specific questions about scope and property reference in actionscript 3

I've been battling with AS3 for a little while now, and I'm working on a simple application using only actionscript and the FlashDevelop/flex-compiler combo. I've hit a bit of a wall in my fledgling OOP understanding, and I'm wondering whether someone might be able to point me in the right direction. I have genuinely read several books, and spent many hours reading online tutorials etc, but something's just not clicking!

What's baffling me is this: When something is declared 'public', according to what I read, it is therefore available anywhere in the application (and should therfore be used with care!) However, when I try to use public properties and methods in my program, they most definitely are not available anywhere other than from the class/object that instantiated them.

This leads me to conclude that even if objects (of different class) are instantiated from the same (say 'main') class, they are not able to communicate with each other at all, even through public members.

If so, then fair enough, but I've honestly not seen this explained properly anywhere. More to the point, how do different objects communicate with other then? and what does Public actually mean then, if it only works through a direct composition hierarchy? If one has to write applications based only on communication from composer class to it's own objects (and presumably use events for, er, everything else?) - isn't this incredibly restrictive?

I'm su开发者_JAVA技巧re this is basic OOP stuff, so my apologies in advance!

Any quick tips or links would be massively appreciated.

There are different topics you are covering in your question. Let me clarify:

• What does the modifier public mean?
• How can instances of the same class communicate to each other?

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1.

In OOP you organize your code with objects. An object needs to be instantiated to provide its functionality. The place where you instantiate the object can be considered as the "context". In Flash the context might be the first frame, in a pure AS3 movie, it might be the main class, in Flex it could be the main mxml file. In fact, the context is always an object, too. Class modifier of your object public class MyClass tells your context whether it is allowed to instantiate the object or not. If set to internal, the context must live in the same directory as the class of the object. Otherwise it is not allowed to create a new object of the class. Private or protected are not valid class modifiers. Public class ... means that any context may create an object of that class. Next: Not only instantiation is controlled by these modifiers but also the visibility of a type. If set to internal, you cannot use an expression like var obj : InternalType in a context that does not live in the same directory as Internal type.

What about methods and properties? Even if your context is allowed to access a type, certain properties and methods might be restricted internal/protected/private var/method and you perhaps are not able to invoke them.

Why we're having such restrictions? Answer is simple: Differnent developers may develop different parts of the same software. These parts should communicate only over defined interfaces. These interfaces should be as small as possible. The developer therefore declares as much code as possible to be hidden from outside and only the necessary types and properties publicly available.

1. Don't mix up with modifiers and global properties. The modifier only tells you if a context is allowed to see a type or method. The global variable is available throughout the code. So even if a class is declared to be public, instances of that class do not know each other by default. You can let them know by:

   • storing the instances in global variables
   • providing setter such as set obj1(obj1 : OBJ1) : void where each object needs to store the reference in an instance variable
   • passing the object as method arguments: doSomething(obj1 : OBJ1)

Hope this helps you to more understand OOP. I am happy to answer your follow up questions.

Jens

@Jens answer (disclaimer: I skimmed) appears to be completely correct.

However, I'm not sure it answers your question very directly, so I'll add a bit here.

A public property is a property of that class instance that is available for other objects to use(function: call, variable: access, etc). However, to use them you must have a reference (like a very basic pointer, if that helps?) to that object instance. The object that instantiates (creates, new ...) that object can take that reference by assigning it to a variable of that class type.

// Reference is now stored in 's'
public ExampleClass s = new ExampleClass();

If you'd like to, you do have the option of making a static property, which is available just by knowing the class name. That property will be shared by all instances of that class, and any external class can refer to it (assuming it's public static) by referring to the class name.

A public property is referred to by the reference you stored.

//public property access
s.foo
s.bar(var)

A static property is referred to by the class name.

//static property access
ExampleClass.foo
ExampleClass.bar(var)

Once you've created the instance, and stored the reference, to an object, you can pass it around as you'd like. The below object of type OtherExampleClass would receive the reference to 's' in its constructor, and would have to store it in a local variable of its own to keep the reference.

public OtherExampleClass s2 = new OtherExampleClass(s);