part 2 in a "Understanding the AS3 Event System" series. It continues on the "office" illustration used in part 1. If you have not read part 1, it is recommended that you do so:

I originally wrote this thread as a response to a Kirupa forum thread:

This is my first draft, so any opinions or thoughts are deeply appreciated, especially if there is anything you still don't fully understand or would like me to clarify further.

Listen, don't tell my boss, but those days when work gets slow, I fire up some StarCraft! StarCraft is no fun alone, so Nico, Bob, Larry, and I all play against eachother. The problem is that we all need to be logged on at the same time in order to play together. I'm the one who plays the most, so I am the "Game Master" - the one who starts up the server, chooses a map, and waits for everyone else to join in.

We need some way to alert eachother when I start up a StarCraft game, AND keep it a secret from my boss (he reads all our emails, so I can't tell them via email). So, we use the Event system!

Custom Event Strings
We have planned that whenever I am about to start up a new game, I stand up and yell out to everyone "I am about to call Yamato!" (who our boss assumes is one of my Japanese clients)

Everyone knows that the custom event string (or event type) for when everyone listening should play starcraft is "Yamato". So, ahead of time, Nico, Bob, and Larry listen for my "Yamato" event:

homeButton.addEventListener("Yamato", startPlayingStarScraft);
Now, we may start playing dozens of different games, and I have chosen a different "code name" as the event string for each type of game:
homeButton.addEventListener("MarcoPolo", startPlayingAOE);
homeButton.addEventListener("Gelinor", startPlayingRuneScape);
homeButton.addEventListener("Germany", startPlayingCOD);
homeButton.addEventListener("Orcish", startPlayingWOW);

Custom Events
Now, I could create a different type of Event Folder for each type of game, such as StarCraftEvent, RuneScapeEvent, AOEEvent etc.

Each type of event file would have information inside of it, for instance, the "StarCraftEvent Folder" may have the following properties:
> the target - Me, since I'm the one "dispatching" the event
> the type - the custom event string, in this case "Yamato"
> map - the StarCraft map we will be playing in
> players - a list of all players
> settings - the game settings

And our class would look something like this (note that "target" and "type" are automatically inherited by the Event since you "extend" it)
public class StarCraftEvent extends Event
    public function StarCraftEvent(the_type:String, the_map:SCMap, the_players:Array, the_settings:GameSettings)
        //Since you extend the Event, let the super class know a few more additional details
        map = the_map;
        players = the_players;
        settings = the_settings;
    public var map:SCMap;
    public var players:Array;
    public var settings:GameSettings
This would be my totally custom made event class! Perfectly customized for whenever we want to start a StarCraft game, allowing everyone to get the information they need!

Dispatching the Custom Event
This is exactly as simple as it was dispatching the "clicked" event:
//Create the event
var players:Array = [Andreas, Nico, Bob, Larry];
var scEvent:StarCraftEvent = new StarCraftEvent("Yamato", lostTemple, players, defaultSettings);

//Dispatch the event (and it's folder containing all the info)
That dispatches an event which alerts everyone who is listening that I am about to start up a StarCraft game.

So, Nico, Bob, and Larry run up to my cubicle and I hand them all the StarCraftEvent Folder containing ALL the information they need to join in the game.

Do you really need to create Custom Events?
Now I can create a new Custom Event for each and every game we play and that would alright.

But I noticed, no one really reads the information in the Event Folder! I spent good money putting together and printing out all the information for those folders, and no one reads them! They get the file from me, then they run back to their own cubicles, fire up the game, and throw the Event Folder directly into the trash.

When you think about it, do they really NEED all that extra information such as maps or players? They will find all that information out anyway when they start up StarCraft. There really is no use going into all the hassle of creating custom events!

Instead, what if I print out a plain old Event Folder? All it says is the "target" and the "type", but if they want more information (which one one does) they can ask me for it directly.

FORGET about the hassle with the StarCraftEvent class, and just do this:
var eventFolder:Event = new Event("Yamato");
99% of the time. That is all you will ever need. You save a lot of unnecessary work.

You don't always need to create custom Events, usually it is enough just using custom Event Strings.

That is Custom Events 101


  1. You can further expand the capabilities of custom events like sending custom data by overriding the clone method and preferably setting bubble to true. That makes the events more flexible. You can include it in the next series.
    Nice tutorials btw.

  2. Oh I see that you're sending custom data. So its okay. My bad.

  3. Ah, yes, I still forgot to override the clone method though. I probably should sneak a paragraph about that in somewhere...

    Anyway, bubbling is in the next lesson. Though, I still haven't written it, but it's on it's way.

    Any ideas on a fourth article after that, or does that pretty much cover events? Perhaps one about signals?

  4. Okay here is an article that describes why its important that we override clone method. Even if we didn't still it works. But sometimes it won't.

    Okay you write about signals. I didn't use those custom libraries.