How to share an Array between all Classes in an application?

Question

I want to share an Array which all classes can \"get\" and \"change\" data inside that array. Something like a Global array or Multi Access array. How this is possible with Acti

Answer 1

NOTE: I discourage the use of Global Variables!

But here is your answer

You can go to your default package and create a file with the same name of your global variable and set the global variable public:

//File: GlobalArray.as

package {
    public var GlobalArray:Array = [];
}

And that's it! You have a global variable. You can acces from your code (from anywhere) like this:

function DoSomething() {
    GlobalArray.push(new Object());
    GlobalArray.pop();
    for each (var object:* in GlobalArray) {
        //...
    }
}

Answer 2

There are a couple of ways to solve this. One is to use a global variable (as suggested in unkiwii's answer) but that's not a very common approach in ActionScript. More common approaches are:

Class variable (static variable)

Create a class called DataModel or similar, and define an array variable on that class as static:

public class DataModel {
    public static var myArray : Array = [];
}

You can then access this from any part in your application using DataModel.myArray. This is rarely a great solution because (like global variables) there is no way for one part of your application to know when the content of the array is modified by another part of the application. This means that even if your data entry GUI adds an object to the array, your data list GUI will not know to show the new data, unless you implement some other way of telling it to redraw.

Singleton wrapping array

Another way is to create a class called ArraySingleton, which wraps the actual array and provides access methods to it, and an instance of which can be accessed using the very common singleton pattern of keeping the single instance in a static variable.

public class ArraySingleton {
    private var _array : Array;

    private static var _instance : ArraySingleton;


    public static function get INSTANCE() : ArraySingleton {
        if (!_instance)
            _instance = new ArraySingleton();

        return _instance;
    }


    public function ArraySingleton() {
        _array = [];
    }


    public function get length() : uint {
        return _array.length;
    }

    public function push(object : *) : void {
        _array.push(object);
    }

    public function itemAt(idx : uint) : * {
        return _array[idx];
    }
}

This class wraps an array, and a single instance can be accessed through ArraySingleton.INSTANCE. This means that you can do:

var arr : ArraySingleton = ArraySingleton.INSTANCE;
arr.push('a');
arr.push('b');
trace(arr.length); // traces '2'
trace(arr.itemAt(0)); // trace 'a'

The great benefit of this is that you can dispatch events when items are added or when the array is modified in any other way, so that all parts of your application can be notified of such changes. You will likely want to expand on the example above by implementing more array-like interfaces, like pop(), shift(), unshift() et c.

Dependency injection

A common pattern in large-scale application development is called dependency injection, and basically means that by marking your class in some way (AS3 meta-data is often used) you can signal that the framework should "inject" a reference into that class. That way, the class doesn't need to care about where the reference is coming from, but the framework will make sure that it's there.

A very popular DI framework for AS3 is Robotlegs.

Answer 3

As this question was linked recently I would add something also. I was proposed to use singleton ages ago and resigned on using it as soon as I realized how namespaces and references work and that having everything based on global variables is bad idea.

Aternative

Note this is just a showcase and I do not advice you to use such approach all over the place.

As for alternative to singleton you could have:

public class Global {
    public static const myArray:Alternative = new Alternative();
}

and use it almost like singleton:

var ga:Alternative = Global.myArray;
ga.e.addEventListener(GDataEvent.NEW_DATA, onNewData);
ga.e.addEventListener(GDataEvent.DATA_CHANGE, onDataChange);
ga.push(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, "ten");
trace(ga[5]); // 5

And your Alternative.as would look similar to singleton one:

package adnss.projects.tchqs 
{
    
    import flash.utils.Proxy;
    import flash.utils.flash_proxy;

    public class Alternative extends Proxy
    {
        private var _data:Array = [];
        private var _events:AltEventDisp = new AltEventDisp();
        private var _dispatching:Boolean = false;
        public var blockCircularChange:Boolean = true;
        
        public function Alternative() {}
        
        override flash_proxy function getProperty(id:*):* {var i:int = id;
            return _data[i += (i < 0) ? _data.length : 0];
            //return _data[id]; //version without anal item access - var i:int could be removed. 
        }
        
        override flash_proxy function setProperty(id:*, value:*):void { var i:int = id;
            if (_dispatching) { throw new Error("You cannot set data while DATA_CHANGE event is dipatching"); return; }
            i += (i < 0) ? _data.length : 0;
            if (i > 9 ) { throw new Error ("You can override only first 10 items without using push."); return;}
            _data[i] = value;
            if (blockCircularChange) _dispatching = true;
            _events.dispatchEvent(new GDataEvent(GDataEvent.DATA_CHANGE, i));
            _dispatching = false;
        }
        
        public function push(...rest) {
            var c:uint = -_data.length + _data.push.apply(null, rest);
            _events.dispatchEvent(new GDataEvent(GDataEvent.NEW_DATA, _data.length - c, c));
        }
        
        public function get length():uint { return _data.length; }
        
        public function get e():AltEventDisp { return _events; }
        
        public function toString():String { return String(_data); }
    }

}

import flash.events.EventDispatcher;
/**
 * Dispatched after data at existing index is replaced. 
 * @eventType   adnss.projects.tchqs.GDataEvent
 */
[Event(name = "dataChange", type = "adnss.projects.tchqs.GDataEvent")]
/**
 * Dispatched after new data is pushed intwo array.
 * @eventType   adnss.projects.tchqs.GDataEvent
 */
[Event(name = "newData", type = "adnss.projects.tchqs.GDataEvent")]
class AltEventDisp extends EventDispatcher { }

The only difference form Singleton is that you can actually have multiple instances of this class so you can reuse it like this:

public class Global {
    public static const myArray:Alternative = new Alternative();
    public static const myArray2:Alternative = new Alternative();
}

to have two separated global arrays or even us it as instance variable at the same time.

Note

Wrapping array like this an using methods like myArray.get(x) or myArray[x] is obviously slower than accessing raw array (see all additional steps we are taking at setProperty).

public static const staticArray:Array = [1,2,3];

On the other hand you don't have any control over this. And the content of the array can be changed form anywhere.

Caution about events

I would have to add that if you want to involve events in accessing data that way you should be careful. As with every sharp blade it's easy to get cut. For example consider what happens when you do this this:

private function onDataChange(e:GDataEvent):void {
    trace("dataChanged at:", e.id, "to", Global.myArray[e.id]);
    Global.myArray[e.id]++;
    trace("new onDataChange is called before function exits"); 
}

The function is called after data in array was changed and inside that function you changing the data again. Basically it's similar to doing something like this:

function f(x:Number) {
    f(++x);
}

You can see what happens in such case if you toggle myArray.blockCircularChange. Sometimes you would intentionally want to have such recursion but it is likely that you will do it "by accident". Unfortunately flash will suddenly stop such events dispatching without even telling you why and this could be confusing.

Download full example here

Why using global variables is bad in most scenarios?

I guess there is many info about that all over the internet but to be complete I will add simple example.

Consider you have in your app some view where you display some text, or graphics, or most likely game content. Say you have chess game. Mayby you have separated logic and graphics in two classes but you want both to operate on the same pawns. So you create your Global.pawns variable and use that in both Grahpics and Logic class.

Everything is randy-dandy and works flawlessly. Now You come with the great idea - add option for user to play two matches at once or even more. All you have to do is to create another instance of your match... right?

Well you are doomed at this point because, every single instance of your class will use the same Global.pawns array. You not only have this variable global but also you have limited yourself to use only single instance of each class that use this variable :/

So before you use any global variables, just think twice if the thing you want to store in it is really global and universal across your entire app.