Category Archives: OOP

EventDispatcher docs

I’ve added documentation for EventDispatcher in LDoc format, generated from the source file.

Here’s the basic usage:

All listeners receive the following fields in the parameter event table:
– event.target (the listener itself)
– event.source (the dispatcher)

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EventDispatcher update

Just a quick post that EventDispatcher has been updated with new features including new methods on(), emit(), once() that are similar to Node.js, a new method removeAllListeners(), and a debug function printListeners(). The on() and emit() methods are actually aliases to the addEventListener() and dispatchEvent() methods.

Chris Byerley (aka develephant) who developed Coronium.io and Coronium.gs is using EventDispatcher under the hood for the game server client code. Check out his projects if you’re a Lua / Corona developer.

Here’s the original post for EventDispatcher if you wish to read about it.

Grab the latest code at Github.

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Send custom events with EventDispatcher for Corona SDK / Lua

In Corona SDK, event listeners can only be added to the global Runtime object or to display objects. In order to broadcast messages to event listeners, the event dispatcher is limited to either one or the other. This limitation places messaging in the two scopes instead of between objects that are supposed to be talking to each other. I’ve seen many examples with display objects that are created solely for the purpose of dispatching events. This just doesn’t feel right to me; so I’m releasing my EventDispatcher, perhaps other developers may find it useful too.

Those who came from the good old Flash 5 days may remember FLEM (FLash Event Model). It was the first listener event model for Flash and ActionScript, and was created by Branden Hall and further developed and maintained by me. I’ve adapted the basic event model mechanism found in ActionScript 2/3 to Lua. This EventDispatcher has a similar interface as the listener model in Corona SDK, with some extra features thrown in (such as optional extra parameters when dispatching events, and returning status).

EventDispatcher provides custom event broadcaster/listener mechanism to regular Lua objects, it works as regular Lua 5.1 / 5.2 code, in Corona SDK, and likely other Lua-based frameworks.

[ Update: See http://swfoo.com/?p=718 ]

Basic usage:

Sample code below demonstrates how it can be used:

Here is the output from the code:

iPad is turned off by Artist2 (table)
Artist2 is resting
Rested 1
Cowboy1 is drawing a gun
Cowboy2 is drawing a gun
Artist1 is drawing a picture
iPad is turned on by Artist2 (function)
Artist2 is drawing on the iPad
Removed
iPad is turned off by Artist2 (table)
Artist2 is resting
Artist1 is resting
iPad is turned off by Artist1 (table)
Rested 2
Cowboy2 is drawing a gun and shooting a bandit
iPad is turned on by Artist2 (function)
Artist2 is drawing a bandit on the iPad
Mayor collected 42 pieces of gold from Dave

And here is the EventDispatcher module:

Update (Sept 12, 2014): Added aliases ‘on’ and ‘emit’ for ‘addEventListener’ and ‘dispatchEvent’ for Coronium GS.

EventDispatcher provides a broadcaster/listener event mechanism to regular Lua objects. Corona developers can write cleaner object-oriented messaging code that doesn’t rely on display objects or send messages from the global Runtime.

For the latest code, check out EventDispatcher at github.

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Inheritance in ActionScript 2.0

Back in the good old days of Flash 5 and MX, trying to do inheritance in ActionScript required some knowledge of what goes on behind the scene. Also, the infamous superclass invocation bug haunted developers for almost three years.

Now in ActionScript 2.0, with the new OOP-specific keywords, inheritance is much simpler:

That’s all. And the keyword “super()” in the Child class constructor is not even mandatory because the compiler inserts it for you!

As for the superclass invocation bug, it is now fix for Flash player 7!

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abstract class

Since ActionScript 2.0 does not have the “abstract” modifier, here’s one way to create a class that acts like an abstract class, by making the constructor private:

When the following statement is compiled, the compiler will complain that one can’t instantiate from this class because the constructor is private:

var o:PretendToBeAbstractClass = new PretendToBeAbstractClass();

Although the constructor is private, but because it behaves as protected, you can still extend this class:

class MyClass extends PretendToBeAbstractClass {
}

What is missing is the compiler does not actually know what an abstract class is; therefore there won’t be any warning messages.

As I mentioned in the last post, you can get around all the type-checking at runtime, and even instantiating an “abstract” class at runtime like this:

var o = new _global["PretendToBeAbstractClass"]();

Or even access “private” properties from it:

trace(o.somePrivateVar);

Obviously these actions defeat the purpose of strict-typing, but it is possible (until runtime type-checking is implemented).

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Singleton

Here’s one way to implement a Singleton in ActionScript 2.0:


The count property is for keeping count of the number of Singleton objects referenced (obviously it is not aware of destroyed objects), and is not really a necessary member of the Singleton pattern.

To use this class, one would write something like this:

var s1:Singleton = Singleton.instance;
var s2:Singleton = Singleton.instance;

Now s1 and s2 are the same instance of the Singleton class.

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private, protected & public

In ActionScript 2.0, there are “private” and “public” modifiers, but there is no “protected”. However, “private” behaves like “protected” – i.e. subclasses and instances can access private members; so there is no true private scope.

To make sure the compiler catches access to private properties/functions, strict-typing has to be used:

Because type-checking is only performed at compile time, there is no guarantee that private members cannot be accessed at runtime.

By default, if the “public” modifier is left out, the member is assumed to be public.

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